Updated on 2023/02/05

写真a

 
MURAKAMI, Hideyuki
 
Scopus Paper Info  
Paper Count: 0  Citation Count: 0  h-index: 19

Citation count denotes the number of citations in papers published for a particular year.

Affiliation
Faculty of Science and Engineering, Graduate School of Advanced Science and Engineering
Job title
Professor(without tenure)

Research Institute

  • 2022
    -
    2024

    Waseda Research Institute for Science and Engineering   Concurrent Researcher

Education

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    The University of Tokyo   The Faculty of Engineering  

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    東京大学大学院工学研究科金属工学専攻  

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    Department of Metallurgy, The University of Tokyo  

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    Department of Materials Science and Metallurgy, the University of Tokyo  

Degree

  • Doctor of Engineering

Professional Memberships

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    米国TMS

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    日本鉄鋼協会

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    日本金属学会

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    日本MRS

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    Metals & Materials Society

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    The Minerals

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    The Materials Research Society of Japan

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    The Japan Institute of Metals

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    The Iron and Steel Institute of Japan

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Research Areas

  • Structural materials and functional materials

  • Composite materials and interfaces

  • Metallic material properties

Research Interests

  • 構造材料

  • 耐酸化

  • コーティング

  • 耐熱合金

  • Oxidation

  • Coatings

  • Heat resistant materials

▼display all

Papers

  • Calculation of constitution of nickel base superalloys by cluster variation method

    Masato Enomoto, Hiroshi Harada, Hideyuki Murakami

    Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan   80 ( 6 ) 57 - 62  2023.06

     View Summary

    High temperature performance of nickel-base superalloys vitally depends on the constitution of alloys, i.e. volume fraction and composition etc. of constituent phases. The development of computer models to predict the effects of multiple alloying additions on the alloy constitution may facilitate the designing of alloys and lead to further improvement of high temperature performance. Some of the present authors have demonstrated that the cluster variation method with tetrahedron approximation which utilizes the Lennard-Jones pair potential can predict satisfactorily various characteristics of γ′/γ equilibrium in Ni-Al alloys containing one or two alloying elements without excessively increasing the computational load. In this study, calculations are extended to some practical and experimental multicomponent alloys and the results are compared with experimental data on the volume fraction and composition of γ′ and γ phases, lattice misfit and site occupation of alloying elements in the γ′ lattice. In the present form the model can be used to calculate the constitution of single crystal superalloys in which no interstitial trace elements such as carbon and boron are added to prevent grain boundary embrittlement.

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  • Thermal plasma CVD of SiC under reduced pressure

    Hideyuki Murakami, Kimihiro Higuchi, Toyonobu Yoshida

    Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals   56 ( 4 ) 452 - 458  2023.04

     View Summary

    The main purpose of this study is to develop a novel coating process by thermal plasma chemical vapor deposition (TPCVD) under reduced pressure around 3.5×104Pa. We began with the calculation of temperature and flow fields in hybrid Ar plasmas which were operated under reduced pressure and local thermodynamical equilibrium (LTE) conditions in order to investigate its effectiveness for TPCVD. The derived results suggested that the reduced pressure operation causes two main effects; (1) heating efficiency of the exit gas is raised to about 73% because of the reduction of radiation energy loss, and (2) plasma velocity is drastically accelerated to the order of 100 m/s at the torch exit. Moreover, the excellent stability and velocity controllability of hybrid plasma were also shown. Based on these modeling works, high rate and large area coating of SiC was tried at 3.5×104Pa by injecting SiCl4 and CH4 as reactants into Ar plasmas. During deposition, plasma tail flame revealed two distinct zones, that is, a central high intensity luminous zone and a relatively low intensity greenful one. Dense grossy SiC films with grain size of 3 to 7 nm and Vickers hardness over 3000 kgf/mm2 were deposited successfully at the maximum deposition rate of 70 nm/s on a graphite substrate in the latter zone at the deposition temperature less than 1470 K. Particularly noteworthy is that the dense film depositing area spread over 150 mm in diameter. In conclusion, these theoretical and experimental investigations have confirmed the possibility of high rate and large area ceramics deposition by TPCVD using a hybrid plasma system.

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  • Elemental effects on the oxidation of refractory compositionally complex alloys

    Kai Chi Lo, Hideyuki Murakami, Uwe Glatzel, Jien Wei Yeh, Stéphane Gorsse, An Chou Yeh

    International Journal of Refractory Metals and Hard Materials   108  2022.11

     View Summary

    The oxidation of refractory compositionally complex alloys (RCCAs) is one of the most critical subjects for their high temperature applications. In this work, several RCCAs were systematically designed and isothermally oxidised at 1000 °C for up to 24 h; the elemental effects of Al, Cr, Mo, Nb Ta, Ti, V, and Zr on the oxidation of the RCCAs were studied. It was found that Al, Cr, Ti could enhance the oxidation resistance of the RCCAs by promoting the formation of rutile-type AlTaO4, CrTaO4, and their related oxides, while Zr and V formed non-protective oxides such as Nb2Zr6O17, Ta2Zr6O17, ZrTiO4, and VTa9O25, resulting in severe internal oxidation. Mo appeared to be effective in suppressing internal oxidation, but it could form non-protective MoTiTa8O25. Ta was slightly more beneficial than Nb for reducing the oxidation mass gain. This study provides insights on the elemental effects on the oxidation and guidelines for improving the oxidation resistance of RCCAs.

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  • Effects of yttrium and silicon contents in Hastelloy-X built by selective laser melting process

    Santhosh Banoth, Thaviti Naidu Palleda, Takuma Saito, Hideyuki Murakami, Koji Kakehi

    Journal of Alloys and Compounds   896   163050 - 163050  2022.03  [Refereed]

     View Summary

    The effects of the addition of yttrium (Y) on the creep and tensile properties of Hastelloy-X built by a selective laser melting (SLM) process were studied. Two alloys were chosen: Hastelloy-X (Y-free) as a reference material and a specimen with 0.046 wt% yttrium (Y-added). Post-processing solution heat treatment (ST) was conducted at 1177 °C in air for 2 h, followed by air cooling to obtain appreciable mechanical properties. In the as-built condition, the Y-free specimen showed the formation of cracks; this was mainly due to the segregation of solute elements such as W, Si, and C at the grain boundaries and interdendritic regions. The Y-added specimen was free of cracks and showed longer creep life and rupture elongation than the Y-free specimen because the reduced Si content in the Y-added alloy eliminated cracking. In addition, the formation of Y2O3 oxides improved tensile elongation and creep properties by stabilizing solute oxygen contamination. After ST, both vertical and horizontal Y-added specimens presented better creep properties than ST Y-free specimens. This was mainly due to grain morphological change, carbides, and oxides formation at the grain boundary after ST heat treatment in the Y-added specimens.

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  • Development of platinum-group-metals-containing high entropy alloys with outstanding thermal capability and hot hardness

    Te Kang Tsao, Saad Sheikh, Hideyuki Murakami

    Applied Physics Letters   119 ( 19 )  2021.11

     View Summary

    Four equi- or near equi-molar platinum-group-metals-containing high entropy alloys (PGM-HEAs) have been developed with the aim to identify promising materials for ultra-high temperature applications. Experimental results indicate that IrNiPtRh and IrNiPtRhCo HEAs can form single phase fcc solid-solution, while secondary phases are also formed within the fcc matrix in IrNiPtRhAg0.5 and IrNiPtRhAl0.5 alloys, which are identified as (Ag, Pt)-rich fcc2 and (Pt, Ni)3Al-L12 phases, respectively. For mechanical performance, the room to high temperature hardness of PGM-HEAs is outstanding, especially for IrNiPtRhAl0.5, which shows significantly improved hot hardness than those of Ni-based superalloys, such as IN718 and Udimet 720Li, at temperatures above 800 °C, while it also exhibits much higher melting tolerance. It is related to the contributions from the advantages of platinum-group-metals, strong solution hardening, and L12 precipitation strengthening, thus providing superior resistance against thermal softening. It is believed that the current findings will shed light toward a promising research direction for ultrahigh-temperature applications, which is hard to achieve for the traditional platinum-group-alloys. In addition, the design space of compositionally complex alloys for future investigations would be significantly broadened.

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  • Dimensional stability of a metastable FCC high entropy alloy

    Chun Lin Lin, Tso Wei Chen, Yao Jen Chang, Hideyuki Murakami, Seiji Mitani, An Chou Yeh

    Applied Physics Letters   119 ( 17 )  2021.10

     View Summary

    In this work, a metastable face-centered cubic High Entropy Alloy (HEA) Fe35.0Co28.7Ni24.8Cr1.1Al1.3V4.5Ti1.2Nb2.8Si0.6 was found to exhibit significantly suppressed thermal expansion coefficient (6.0 ppm/K) and stable Young's modulus over a wide temperature range. Experimental and theoretical analyses suggest that both Invar and Elinvar effects were present; in addition, the metastability of this alloy with respect to the first-order phase transition and the magnetic second-order phase transition led to abrupt changes in thermal expansion behavior. The first-order phase transition was associated with the formation of a plate-like D019 phase. The magnetic second-order phase transition decreased the Invar effect and the Elinvar effect and resulted in a significant magnetic entropy difference (-3.12 J kg-1 K-1) in this HEA of interest.

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  • Investigation on the thermal expansion behavior of FeCoNi and Fe<inf>30</inf>Co<inf>30</inf>Ni<inf>30</inf>Cr<inf>10-x</inf>Mn<inf>x</inf> high entropy alloys

    Chun Lin Lin, Jhuo Lun Lee, Shih Ming Kuo, Ming Yen Li, Lu Gan, Hideyuki Murakami, Seiji Mitani, Stéphane Gorsse, An Chou Yeh

    Materials Chemistry and Physics   271  2021.10

     View Summary

    This work investigates the thermal expansion behaviors of Fe30Co30Ni30Cr10-xMnx High Entropy Alloys (HEAs) (x = 0, 5, 10 at.%) from 400 K to 1200 K. Interestingly, comparing to that of Cantor alloy, a decrease in Cr and Mn in Co–Cr–Fe–Mn–Ni system could significantly decrease the thermal expansion coefficient by 45.2%; Fe30Co30Ni30Cr10-xMnx (x = 0, 5, 10) also showed an abrupt change in thermal expansion behaviors similar to that of Invar alloys. Experimental and theoretical analysis suggest the abrupt change in thermal expansion behaviors of these HEAs were associated with the transition from ferromagnetism to paramagnetism, and these alloys appear to exhibit the dimensional stability of the Invar effect. Since the Invar effect is related to ferromagnetic properties, the amount of Cr and Mn in the HEAs would influence the suppression of thermal expansion due to different intensity of anti-ferromagnetic coupling effect. This research contributes to the understanding of the thermal expansion behaviors of Co–Cr–Fe–Mn–Ni high entropy alloys and the effects of antiferromagnetic elements.

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  • Correlation between solution treatment temperature, microstructure, and yield strength of forged ti-17 alloys

    Yoko Yamabe-Mitarai, Syuji Kuroda, Norie Motohashi, Takanobu Hiroto, Akira Ishida, Hideyuki Murakami, Yoshio Itsumi

    Crystals   11 ( 6 )  2021.06

     View Summary

    The Ti compressor disks of aviation jet engines are produced by forging. Their microstructure, which depends on the forging conditions, strongly affects their mechanical properties. In this study, changes in the microstructure of Ti-17 alloy as a result of different solution-treatment (ST) temperatures and the related tensile yield strengths were investigated to elucidate the correlation between the ST temperature, microstructure, and yield strength. Ti-17 alloys ingots were isothermally forged at 800 C and solution-treated at 750, 800, and 850 C. The microstructure and yield strength were investigated for samples subjected to different ST temperatures. The primary a phase formed during the ST, and the secondary a phase formed during the aging treatment at 620 C. The yield strength increased with increasing volume fraction of the primary a phase and increased further upon formation of the secondary a phase during the tensile test at room temperature. The correlation of the primary and secondary a phases with yield strength was clarified for tensile properties at room temperature, 450, and 600 C. An equation to predict the yield strength was constructed using the volume fraction of the primary and secondary a phases.

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  • Modeling the precipitation processes and the formation of hierarchical microstructures in a single crystal high entropy superalloy

    Stéphane Gorsse, Yung Ta Chen, Wei Che Hsu, Hideyuki Murakami, An Chou Yeh

    Scripta Materialia   193   147 - 152  2021.03

     View Summary

    Although superior high temperature tensile yield strength of high entropy superalloys (HESAs) arises from their hierarchical microstructure, the precipitation processes driving its formation remains unclear. In the present study, we analyze the kinetics of γ’ and γ precipitations by treating the concurrent nucleation, growth and coarsening using common computational thermodynamic and kinetic tools to simulate the microstructure genesis and evolution in HESA during thermal treatments. The ability of the simulations to reproduce the experimentally observed microstructure parameters is evaluated. Temperature-time-transformation (TTT) diagrams are calculated to serve as guidelines for further optimization of the hierarchical microstructure of HESAs.

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  • Microstructure and area specific resistance of cathodic half cells for solid oxide fuel cells composed of perovskite-type cathodes and Co-alloy-coated ferritic stainless steel interconnects

    Lu Gan, Xabier Montero, Saad Ahmed Sheikh, Isao Saeki, Hideyuki Murakami

    Surface and Coatings Technology   406  2021.01

     View Summary

    In order to prevent oxidation and Cr poisoning, a Co-W coating has been applied on ferritic stainless steel (FSS), which is used as the interconnect parts of solid oxide fuel cells (SOFCs). However, the electrical properties of the Co–W-coated stainless steels have not yet been evaluated. In this study, cathodic half-cells were experimentally manufactured with La0.8Sr0.2MnO3 (LSM) and La0.8Sr0.2Co0.8Fe0.2O3 (LSCF) as cathodes, and uncoated, Co-coated, and Co–W-coated FSSs as interconnects, respectively. Then their oxidation property and area-specific resistance (ASR) were evaluated at 800 °C. Based on phase identification and microstructural analysis, the Co–W-coating was confirmed to effectively suppress the Cr poisoning of the cathode. The ASR values of the Co–W-coated steel with LSM and LSCF as cathodes were 102 and 97 mΩ cm2, respectively, which are adequate for SOFC application. Furthermore, when the LSM was applied as a cathode material, the formation of Mn spinels enhanced the adherence between the cathode and interconnected parts. Thus, the combination of Co–W-coated FSS steel with LSM as the cathode material exhibited the optimal combination of electrical conductivity and microstructural stability. Graphic abstract.

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  • Martensitic transformation of high-entropy and medium-entropy shape memory alloys

    Hiromichi Matsuda, Masayuki Shimojo, Hideyuki Murakami, Yoko Yamabe-Mitarai

    Materials Science Forum   1016 MSF   1802 - 1810  2021

     View Summary

    As new generation of high-temperature shape memory alloys, high-entropy alloys (HEAs) have been attracted for strong solid-solution hardened alloys due to their severe lattice distortion and sluggish diffusion. TiPd is the one potential high-temperature shape memory alloys because of its high martensitic transformation temperature above 500 °C. As constituent elements, Zr expected solid-solution hardening, Pt expected increase of transformation temperature, Au expected keeping transformation temperature, and Co expected not to form harmful phase. By changing the alloy composition slightly, two HEAs and two medium entropy alloys (MEAs) were prepared. Only two MEAs, Ti45Zr5Pd25Pt20Au5, and Ti45Zr5Pd25Pt20Co5 had the martensitic transformation. The perfect recovery was obtained in Ti45Zr5Pd25Pt20Co5 during the repeated thermal cyclic test, training, under 200 MPa. On the other hand, the small irrecoverable strain was remained in Ti45Zr5Pd25Pt20Au5 during the training under 150 MPa because of the small solid-solution hardening effect. It indicates that Ti45Zr5Pd25Pt20Co5 is the one possible HT-SMA working between 342 and 450 °C.

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  • Tensile creep behavior of single-crystal high-entropy superalloy at intermediate temperature

    Takuma Saito, Akira Ishida, Michinari Yuyama, Yuji Takata, Kyoko Kawagishi, An Chou Yeh, Hideyuki Murakami

    Crystals   11 ( 1 )  2021.01

     View Summary

    In this study, we investigated the creep deformation mechanism of a single-crystal highentropy superalloy (HESA) with the spherical 0 precipitates at 760 °C. Before the creep tests, long-term aging tests at 760 °C without load were conducted, which showed Ostwald ripening of the secondary 0 precipitates up to 50 h. The creep tests revealed that in the range of 500 and 600 MPa at 760 °C, the creep deformation mechanism of HESA was independent of applied stress in both the primary and secondary creep regions. The deformation mechanism of HESA was further investigated under the condition of 760 °C and 520 MPa by performing creep interrupted tests and microstructural analysis. Scanning electron microscope observation showed elongated 0 precipitates along the applied stress axis near the ruptured surface. This could have been caused by the multi-slip around <100> preceded by the lattice rotation into <100> along the tensile axis, which was confirmed by the electron backscatter diffraction analysis. Transmission electron microscope observation of the creep interrupted and ruptured specimens showed bypass and climb motion of dislocations in the 2-h interrupted, shearing of the 0 precipitates by the paired straight dislocations in the 50-h interrupted, and shearing of the 0 precipitates by both the straight and the curved paired dislocations in the ruptured specimens, respectively. The secondary 0 precipitates do not affect creep behavior as long as the deformation mechanism is a bypass and climb motion of dislocations.

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  • Correction to: Fabrication and Oxidation Resistance of TiAl Matrix Coatings Reinforced with Silicide Precipitates Produced by Heat Treatment of Warm Sprayed Coatings (Journal of Thermal Spray Technology, (2018), 27, 7, (1165-1176), 10.1007/s11666-018-0751-x)

    Judyta Sienkiewicz, Seiji Kuroda, Hideyuki Murakami, Hiroshi Araki, Maciej Giżyński, Krzysztof J. Kurzydłowski

    Journal of Thermal Spray Technology   29 ( 8 ) 2080  2020.12

     View Summary

    The primary affiliation of author Judyta Sienkiewicz should be listed as: Research Center for Structural Materials, National Institute for Materials Science, Tsukuba-Shi, Japan, and the secondary affiliation as: Institute of Armament Technology, Faculty of Mechatronics and Aerospace Military University of Technology, Warsaw, Poland.

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  • Effect of heat treatments on the microstructural evolution of a single crystal high-entropy superalloy

    Takuma Saito, Yung Ta Chen, Yuji Takata, Kyoko Kawagishi, Wei Che Hsu, An Chou Yeh, Hideyuki Murakami

    Metals   10 ( 12 ) 1 - 17  2020.12

     View Summary

    The effect of solution heat treatment as well as primary and secondary aging treatment conditions on the microstructural evolution of a high-entropy superalloy was investigated. The as-cast sample shows coarsened γ′ precipitates and other extra phases at interdendrite region due to microsegregation. This microsegregation makes γ′ solvus unclear and decreases the solidus. After conducting the solution treatment determined in this study, primary aging conditions showing an aligned cubic γ′ phase at 1050◦C for 4 h and random spherical γ′ precipitates at 950◦C for 20 h in similar size were found. By using both samples, secondary aging conditions showing coarsened secondary γ′ precipitates and γ particles inside the γ′ precipitates at 800◦C for 20 h and fine secondary γ′ precipitates at 850 and 870◦C for 20 h were found.

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  • Hierarchical microstructure strengthening in a single crystal high entropy superalloy

    Yung Ta Chen, Yao Jen Chang, Hideyuki Murakami, Taisuke Sasaki, Kazuhiro Hono, Chen Wei Li, Koji Kakehi, Jien Wei Yeh, An Chou Yeh

    Scientific Reports   10 ( 1 )  2020.12

     View Summary

    A hierarchical microstructure strengthened high entropy superalloy (HESA) with superior cost specific yield strength from room temperature up to 1,023 K is presented. By phase transformation pathway through metastability, HESA possesses a hierarchical microstructure containing a dispersion of nano size disordered FCC particles inside ordered L12 precipitates that are within the FCC matrix. The average tensile yield strength of HESA from room temperature to 1,023 K could be 120 MPa higher than that of advanced single crystal superalloy, while HESA could still exhibit an elongation greater than 20%. Furthermore, the cost specific yield strength of HESA can be 8 times that of some superalloys. A template for lighter, stronger, cheaper, and more ductile high temperature alloy is proposed.

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  • Thermal stability of the CoWO<inf>4</inf> layer formed on ferritic stainless steel

    Lu Gan, Toshiyuki Nishimura, Saad Ahmed Sheikh, Isao Saeki, Hideyuki Murakami

    Corrosion Science   176  2020.11

     View Summary

    This study focuses on the temperature capability of CoWO4 as the oxidation protection layer of ferritic stainless steel (FSS). The thermogravimetric-differential thermal analysis revealed that until CoWO4 started to melt at 1297 °C, little mass change was observed. In addition, the coefficient of thermal expansion of CoWO4 was 11.5 ± 0.2 (×10−6 K−1), which was similar to that of FSS, evidencing its good thermal stability and compatibility for the protection of FSS. The oxidation test of the Co–W coated specimens at 700 °C – 1000 °C in air confirmed effectiveness of the CoWO4 layer at T ≤ 850 °C.

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  • Designing high entropy superalloys for elevated temperature application

    Yung Ta Chen, Yao Jen Chang, Hideyuki Murakami, Stéphane Gorsse, An Chou Yeh

    Scripta Materialia   187   177 - 182  2020.10

     View Summary

    In the context of cast alloy development for high temperature applications, high entropy superalloys (HESA) have exhibited superior cost specific tensile strength than that of superalloys such as CM247LC. Compositions of HESA are distinctively different from those of cast superalloys with higher contents of Fe and Ti, making HESA cheaper and lighter. Comparing to superalloys, although HESA has adopted the template of FCC-structured (γ) matrix and coherent L12-structured (γ′) precipitates, γ′ is enriched with solutes with higher intrinsic strength, rendering positive lattice misfit, and the high entropy γ matrix may have attributed to a good combination of strength and ductility.

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  • Electroplating of copper on low carbon steel from alkaline citrate complex baths

    Isao Saeki, Takuma Harada, Isamu Tanaka, Tetsuya Ando, Lu Gan, Hideyuki Murakami

    ISIJ International   60 ( 9 ) 2031 - 2037  2020.09

     View Summary

    The use of alkaline electroplating baths is the essential requirement to deposit Cu directly onto steels because of non-adherent Cu formation by replacement reaction between Cu2+ and Fe in acidic solution. For the development of such an electroplating bath, complexing agents to form soluble Cu complex in alkaline pH is necessary at first. Secondary, the soluble Cu complex must be reduced electrochemically. Cyanide-based baths meet these requirements, but the bath is toxic. In this study, the survey of complexing agents revealed that citric and tartaric acids form soluble copper complex solutions in alkaline pH, and electroplating is possible. The cathodic current density range to obtain smooth and adhesive electroplating with citrate complexed bath was extensive than that with a tartrate bath. It was found that 0.1 mol dm -3 CuSO4 - 0.5 mol dm -3 citric acid baths with pH of 9-11 are optimum to obtain adhesive and uniform Cu layer. Copper electroplating with an acidic CuSO4-H2SO4 bath was possible on 1 μm Cu layer with the alkaline citrate bath. Because the plating rate is high with the acidic bath, the multilayer Cu electroplating from the citrate bath and then an acidic sulfate bath gives a reasonable way for Cu coating onto steels. Elongation test of the steel sheet electroplated with the multilayer Cu showed that detachment of the Cu layer was limited in the vicinity of the broken part of the sheet. It is concluded that the toxic cyanide Cu plating bath can be replaced with a citrate bath.

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  • The Effects of Chemistry Variations in New Nickel-Based Superalloys for Industrial Gas Turbine Applications

    Sabin Sulzer, Magnus Hasselqvist, Hideyuki Murakami, Paul Bagot, Michael Moody, Roger Reed

    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science   51 ( 9 ) 4902 - 4921  2020.09

     View Summary

    Industrial gas turbines (IGT) require novel single-crystal superalloys with demonstrably superior corrosion resistance to those used for aerospace applications and thus higher Cr contents. Multi-scale modeling approaches are aiding in the design of new alloy grades; however, the CALPHAD databases on which these rely remain unproven in this composition regime. A set of trial nickel-based superalloys for IGT blades is investigated, with carefully designed chemistries which isolate the influence of individual additions. Results from an extensive experimental characterization campaign are compared with CALPHAD predictions. Insights gained from this study are used to derive guidelines for optimized gas turbine alloy design and to gauge the reliability of the CALPHAD databases.

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  • Alloying effect on the oxidation behavior of a ductile Al<inf>0.5</inf>Cr<inf>0.25</inf>Nb<inf>0.5</inf>Ta<inf>0.5</inf>Ti<inf>1.5</inf> refractory high-entropy alloy

    S. Sheikh, L. Gan, A. Ikeda, H. Murakami, S. Guo

    Materials Today Advances   7  2020.09

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    Refractory high-entropy alloys (RHEAs) are widely studied because of their promising potential for ultrahigh-temperature applications. One key challenge towards the development of RHEAs as high-temperature structural materials is to concurrently achieve optimum oxidation resistance and mechanical properties. Here in this work, the effect of alloying on the oxidation behavior of ductile RHEAs was studied. Specifically, a ductile RHEA, Al0.5Cr0.25Nb0.5Ta0.5Ti1.5, was alloyed with Al and Zr aiming to improve its oxidation resistance. The two modified RHEAs, Al0.75Cr0.25Nb0.5Ta0.5Ti1.5 and Al0.5Cr0.25Nb0.5Ta0.5Ti1.5Zr0.01, indeed show enhanced oxidation resistance at 800 °C and 1,100 °C, compared with Al0.5Cr0.25Nb0.5Ta0.5Ti1.5. In addition, all three RHEAs studied here show an excellent oxidation resistance at 800 °C compared with other RHEAs, although there is still a large space to further improve their performance at 1,100 °C. Internal oxidation and even nitridation are still present after oxidation exposure, indicating further efforts are required to form protective oxide scales on the surface of ductile RHEAs. Nevertheless, the work is expected to shed some light on future directions of improving the oxidation of ductile RHEAs, via the alloying route.

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  • Breakaway corrosion of austenitic steel induced by fireside corrosion

    X. Montero, A. Ishida, M. Rudolphi, H. Murakami, M. C. Galetz

    Corrosion Science   173  2020.08

     View Summary

    The long-term fireside corrosion in biomass co-firing of Sanicro 25 was characterized employing advanced high-resolution electron microscopy methods. The analysed sample was exposed for 7000 h at 650 °C showing breakaway corrosion. The corrosion mode differed from type II hot corrosion. The main effects were the formation of a thick nickel sulphide layer and accelerated metal loss. This sulphide formation became possible due to subsurface zone chromium depletion by the formation of stable chromium carbides. With decreasing Cr activity both the ability to maintain the protective scale and the possibility to trap sulphur in Cr sulphides are reduced.

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  • Forming protective alumina scale for ductile refractory high-entropy alloys via aluminizing

    Saad Sheikh, Lu Gan, Xabier Montero, Hideyuki Murakami, Sheng Guo

    Intermetallics   123  2020.08

     View Summary

    Refractory high-entropy alloys (RHEAs), or multi-principal-element refractory alloys, have been intensively studied in recent years, due to their attractive potential for ultrahigh-temperature structural applications. One formidable challenge facing the materials development for RHEAs though, is to simultaneously achieve good oxidation resistance and good mechanical properties, which unfortunately often impose contradictory microstructural requirements. So far, there exist no ductile RHEAs that possess reasonable oxidation resistance, and the formation of protective oxide scales such as alumina in them is never seen. Here we report, for the first time, a strategy to form protective alumina scale in ductile RHEAs upon high temperature exposure, using a tailor-designed two-step pack aluminizing process. Very importantly, the oxidation resistance of ductile RHEAs improves tremendously, as evidenced from harsh cyclic oxidation tests, thus providing a huge impetus to push forward the further development of RHEAs, towards ultrahigh-temperature applications.

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  • Microstructure and diffusion behavior in the multilayered oxides formed on a Co–W electroplated ferritic stainless steel followed by oxidation treatment

    Lu Gan, Tomoyuki Yamamoto, Hideyuki Murakami

    Acta Materialia   194   295 - 304  2020.08

     View Summary

    Numerous attempts have been made to improve the oxidation resistance and electrical conductivity of the interconnectors in solid oxide fuel cells. A Co–W alloy coating on ferritic stainless steel has attracted attention because the Co–W oxide layer formed by the oxidation treatment of the Co–W alloy coating has proven effective in reducing the outward diffusion of Cr and improving oxidation resistance. This study was designed to elucidate the diffusion behavior of elements and the barrier mechanism of the CoWO4 layer. After oxidation in air at 750°C, a dense, multilayered oxide formed, comprising (from the stainless steel substrate to the outer layer) Cr2O3, (Cr,Fe,Co)3O4, CoWO4, (Co,Fe)3O4, and Co3O4 layers. The CoWO4 layer and neighboring oxide layers were carefully analyzed by scanning electron microscopy and transmission electron microscopy with electron energy-loss spectroscopy, confirming the absence of trivalent cations (Co3+, Fe3+, and Cr3+) and the presence of Fe2+ ions in the CoWO4 layer; thus, CoWO4 functions as a selective diffusion barrier to trivalent cations, as hypothesized. The Cr-containing oxide layer grows based on the reaction between the metallic cations from the substrate and the inward-diffusing oxide ions, whereas Fe and Co can diffuse outward through CoWO4 as Fe2+ and Co2+ ions.

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  • Microstructure and oxidation behavior of pt and ptir diffusion coatings on ni-based single crystal superalloy

    Dao Chi Tue, Te Kang Tsao, Akira Ishida, Hideyuki Murakami, Le Thi Hong Lien

    Materials Transactions   61 ( 8 ) 1671 - 1678  2020.08

     View Summary

    The effects of Ir addition and coating method on the microstructure and oxidation behavior of PtxIr (x = 030 at%) diffusion coatings were investigated. A nickel-based single crystal superalloy UCSX-8 was used as a substrate material, while the coatings were developed by either an electroplating or a paste coating method followed by an annealing heat treatment. The phase identification and microstructure analyses by XRD and SEM/EDS revealed that the alloying of Ir in Pt diffusion coating resulted in the formation of L10 ordered ¡-NiPt2Al structure. Cyclic oxidation tests were carried out at 1423 K in still air in order to investigate the thermal stability and oxidation behavior of the coatings. It was found that Ir can significantly retard the formation of voids in both the coating and substrate. In addition, by replacing electroplating method to the paste coating method, the crack problem due to the brittle feature of electroplated PtIr coatings could be solved. Therefore, the Pt20Ir diffusion coating prepared by the paste-coating method is promising as the bond-coating material due to formation of less voids, no crack and stable Al2O3 on the surface.

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  • Oxidation behaviour of a novel refractory high entropy alloy at elevated temperatures

    Kai Chi Lo, Hideyuki Murakami, Jien Wei Yeh, An Chou Yeh

    Intermetallics   119  2020.04

     View Summary

    The refractory high entropy alloy (RHEA) has shown great potentials for high temperature applications beyond modern Ni-based superalloy. However, its oxidation behaviours are rarely reported and understood. In this work, the oxidation behaviours of a novel RHEA “NV1”, Cr-17.6Al-20.3Mo-15.2Nb-2.9Si-13.4Ta-5.4Ti (in at%), were examined at 1200, 1300, and 1400 °C up to 100 h. At 1200 °C, the oxidation kinetics curve yielded toward parabolic behaviour owing to the formation of a rutile-type complex oxide layer with Al2O3 and Cr2O3 dispersions; breakaway oxidation contributed by Cr2O3 evaporation occurred at 1300 °C; a single power-law behaviour governed the oxidation kinetics curve at 1400 °C, and mullite was identified within the oxide layer. This work provides guidelines for understanding the oxidation mechanisms and improving oxidation resistance of RHEA at elevated temperature.

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  • Effect of surface treatment and crystal orientation on hot corrosion of a Ni-based single-crystal superalloy

    X. Montero, A. Ishida, T. M. Meißner, H. Murakami, M. C. Galetz

    Corrosion Science   166  2020.04

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    The effect of surface preparation and crystal orientation was investigated for a TMS-138 single crystal superalloy under Type I and Type II hot corrosion conditions. The superalloy was cut along the {100} and {110} planes on which grit blasting, mechanical polishing and electro-polishing surface finishes were conducted. At 900 °C grit blasting or electropolishing distorted the surface and altered the γ-γ’ structure of the alloy provoking higher corrosion rates than on mechanically polished surfaces. The electropolished surface revealed that crystal planes with higher atomic number density have an increased corrosion resistance. The anisotropy effects were not observed with Type II exposures.

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  • Effect of W content on the oxidation and electrical behaviors of CoW coatings for SOFC interconnects fabricated by electrodeposition

    Lu Gan, Hideyuki Murakami, Isao Saeki

    Materials Transactions   61 ( 3 ) 490 - 496  2020

     View Summary

    Vaporization of Cr species from solid oxide fuel cell (SOFC) interconnectors must be avoided to prevent the degradation of the electrocatalytic activity of cathodes. A diffusion barrier against Cr ions in the thermally grown oxide is required for the prevention of vaporization because the origin of Cr is an interconnector substrate made of ferritic stainless steel. Additionally, the diffusion barrier should not decrease the electron conductivity of the interconnector. We found that a CoWO4 layer formed on type 430 stainless steel (18 mass% Cr) that was covered with a Co2.4 at% W alloy via the electroplating process. The layer effectively blocked the outward diffusion of Cr because the trivalent Cr ion, Cr3+, cannot penetrate this layer. In this report, further understanding of CoWelectroplated ferritic stainless steels is described. First, a CoW co-electroplating method with a high W content was developed. The W content increased with increasing W concentration in the bath and with the bath pH, and Co30 at% W plating was obtained. Then, electroplating was conducted on types 430 and 445 (23 mass% Cr) for the comparative study of oxidation properties. A thick and compact CoWO4 layer was formed on the Co5at% Welectroplated steel. Excess W in the plating with more than 10 at% W resulted in the formation of a FeCoCr intermetallic layer, which may cause a decrease in substrate toughness. The structures of the multi-layered oxides that formed on both alloy substrates were similar to each other. The low specific resistance of CoWO4 and the thin Cr2O3 that formed on the coated specimens resulted in the low area-specific resistance of the CoW- coated specimen at 800°C, and the value was lower than for the uncoated steels. We concluded that Co5at% Welectroplating on 23 mass% Cr stainless steels is optimal for SOFC interconnectors.

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  • Development of a Prediction Model and Process–Microstructure–Property Database on Forging and Heat Treatment of Superalloy 720Li

    Nobufumi Ueshima, Chuya Aoki, Toshio Osada, Satoko Horikoshi, Akira Yanagida, Hideyuki Murakami, Toshiki Ishida, Yoko Yamabe-Mitarai, Katsunari Oikawa, Nobuki Yukawa, Jun Yanagimoto

    Minerals, Metals and Materials Series     491 - 499  2020

     View Summary

    A process–microstructure–property database on forging and heat treatment of superalloy 720Li was established by high precision large-scale 1500 ton forging simulator and laboratory-scale forging simulator. The database was utilized to determine the parameters of flow stress, microstructure, and strength prediction models. The models were integrated to CAE software to predict process–microstructure–property relationships. In the integrated model, the stress, strain, and temperature distributions and their temporal development are calculated by using flow stress model and thermophysical properties. The calculated stress, strain, and temperature data are inputted into the microstructure model. The microstructure model considers grain growth, recrystallization, and precipitation of γ′ and calculates the temporal evolution of microstructural features. The strength model considers solution, grain boundary, and precipitation strengthening and calculates high-temperature 0.2% tensile proof stress, which is related to creep and low-cycle fatigue properties, from the calculated microstructural features. The integrated model successfully predicted the load, microstructure, and strength distribution of a prototype forging experiment conducted by the Hitachi Metals 6000 ton forging machine. The integrated model is a promising tool to design the forging and heat treatment process of the alloy.

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  • An oxidation resistant refractory high entropy alloy protected by CrTaO <inf>4</inf> -based oxide

    Kai Chi Lo, Yao Jen Chang, Hideyuki Murakami, Jien Wei Yeh, An Chou Yeh

    Scientific Reports   9 ( 1 )  2019.12

     View Summary

    Although refractory high entropy alloys (RHEAs) have shown potentials to be developed as structural materials for elevated temperature applications, most of the reported oxidation behaviours of RHEA were associated with short term exposures for only up to 48 hours, and there is a lack of understanding on the oxidation mechanism of any RHEA to-date. In this work, by using thermogravimetric analysis, isothermal oxidation was conducted on a novel RHEA at 1000 °C and 1100 °C for up to 200 hours, which is an unprecedented testing duration. The external oxide layer strongly influenced the weight gain behaviours, and it consisted of CrTaO 4 -based oxide with some dispersion of Al 2 O 3 and Cr 2 O 3 . At 1000 °C, the inability to form dense CrTaO 4 -based oxide layer resulted an exponential dependence of weight gain throughout 200 hours. At 1100 °C, mass gain curve showed two parabolic dependences associated with the formation of protective CrTaO 4 -based oxide layer and the weight gain after 200 hours was 4.03 mg/cm 2 , which indicates that it is one of the most oxidation resistant RHEAs comparing to literature data to-date. This work can also provide insights on how to further develop RHEA to withstand long term oxidation at elevated temperatures.

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  • A strategy of designing high-entropy alloys with high-temperature shape memory effect

    Je In Lee, Koichi Tsuchiya, Wataru Tasaki, Hyun Seok Oh, Takahiro Sawaguchi, Hideyuki Murakami, Takanobu Hiroto, Yoshitaka Matsushita, Eun Soo Park

    Scientific Reports   9 ( 1 )  2019.12

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    Shape memory effect, the ability to recover a pre-deformed shape on heating, results from a reversible martensitic transformation between austenite and martensite phases. Here, we demonstrate a strategy of designing high-entropy alloys (HEAs) with high-temperature shape memory effect in the CrMnFeCoNi alloy system. First, we calculate the difference in Gibbs free energy between face-centered-cubic (FCC) and hexagonal-close-packed (HCP) phases, and find a substantial increase in thermodynamic equilibrium temperature between the FCC and HCP phases through composition tuning, leading to thermally- and stress-induced martensitic transformations. As a consequence, the shape recovery temperature in non-equiatomic CrMnFeCoNi alloys can be increased to 698 K, which is much higher than that of conventional shape memory alloys (SMAs) and comparable to that of B2-based multi-component SMAs containing noble metals (Pd, Pt, etc.) or refractory metals (Zr, Hf, etc.). This result opens a vast field of applications of HEAs as a novel class of cost-effective high-temperature SMAs.

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  • Microstructural, mechanical and electrochemical characterization of TiZrTaHfNb and Ti<inf>1.5</inf>ZrTa<inf>0.5</inf>Hf<inf>0.5</inf>Nb<inf>0.5</inf> refractory high-entropy alloys for biomedical applications

    Amir Motallebzadeh, Naeimeh Sadat Peighambardoust, Saad Sheikh, Hideyuki Murakami, S. Guo, Demircan Canadinc

    Intermetallics   113  2019.10

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    The state-of-the-art metallic biomaterials are 316L, CoCrMo and Ti6Al4V but they all suffer from known issues relating to biocompatibility, wear resistance and corrosion resistance. Therefore, there is always the motivation to identify novel superior metallic biomaterials to 316L, CoCrMo and Ti6Al4V. The concept of refractory high-entropy alloys (RHEAs) provides an interesting research direction towards developing novel metallic biomaterials, initially because RHEAs consist of purely biocompatible elements, but a systematic study of the performance of RHEAs targeting biomedical applications, while comparing to that of the state-of-the-art 316L, CoCrMo and Ti6Al4V, was not existing before and constitutes the theme of the current work. Two exemplary RHEAs that are studied in detail in this work, TiZrTaHfNb and Ti1.5ZrTa0.5Hf0.5Nb0.5, show highly promising characteristics as novel superior metallic biomaterials in that they possess a desirable combination of wear resistance, wettability and pitting and general corrosion resistance, outperforming 316L, CoCrMo and Ti6Al4V almost in all these important aspects. In addition, it is also shown in this work that how appropriate alloying in RHEAs can be utilized to fine-tune their performance as better metallic biomaterials, such as the correlation between lattice strain and corrosion resistance.

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  • Mechanical behaviour of conventional, Pt- and Pt/Ir-modified NiAl diffusion coatings after thermocyclic exposure at 1100°C

    Ceyhun Oskay, Mathias Christian Galetz, Hideyuki Murakami

    Materials at High Temperatures   36 ( 5 ) 404 - 416  2019.09

     View Summary

    The influence of Pt- and Pt/Ir-modifications on the mechanical properties of NiAl coatings after thermocyclic exposure at 1100°C was investigated. Four-point-bending tests with in-situ acoustic emission measurement were used to determine the fracture strain. After 100 h of exposure a steep increase in fracture strain was observed for the NiAl coating. In contrast, the increase was found to be moderate for modified coatings. For all investigated coatings the fracture strain was correlated with the mean Al-concentration in the diffusion zone, and a moderate increase in the β-phase field with decreasing Al-concentration was found. In the two-phase (β/γ') stability region fracture strain increased steeply. Indentation hardness instead showed a steep decrease within the single β-phase field, but remained almost constant in the two-phase region. Reduced elastic modulus followed a V-curve behaviour, in which a decrease with ongoing Al-depletion in the β-stability region was followed by an increase in the two-phase region.

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  • Correction to: Microstructure and Oxidation Performance of TiAl-(Cr, Nb, Ta) Coatings Fabricated by Warm Spray and High-Velocity Oxy-Fuel Spraying (Journal of Thermal Spray Technology, (2019), 28, 3, (563-579), 10.1007/s11666-019-00837-5)

    Judyta Sienkiewicz, Seiji Kuroda, Hideyuki Murakami, Hiroshi Araki, Maciej Giżyński, Krzysztof J. Kurzydłowski

    Journal of Thermal Spray Technology   28 ( 4 ) 880  2019.04

     View Summary

    The primary affiliation of author Judyta Sienkiewicz should be listed as: Research Center for Structural Materials, National Institute for Materials Science, Tsukuba-Shi, Japan, and the secondary affiliation as: Institute of Armament Technology, Faculty of Mechatronics and Aerospace Military University of Technology, Warsaw, Poland.

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  • Microstructure and Oxidation Performance of TiAl-(Cr, Nb, Ta) Coatings Fabricated by Warm Spray and High-Velocity Oxy-Fuel Spraying

    Judyta Sienkiewicz, Seiji Kuroda, Hideyuki Murakami, Hiroshi Araki, Maciej Giżyński, Krzysztof J. Kurzydłowski

    Journal of Thermal Spray Technology   28 ( 3 ) 563 - 579  2019.02

     View Summary

    To improve the oxidation resistance of near α-titanium alloy IMI834, TiAl-(Cr, Nb, Ta) coatings were deposited by applying high-velocity oxy-fuel (HVOF) and warm spray (WS). Comparison was made in terms of microstructure, surface morphology as well as isothermal and cyclic oxidation behaviors in the air at 750 °C up to 100 h and 100 cycles, respectively. The results show that smoother and less oxidized coatings were deposited by warm spraying. The microstructure of all coatings underwent an appreciable change during the oxidation tests, as in as-sprayed state it occurred in the nonequilibrium state. It was revealed that a small difference in the initial oxidation between the two spraying processes as well as microstructure, the level of porosity and surface roughness significantly influences the oxidation kinetics of the sprayed coatings at high temperature, which should affect the service lifetime as an oxidation-resistant layer for potential applications. After exposure at 750 °C in air, rutile TiO2 was found in addition to α-Al2O3 in the oxide scale formed on the HVOF and warm sprayed coatings. However, isothermal and cyclic oxidation tests of all WS TiAl-(Cr, Nb, Ta) coatings showed improved oxidation resistance of IMI 834 as well as good adherence to the substrate alloy.

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  • Development of multi-value circulation based on remanufacturing

    Kenichi Nakajima, Mitsutaka Matsumoto, Hideyuki Murakami, Masao Hayakawa, Yasunari Matsuno, Wataru Takayanagi

    Materiaux et Techniques   107 ( 1 )  2019

     View Summary

    Remanufacturing is an industrial process that turns used products into new ones with the same quality, functionality, and warranty as new products; it is a critical element for realizing a resource-efficient manufacturing industry and a circular economy. Remanufacturing may involve adding new and better functionality to used products, such as adding more wear-resistant materials to the surface or new sensor systems. Remanufacturing has been undertaken for products such as: Automobile parts, machinery, photocopiers, single-use cameras, furniture, and turbine components, etc. It is generally superior to material recycling in terms of energy and material savings. Our project aims to develop technologies necessary for the promotion of remanufacturing and to establish a cooperative network related to remanufacturing. As technical development items, our aim is to develop methods to assess the reliability of parts/components, develop technologies to restore deteriorated metal surfaces of used products, introduce production management methods for remanufacturing, and design a circulation system to retain the added values of products. In this paper, we introduce an outline of the project and present some preliminary results. This paper shows the possibility to quantitatively evaluate the carbide distribution (size and density) of the carburized surface of a gear, and also shows the potential to repair materials exposed to a high-Temperature oxidative atmosphere by Pr-Ir coating technology.

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  • Aluminizing for enhanced oxidation resistance of ductile refractory high-entropy alloys

    Saad Sheikh, Lu Gan, Te Kang Tsao, Hideyuki Murakami, Samrand Shafeie, Sheng Guo

    Intermetallics   103   40 - 51  2018.12

     View Summary

    Refractory high-entropy alloys (RHEAs) emerge as promising candidate materials for ultrahigh-temperature applications. One critical issue to solve for RHEAs is their balanced oxidation resistance and mechanical properties, mainly room-temperature ductility for the latter. Recently, it was found that existing ductile RHEAs are subject to catastrophic accelerated oxidation, also known as pesting. In this work, both alloying and surface coating, are applied to enhance the oxidation resistance of ductile RHEAs, with the focus on surface coating using the pack cementation method and more specifically, aluminizing. The oxidation resistance of two RHEAs, Hf0.5Nb0.5Ta0.5Ti1.5Zr, one recently identified ductile RHEA which pests in the temperature range of 600–1000 °C, and Al0.5Cr0.25Nb0.5Ta0.5Ti1.5, the newly designed ductile RHEA which does not pest but embrittles after oxidation, are studied after aluminizing at 900 °C using three different pack components. Aluminizing, if using the appropriate pack cementation parameters, can avoid pesting in Hf0.5Nb0.5Ta0.5Ti1.5Zr and alleviate the oxidation induced embrittlement in Al0.5Cr0.25Nb0.5Ta0.5Ti1.5, and holds the promise for further improving the RHEAs as potential ultrahigh-temperature materials.

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  • Oxide scale formation and microstructural degradation of conventional, Pt- and Pt/Ir-modified NiAl diffusion coatings during thermocyclic exposure at 1100 °C

    C. Oskay, M. C. Galetz, H. Murakami

    Corrosion Science   144   313 - 327  2018.11

     View Summary

    The microstructural degradation of Pt- and Pt/Ir-modified NiAl coatings during thermocyclic exposure at 1100 °C was investigated. Detailed microstructural investigation revealed the different beneficial effects of Pt and Ir. Platinum was found to be active in the healing mechanism of the oxide scale, whereas Ir incorporation provided an obstruction to interdiffusion. Oxidized surface morphologies exhibited the formation of a dense alumina scale on top of the PtAl coating, whereas Al-depletion in the NiAl coating was accelerated due to scale spallation and interfacial void formation. Additionally, Pt inward- and Ni outward diffusion was substantially reduced in the Ir-containing coatings.

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  • Fabrication and Oxidation Resistance of TiAl Matrix Coatings Reinforced with Silicide Precipitates Produced by Heat Treatment of Warm Sprayed Coatings

    Judyta Sienkiewicz, Seiji Kuroda, Hideyuki Murakami, Hiroshi Araki, Maciej Giżyński, Krzysztof J. Kurzydłowski

    Journal of Thermal Spray Technology   27 ( 7 ) 1165 - 1176  2018.10

     View Summary

    Ti-Al-based intermetallics are promising candidates as coating materials for thermal protection systems in aerospace vehicles; they can operate just below the temperatures where ceramics are commonly used, and their main advantage is the fact that they are lighter than most other alloys, such as MCrAlY. Therefore, Ti-Al-Si alloy coatings with five compositions were manufactured by spraying pure Ti and Al-12 wt.% Si powders using warm spray process. Two-stage hot pressing at 600 and 1000 °C was applied to the deposits in order to obtain titanium aluminide intermetallic phases. The microstructure, chemical composition, and phase composition of the as-deposited and hot-pressed coatings were investigated using SEM, EDS, and XRD. Applying of hot pressing enabled the formation of dense coatings with porosity around 0.5% and hard Ti5(Si,Al)3 silicide precipitates. It was found that the Ti5(Si,Al)3 silicides existed in two types of morphologies, i.e., as large particles connected together and as small isolated particles dispersed in the matrix. Furthermore, the produced coatings exhibited good isothermal and cyclic oxidation resistance at a temperature of 750 °C for 100 h.

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  • Accelerated oxidation in ductile refractory high-entropy alloys

    Saad Sheikh, Muhammad Kurnia Bijaksana, Amir Motallebzadeh, Samrand Shafeie, Adrianna Lozinko, Lu Gan, Te Kang Tsao, Uta Klement, Demircan Canadinc, Hideyuki Murakami, Sheng Guo

    Intermetallics   97   58 - 66  2018.06

     View Summary

    Refractory high-entropy alloys (RHEAs) are promising candidates for new-generation high temperature materials, but they generally suffer from room temperature brittleness and unsatisfactory high-temperature oxidation resistance. There currently lack efforts to address to these two critical issues for RHEAs at the same time. In this work, the high temperature oxidation resistance of a previously identified ductile Hf0.5Nb0.5Ta0.5Ti1.5Zr RHEA is studied. An accelerated oxidation or more specifically, pesting, in the temperature range of 600–1000 °C is observed for the target RHEA, where the oxidation leads the material to catastrophically disintegrate into powders. The pesting mechanism is studied here, and is attributed to the failure in forming protective oxide scales accompanied by the accelerated internal oxidation. The simultaneous removal of zirconium and hafnium can eliminate the pesting phenomenon in the alloy. It is believed that pesting can also occur to other equiatomic and non-equiatomic quinary Hf-Nb-Ta-Ti-Zr or quaternary Hf-Nb-Ti-Zr and Hf-Ta-Ti-Zr RHEAs, where all currently available ductile RHEAs are identified. Therefore, the results from this work will provide crucial perspectives to the further development of RHEAs as novel high-temperature materials, with balanced room-temperature ductility and high-temperature oxidation resistance.

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  • High temperature oxidation of Co-W electroplated type 430 stainless steel for the interconnect of solid oxide fuel cells

    Lu Gan, Hideyuki Murakami, Isao Saeki

    Corrosion Science   134   162 - 168  2018.04

     View Summary

    Cobalt–tungsten alloy layer containing 2.4 at% W was electroplated on type 430 stainless steel (Fe–16 mass% Cr), and it was subjected to high-temperature oxidation at 800 °C in Air–3 vol% H2O atmosphere for 1000 h. A five-layered oxide of Co3O4, FeCo2O4, CoWO4, (Co,Fe,Cr)3O4, and Cr2O3 formed within 1 h, and the structure retained up to 1000 h. The outward diffusion of Cr3+ was effectively prevented by the CoWO4 layer. In the oxide, Co and W exist as divalent and hexavalent ions respectively, and there is no trivalent cation sites or trivalent cation vacancy through which Cr3+ diffuses.

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  • Microstructural investigation of oxidized complex refractory high entropy alloys

    Kai Chi Lo, An Chou Yeh, Hideyuki Murakami

    Materials Transactions   59 ( 4 ) 556 - 562  2018

     View Summary

    The microstructures of two newly developed refractory high entropy alloys were examined after isothermal oxidation at 1200°C for 10 hours. Scanning electron microscope analysis showed the formation of aluminosilicate layer on the sample surface, and the structure of oxide layers appears to be greatly affected by the content of Al and V. With increased Al content and decreased V content, the size of pores within the internal oxidation zone was decreased. Future directions to improve the oxidation resistance of complex refractory alloys were proposed.

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  • A study of NiCo<inf>0.6</inf>Fe<inf>0.2</inf>Cr<inf>x</inf>SiAlTi<inf>y</inf> high-entropy alloys for applications as a high-temperature protective coating and a bond coat in thermal barrier coating systems

    Wei Lin Hsu, Hideyuki Murakami, Hiroshi Araki, Makoto Watanabe, Seiji Kuroda, An Chou Yeh, Jien Wei Yeh

    Journal of the Electrochemical Society   165 ( 9 ) C524 - C531  2018

     View Summary

    Atmospheric plasma spraying, high-velocity oxy-fuel spraying and warm spraying processes were used to prepare NiCo0.6Fe0.2Cr1.5SiAlTi0.2 and NiCo0.6Fe0.2Cr1.3SiAl high-entropy alloy coatings for the investigation of their high temperature properties. The best-performer, the warm-sprayed high-entropy alloy coating was then applied to a thermal barrier coating system as a bond coat over an Incoloy 800H substrate. Experimental results show that the investigated high-entropy alloy coatings have high hardness, excellent wear resistance and good oxidation resistance at 1100◦C that makes them promising overlay coatings for elevated temperature applications.

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  • Strain controlled low cycle fatigue behavior of U720Li disk superalloy above 700 °C

    Fei Sun, Yuefeng Gu, Kyoko Kawagishi, Hideyuki Murakami, Yoko Yamabe-Mitarai

    Minerals, Metals and Materials Series   2018-June   779 - 788  2018

     View Summary

    The low cycle fatigue behaviors of Udimet 720Li (U720Li) and the related microstructure evolution have been investigated at 725 °C under strain control tests. The interrelationships between microstructure factors and properties were analyzed using transmission electron backscatter diffraction (t-EBSD) in scanning electron microscopy (SEM) and transmission electron microscopy (TEM). For comparison, LCF behaviors at 650 °C were also presented to find the inferior aspects of U720Li alloy at relatively higher service temperatures. The results show that recrystallization occurs during the LCF testing at 725 °C. The extent of recrystallization increases with the strain. The LCF property at 725 °C is weakened to a large extent after recrystallization, which could be the significant factor that causes LCF degradation. Combination t-EBSD and TEM are beneficial for characterizing and analyzing the microstructure evolution in terms of primary y' precipitates and dislocations, proposing that gliding dislocations could concentrate into walls to form sub-grain boundaries, with the combination of primary y' precipitates to form sub-grains.

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  • The High Temperature Tensile and Creep Behaviors of High Entropy Superalloy

    Te Kang Tsao, An Chou Yeh, Chen Ming Kuo, Koji Kakehi, Hideyuki Murakami, Jien Wei Yeh, Sheng Rui Jian

    Scientific Reports   7 ( 1 )  2017.12

     View Summary

    This article presents the high temperature tensile and creep behaviors of a novel high entropy alloy (HEA). The microstructure of this HEA resembles that of advanced superalloys with a high entropy FCC matrix and L12 ordered precipitates, so it is also named as "high entropy superalloy (HESA)". The tensile yield strengths of HESA surpass those of the reported HEAs from room temperature to elevated temperatures; furthermore, its creep resistance at 982 °C can be compared to those of some Ni-based superalloys. Analysis on experimental results indicate that HESA could be strengthened by the low stacking-fault energy of the matrix, high anti-phase boundary energy of the strengthening precipitate, and thermally stable microstructure. Positive misfit between FCC matrix and precipitate has yielded parallel raft microstructure during creep at 982 °C, and the creep curves of HESA were dominated by tertiary creep behavior. To the best of authors' knowledge, this article is the first to present the elevated temperature tensile creep study on full scale specimens of a high entropy alloy, and the potential of HESA for high temperature structural application is discussed.

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  • The Microstructure Stability of Precipitation Strengthened Medium to High Entropy Superalloys

    Te Kang Tsao, An Chou Yeh, Hideyuki Murakami

    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science   48 ( 5 ) 2435 - 2442  2017.05

     View Summary

    Medium and high entropy superalloys based on the Ni-Co-Fe system with strengthening L12 γ′ precipitates have been developed. The present study has shown that by controlling the elemental partitioning between γ/γ′, thermal stability of γ′ can be enhanced in the high entropy γ matrix. Most importantly, high entropy superalloys exhibit stable γ–γ′ microstructures with no TCP phases after long-term exposure at elevated temperatures. Therefore, a new alloy design space for stable γ–γ′ microstructure has been presented. Furthermore, due to relatively high content of Fe and Ti, their raw materials cost and alloy density can potentially be lower than those of conventional superalloys.

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  • Formation and subsequent phase evolution of metastable Ti-Al alloy coatings by kinetic spraying of gas atomized powders

    M. Gizynski, S. Miyazaki, J. Sienkiewicz, S. Kuroda, H. Araki, H. Murakami, Z. Pakiela, A. Yumoto

    Surface and Coatings Technology   315   240 - 249  2017.04

     View Summary

    High temperature application of modern Ti-alloys is often limited by their insufficient oxidation resistance at temperatures exceeding 650 °C. One way to overcome this obstacle is application of an overlay coating made of TiAl-based alloys. In this study kinetically sprayed coatings of Ti-46Al-8.5Nb-1Ta (at%) on a commercial alloy IMI-834 were investigated. The feedstock powder fabricated by gas atomization consisted of non-equilibrium α-Ti and β-Ti phases. Heat treatment of the powder at 750 °C triggered transformation to the equilibrium ordered γ-TiAl and α2-Ti3Al phases. Mechanical properties of both the as-atomized and the heat treated powders were tested, showing that the as-sprayed one possesses lower hardness and higher ductility. Since such properties of the as-atomized powder are more suited for kinetic spraying, the as-atomized powder was used for coating fabrication. The as-sprayed coatings inherited the non-equilibrium phases of the atomized powder with significant deformation. The coatings were annealed at 750 °C and immediate transformation to a refined microstructure containing the γ-TiAl and α2-Ti3Al phases was observed in < 20 min. Longer annealing time resulted in significant grain growth and formation of defect-free grains.

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  • On the study of thermal-sprayed Ni<inf>0.2</inf>Co<inf>0.6</inf>Fe<inf>0.2</inf>CrSi<inf>0.2</inf>AlTi<inf>0.2</inf> HEA overlay coating

    Wei Lin Hsu, Hideyuki Murakami, Jien Wei Yeh, An Chou Yeh, Kazuya Shimoda

    Surface and Coatings Technology   316   71 - 74  2017.04

     View Summary

    The present works investigated the microstructure, mechanical properties and oxidation behavior of the Ni0.2Co0.6Fe0.2CrSi0.2AlTi0.2 high-entropy alloy (HEA) coating fabricated by atmospheric plasma spraying (APS) and high-velocity oxygen-fuel spraying (HVOF) process. The hardness of HEA coating is about 800 HV and its wear resistance (20 m/mm3) is nearly twice higher than that of the SUJ2 bearing steel (12 m/mm3). The isothermal oxidation weight gain test demonstrates that the HEA coating exhibits good oxidation resistance similar to MCrAlY coating at 1100 °C due to the formation of a protective α-Al2O3 scale. The thermal-sprayed HEA coating is thus promising as an overlay coating for high-temperature applications.

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  • Chromium and iridium effects on the short-term interdiffusion behaviour between Pt rich γ-γ′ bond-coatings and a Ni-Al-Cr alloy

    Pauline Audigié, Aurélie Rouaix-Vande Put, Hideyuki Murakami, Daniel Monceau

    Surface and Coatings Technology   309   258 - 265  2017.01

     View Summary

    The interdiffusion behaviour of a 5 μm thick layer of Pt deposited by electroplating on a γ-Ni-12Al-10Cr model alloy was studied in order to assess the effect of Cr. Heat treatments were performed for 1 min up to 1 h at 1100 °C under argon. Cr addition increased the uphill diffusion of Al to the surface when compared with Pt/γ-(Ni,Al) systems. Al and Cr had a positive chemical interaction in presence of Pt, as shown by the positive values of the DAlCrNi and DCrAlNi diffusion coefficients determined by modelling. Pt had a negative chemical interaction with Al and with Cr in such a way that Pt decreased their activities. According to the diffusion coefficient values, Pt had a greater influence on the Al activity than on the Cr one. Similarly, 2 μm of Pt and 3 μm of Pt-25Ir were deposited by electroplating on the same model alloy to investigate the effect of Ir. Heat treatments were performed in the same conditions as for Cr. Iridium slowed down the interdiffusion when compared with systems with Pt only. Iridium diffused slower toward the substrate than Pt and a lower Pt + Ir flux toward the substrate was found. As voids formed at the interdiffusion zone/substrate interface due to Kirkendall effect, this lower inward Pt + Ir flux resulted in a lower outward vacancy flux and then Ir reduced Kirkendall voids formation. Moreover, Ir decreased the Pt effect on Al activity by dilution or even gave an opposite contribution to the Pt one. This reduced the uphill diffusion of Al, delaying the α-NiPtAl phase formation. Diffusion paths of each model system were also identified after 15 min at 1100 °C and all highlighted the α-NiPtAl phase formation and its aptitude to be used in TBC systems.

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  • On The Superior High Temperature Hardness of Precipitation Strengthened High Entropy Ni-Based Alloys

    Te Kang Tsao, An Chou Yeh, Chen Ming Kuo, Hideyuki Murakami

    Advanced Engineering Materials   19 ( 1 )  2017.01

     View Summary

    The microstructure and high temperature hardness of two face-centered cubic high entropy Ni-based alloys with L12 γ′ precipitates have been studied. Both alloys exhibit higher mixing entropy and with the advantages in lower density and lower cost of raw materials than conventional Ni-based superalloys. Their γ′ solvus are above 1 150 °C, and the γ–γ′ microstructure can be thermodynamically stable after isothermal ageing from 700 to 1 100 °C for at least 500 h. By XRD peak deconvolution, positive lattice misfits between γ and γ′ have been shown till elevated temperatures. The results from nano-indentation test indicate that their highly alloyed γ′ phase have rendered more significant strengthening, and the underlying mechanism can be attributed to the higher anti-phase boundary energy. Therefore, with minor refractory additions, the bulk hardness of present alloys can surpass that of commercial superalloy from room to high temperature.

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  • Research of application of iridium-hafnium alloy for a chamber of bipropellant thruster

    Go Fujii, Hideyuki Kobayashi, Hirohide Ikeda, Kazuya Shimoda, Hideyuki Murakami, Masahiro Sasaki, Yuichi Iwamoto, Tomoaki Miyazawa, Tomohiro Maruko

    53rd AIAA/SAE/ASEE Joint Propulsion Conference, 2017    2017

     View Summary

    This research showed the possibility of applying iridium hafnium alloy for a bipropellant thruster chamber. It also revealed that the heat resistant temperature of iridium hafnium alloy with aluminizing exceeded almost 1600°C, and that the alloy had excellent resistance to oxidation. A chamber made of the iridium hafnium alloy with aluminizing was then manufactured and its endurance for almost 1600°C was demonstrated in the hot firing tests. Through these tests, this alloy could be a good candidate material for the chamber of a high performance thruster and could be provide several advantages for customers.

  • Highly Segmented Thermal Barrier Coatings Deposited by Suspension Plasma Spray: Effects of Spray Process on Microstructure

    Xiaolong Chen, Hiroshi Honda, Seiji Kuroda, Hiroshi Araki, Hideyuki Murakami, Makoto Watanabe, Yoshio Sakka

    Journal of Thermal Spray Technology   25 ( 8 ) 1638 - 1649  2016.12

     View Summary

    Effects of the ceramic powder size used for suspension as well as several processing parameters in suspension plasma spraying of YSZ were investigated experimentally, aiming to fabricate highly segmented microstructures for thermal barrier coating (TBC) applications. Particle image velocimetry (PIV) was used to observe the atomization process and the velocity distribution of atomized droplets and ceramic particles travelling toward the substrates. The tested parameters included the secondary plasma gas (He versus H2), suspension injection flow rate, and substrate surface roughness. Results indicated that a plasma jet with a relatively higher content of He or H2 as the secondary plasma gas was critical to produce highly segmented YSZ TBCs with a crack density up to ~12 cracks/mm. The optimized suspension flow rate played an important role to realize coatings with a reduced porosity level and improved adhesion. An increased powder size and higher operation power level were beneficial for the formation of highly segmented coatings onto substrates with a wider range of surface roughness.

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  • Columnar and DVC-structured thermal barrier coatings deposited by suspension plasma spray: high-temperature stability and their corrosion resistance to the molten salt

    Xiaolong Chen, Takuma Ohnuki, Seiji Kuroda, Maciej Gizynski, Hiroshi Araki, Hideyuki Murakami, Makoto Watanabe, Yoshio Sakka

    Ceramics International   42 ( 15 ) 16822 - 16832  2016.11

     View Summary

    High-temperature stability of SPS YSZ coatings with the columnar and deep vertically cracked (DVC) structures and their corrosion resistance to 56 wt% V2O5+44 wt% Na2SO4 molten salt mixture were investigated. Both the columnar and DVC-structured YSZ coatings were sintered at 1000 °C, but a significant increase in porosity in combination with significant reductions in Vickers’ hardness and Young's modulus were observed at the temperatures from 1200 °C to 1400 °C. The DVC-structured YSZ coating exhibited superior corrosion resistance against the molten salt mixture attack to the columnar-structured one due to its higher density behaving as a sealing protective top layer at 950 °C.

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  • High temperature oxidation and corrosion properties of high entropy superalloys

    Te Kang Tsao, An Chou Yeh, Chen Ming Kuo, Hideyuki Murakami

    Entropy   18 ( 2 )  2016

     View Summary

    The present work investigates the high temperature oxidation and corrosion behaviour of high entropy superalloys (HESA). A high content of various solutes in HESA leads to formation of complex oxides, however the Cr and Al activities of HESA are sufficient to promote protective chromia or alumina formation on the surface. By comparing the oxidation and corrosion resistances of a Ni-based superalloy-CM247LC, Al2O3-forming HESA can possess comparable oxidation resistance at 1100 °C, and Cr2O3-forming HESA can exhibit superior resistance against hot corrosion at 900 °C. This work has demonstrated the potential of HESA to maintain surface stability in oxidizing and corrosive environments.

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  • A heat-resistant NiCo<inf>0.6</inf>Fe<inf>0.2</inf>Cr<inf>1.5</inf>SiAlTi<inf>0.2</inf> overlay coating for high-temperature applications

    Wei Lin Hsu, Hideyuki Murakami, Jien Wei Yeh, An Chou Yeh, Kazuya Shimoda

    Journal of the Electrochemical Society   163 ( 13 ) C752 - C758  2016

     View Summary

    This paper proposes an overlay coating based on the high-entropy alloy composition NiCo0.6Fe0.2Cr1.5SiAlTi0.2 [high-entropy alloy coating (HEAC)]. HEAC samples underwent spark-plasma sintering processes and were subjected to oxidation tests, high-temperature hardness tests, and thermal conductivity and thermal expansion measurements. Experimental results indicated that the HEAC can form protective α-Al2O3 at 1100°C and exhibits oxidation behavior similar to that of MCrAlY. Furthermore, compared to MCrAlY, the HEAC possesses better thermal stability, higher hot hardness, lower thermal conductivity, and smaller thermal expansion. The potential of the HEAC as an overlay coating for high-temperature applications is discussed.

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  • Isothermal oxidation of aluminized coatings on high-entropy alloys

    Che Wei Tsai, Kuen Cheng Sung, Kzauki Kasai, Hideyuki Murakami

    Entropy   18 ( 10 )  2016

     View Summary

    The isothermal oxidation resistance of Al0.2Co1.5CrFeNi1.5Ti0.3 high-entropy alloy is analyzed and the microstructural evolution of the oxide layer is studied. The limited aluminum, about 3.6 at %, leads to the non-continuous alumina. The present alloy is insufficient for severe circumstances only due to chromium oxide that is 10 μm after 1173 K for 360 h. Thus, the aluminized high-entropy alloys (HEAs) are further prepared by the industrial packing cementation process at 1273 K and 1323 K. The aluminizing coating is 50 μm at 1273 K after 5 h. The coating growth is controlled by the diffusion of aluminum. The interdiffusion zone reveals two regions that are the Ti-, Co-, Ni-rich area and the Fe-, Cr-rich area. The oxidation resistance of aluminizing HEA improves outstandingly, and sustains at 1173 K and 1273 K for 441 h without any spallation. The alumina at the surface and the stable interface contribute to the performance of this Al0.2Co1.5CrFeNi1.5Ti0.3 alloy.

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  • Microstructure changes and oxidation resistance of aluminized Ni-based single crystal superalloys

    Hideyuki Murakami, Kazuki Kasai

    Proceedings of the International Symposium on Superalloys   2016-January   719 - 726  2016

     View Summary

    Microstructural changes and oxidation resistance of simply aluminized Ni-based single-crystal superalloys were systematically investigated. There are several kinds of microstructural changes observed in the as-aluminized Ni-based single crystal superalloys. For instance, in the case of a gritblasted specimen, secondary reaction zone (SRZ) was formed in the vicinity of substrate/coating interfaces. On the other hand, in the polished specimens, voids were formed by thermal cyclic heating. It is also found that grit-blasted specimens with SRZ formation exhibited accelerated oxidation. Microstructural changes are slightly accelerated along 〈110〉 direction rather than 〈100〉 direction. These experimental results can be explained by the recrystallization of substrate surfaces and subsequent interdiffusion between the coated layer and the substrate. Cyclic oxidation tests were also performed to assess the oxidation resistance of specimens with different microstructure and it is found that specimens with SRZ formation demonstrated worse oxidation resistance than those with voids formation, probably due to the difference in interdiffusion of solute elements between aluminized coatings and substrates.

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  • High temperature properties of advanced directionally-solidified high entropy superalloys

    Te Kang Tsao, An Chou Yeh, Jien Wei Yeh, Mau Sheng Chiou, Chen Ming Kuo, H. Murakami, Koji Kakehi

    Proceedings of the International Symposium on Superalloys   2016-January   1001 - 1009  2016

     View Summary

    High-Entropy-Superalloys (HESA) with good cost-performance has been proposed. In present work, the high temperature phase stability, oxidation, hot corrosion and mechanical properties of HESA alloys were studied. The microstructure of HESA is composed of stable FCC γ matrix and L12 γ′ precipitates, and both γ and γ′ phases are highly alloyed. This γ/γ′ microstructure can remain stable after long term exposure at inter-mediate to high temperatures. With respect to surface stability, HESAs show high Al and Cr activities to form either protective Al2O3 or Cr2O3 rapidly. With minor additions of refractory elements, the high temperature hardness of HESA can be higher than that of conventional superalloy. HESAs show comparable tensile strength and creep rupture properties comparing to that of commercial superalloys. Furthermore, their densities are below 8.0 g.cm-3, and the cost of raw materials can be 20 % cheaper than that of CM247LC. Therefore, the potential of HESAs for high temperature applications are revealed in present article.

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  • Effects of Al Content and Addition of Third Element on Fabrication of Ti-Al Intermetallic Coatings by Heat Treatment of Warm-Sprayed Precursors

    J. Sienkiewicz, S. Kuroda, K. Minagawa, H. Murakami, H. Araki, K. J. Kurzydłowski

    Journal of Thermal Spray Technology   24 ( 5 ) 749 - 757  2015.06

     View Summary

    Four powder mixtures of titanium and aluminum with 50:50, 40:60, 30:70, and 20:80 atomic ratios were used as feedstock for Warm Spray process to produce composite coatings. A two-stage heat treatment at 600 and 1000 °C was applied to the deposits in order to obtain titanium aluminide intermetallic phases. The microstructure, chemical, and phase composition of the as-deposited and heat-treated coatings were investigated using SEM, EDS, and XRD. It was found that the Al content affects on the thickness expansion of the heat-treated Ti-Al coatings significantly and also has a major influence on the porosity development, which is caused by the Kirkendall effect. The effects of adding a third element Si and heat treatment with pressure to produce denser Ti-Al intermetallic coating were also examined. The investigated hot-pressed coatings with addition of Si exhibited much denser microstructure and contained Ti-Al intermetallic phases with titanium silicide precipitates.

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  • On the oxidation behavior of titanium within coated nickel-based superalloys

    R. J. Bennett, R. Krakow, A. S. Eggeman, C. N. Jones, H. Murakami, C. M.F. Rae

    Acta Materialia   92   278 - 289  2015.06

     View Summary

    Rutile precipitation within alumina scales grown on coated nickel-based superalloy CMSX-4 has been found to occur preferentially at grain boundaries within the scale. Misorientation analysis using Rodrigues-Frank space has revealed clustering of the misorientation between neighboring grains of corundum and rutile about the established 0 0 0 1c{1 1 2¯ 0}c//0 1 0r{1 0 1}r orientation relationship observed in Ti-containing sapphire crystals. The fraction of interfaces found to exist in this configuration is sufficient to explain the nucleation of rutile from a single corundum grain abutting the rutile grain. The diffusive behavior of Ti has been observed to vary considerably within three commercially used coatings, a plain aluminide coating, a plat-aluminide coating and a diffused platinum coating. Titanium diffusion is enhanced by the presence of Pt. However this did not lead to the precipitation of more rutile, which although observed in all three coatings, was present in sufficient quantity to be detected using XRD only within the plain aluminide coated samples.

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  • Microstructure and oxidation behaviour of Ir-rich Ir-Al binary alloys

    Z. B. Bao, H. Murakami, Y. Yamabe-Mitarai

    Corrosion Science   87   306 - 311  2014.10

     View Summary

    In the current study, alloys of Ir-11Al, Ir-23Al, Ir-30Al, Ir-41Al and Ir-45Al (at.%) were prepared to investigate the microstructure and oxidation behaviour of Ir-rich Ir-Al alloys. Ir(Al)<inf>ss</inf> and/or β-IrAl intermetallic phases were found to exist in the prepared alloys. During isothermal oxidation at 1100°C, the Ir(Al)<inf>ss</inf> and β-IrAl individually changed to porous and dense Al<inf>2</inf>O<inf>3</inf>. The microstructure of the oxide scale formed on Ir-23Al was similar to that of its former alloy which possessed a dendrite-like configuration. It was found that the mass change of Ir-45Al followed a parabolic law, showing the best oxidation resistance among the Ir-Al alloys. © 2014 Elsevier Ltd.

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  • Nanostructural characterization of carbon nanotubes in laser-sintered polyamide 12 by 3D-TEM

    Jiaming Bai, Ruth D. Goodridge, Richard J.M. Hague, Mo Song, Hideyuki Murakami

    Journal of Materials Research   29 ( 17 ) 1817 - 1823  2014.06

     View Summary

    Three-dimensional transmission electron microscopy (3D-TEM) is a powerful technology that provides 3D characterization of the internal details of a material. In this work, for the first time, 3D-TEM was used to characterize a laser-sintered polymer nanocomposite. The dispersion of carbon nanotubes (CNTs) in the laser-sintered polyamide 12 (PA12)-CNT nanocomposite parts was evaluated. At first, to prepare 3D-TEM samples at specific locations, a focused ion beam technique was used. Then, high quality two-dimensional (2D)-TEM images were achieved at various scanning angles for the PA12-CNT laser-sintered sample. After that, 3D-TEM images were reconstructed by combining all the 2D-TEM images. Results revealed that the CNTs were agglomerate-free in the PA12-CNT parts after laser sintering, which helps to explain previously reported improvement in mechanical properties of laser-sintered PA12-CNT parts.

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  • Fabrication of TiAl intermetallic phases by heat treatment of warm sprayed metal precursors

    Judyta Sienkiewicz, Seiji Kuroda, Rafał M. Molak, Hideyuki Murakami, Hiroshi Araki, Susumu Takamori, Krzysztof J. Kurzydłowski

    Intermetallics   49   57 - 64  2014.06

     View Summary

    Warm Spraying is an atmospheric coating process based on high-velocity impact bonding of powder particles. By decreasing the temperature of combustion gas via mixing with nitrogen the oxidation of feedstock powder can be effectively controlled. This is particularly important for Ti-based coating materials, which rapidly oxidize at elevated temperatures. In this study, Ti-Al composite coatings were fabricated by the Warm Spray process using a mixture of titanium and aluminum powders as a feedstock and applying a two-stage heat treatment at 600 and 1000 C to obtain intermetallic phases. The microstructure, chemical and phase composition of the deposited and heat-treated coatings were investigated using SEM, EDS and XRD. The experimental results show that TiAl3 was the first intermetallic phase formed during the first-stage heat treatment. The growth of TiAl3 layer occurred mainly by diffusion of Al into Ti particles. Significant porosity that developed during the heat treatment was caused mainly by Kirkendall effect. After the second-stage heat treatment, a coating layer with TiAl as the dominant phase was obtained with about 20 vol % porosity. © 2014 Elsevier Ltd. All rights reserved.

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  • Synthesis and electrocatalytic performance of atomically ordered nickel carbide (Ni<inf>3</inf>C) nanoparticles

    Nor A. Fadil, Govindachetty Saravanan, Gubbala V. Ramesh, Futoshi Matsumoto, Hideki Yoshikawa, Shigenori Ueda, Toyokazu Tanabe, Toru Hara, Shinsuke Ishihara, Hideyuki Murakami, Katsuhiko Ariga, Hideki Abe

    Chemical Communications   50 ( 49 ) 6451 - 6453  2014.05

     View Summary

    Atomically ordered nickel carbide, Ni3C, was synthesized by reduction of nickel cyclopentadienyl (NiCp2) with sodium naphthalide to form Ni clusters coordinated by Cp (Ni-Cp clusters). Ni-Cp clusters were thermally decomposed to Ni3C nanoparticles smaller than 10 nm. The Ni3C nanoparticles showed better performance than Ni nanoparticles and Au nanoparticles in the electrooxidation of sodium borohydride. © The Partner Organisations 2014.

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  • Mechanical properties at 2223 K and oxidation behavior of Ir alloys

    Yoko Yamabe-Mitarai, Hideyuki Murakami

    Intermetallics   48   86 - 92  2014.05

     View Summary

    Compressive strengths of twelve Ir-1at%X binary alloys were evaluated at 2223 K to understand the strengthening effects of alloying elements. The alloying elements were selected from group 4 to group 10 in the periodic table. The maximum strength of about 65 MPa was obtained in Ir-Hf and Ir-Zr alloys. The effect of adding multiple alloying elements on strength was also investigated. Isothermal oxidation behavior was investigated at 1373 K for Ir alloys including Nb and/or Hf. The weight of all the tested alloys decreased linearly during the oxidation test, indicating that Ir oxide evaporated during oxidation. © 2013 Elsevier Ltd. All rights reserved.

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  • Influences of ruthenium and crystallographic orientation on creep behavior of aluminized nickel-base single crystal superalloys

    F. H. Latief, K. Kakehi, H. An-Chou Yeh, H. Murakami

    Materials Science and Engineering A   592   143 - 152  2014.01

     View Summary

    The influences of ruthenium and surface orientation on creep behavior of aluminized Ni-base single crystal superalloys were investigated by comparing two different types of NKH superalloys. The aluminized coated specimens were then subjected to creep rupture tests at a temperature of 900. °C and a stress of 392. MPa. The coating treatment resulted in a significant decrease in creep rupture lives for both superalloys. The diffusion zones between the coating and substrate led to changes in microstructure, which diminished the creep behavior of the aluminized superalloys. Because of the interdiffusion of Ru, Al and Ni, the solubility of some of the refractory elements, such as W, Re. Mo, Co and Cr decreased in the diffusion zone; the precipitation of topologically close-packed (TCP) phases was thus inevitable. In the present study, the addition of Ru increased the degree of Re and Cr supersaturation in the γ matrix. Consequently, the addition of Ru indirectly promoted the precipitation of TCP phases in aluminized Ni-base single crystal superalloys. Furthermore, the growth of TCP precipitates was greatly influenced by the specific surface orientations of the Ni-base single crystal superalloys. In conclusion, the {110} specimens showed shorter creep rupture life than the {100} specimens, this was due to the difference in the crystallographic geometry of {111}〈101〉 slip system and TCP precipitates between the two side-surface orientations of the specimens. © 2013 Elsevier B.V.

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  • Investigation of multilayer superhard Ti-Hf-Si-N/NbN/AI<inf>2</inf>O<inf>3</inf> coatings for high performance protection

    A. D. Pogrebnjak, A. S. Kaverina, V. M. Beresnev, Y. Takeda, K. Oyoshi, H. Murakami, A. P. Shypylenko, M. G. Kovaleva, M. S. Prozorova, O. V. Kolisnichenko, B. Zholybekov, D. A. Kolesnikov

    Ceramic Engineering and Science Proceedings   35 ( 6 ) 163 - 171  2014

     View Summary

    This paper presents a new type of multi-layer (multicomponent) nano- and microstructured coatings Ti-Hf-Si-N/NbN/Al2O3 with thickness up to 200 m. It demonstrates high physicalmechanical and tribological properties. It was found that the investigated coatings, along with high hardness from 47 to 56 GPa and modulus of elasticity from 435 to 570 GPa, the plasticity index 0.08-0.11, have a fairly low coefficient of friction, which varies from 0.02 to 0.001 for a given mode of deposition. It is also shown that these multilayered coatings have high thermal stability (above 1000°C). The annealing temperature up to 1070°C in a vacuum about 10-2 Pa showed that the coating in the upper layers consisting of Ti-Hf-Si-N/NbN the size of nanograins increases from 25 to 56 nm (for the Ti-Hf-Si-N) and from 14-15 to 35-37 nm for NbN. The increase of steel microhardness at the "coating-substrate" interface, is due to the presence of the hardening near the border with coating formed as a result of impact during abrasive blasting and deposition of coating.

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  • Hot corrosion behavior of plasma sprayed 4 mol% Y<inf>2</inf>O <inf>3</inf>-ZrO<inf>2</inf> thermal barrier coatings with volcanic Ash

    Won Jun Lee, Byung Koog Jang, Dae Soon Lim, Yoon Suk Oh, Seong Won Kim, Hyung Tae Kim, Hiroshi Araki, Hideyuki Murakami, Seiji Kuroda

    Journal of the Korean Ceramic Society   50 ( 6 ) 353 - 358  2013.11

     View Summary

    The hot corrosion behavior of plasma sprayed 4 mol% Y2O 3-ZrO2 (YSZ) thermal barrier coatings (TBCs) with volcanic ash is investigated. Volcanic ash that deposited on the TBCs in gas-turbine engines can attack the surface of TBCs itself as a form of corrosive melt. YSZ coating specimens with a thickness of 430-440 μm are prepared using a plasma spray method. These specimens are subjected to hot corrosion environment at 1200°C with five different duration time, from 10 mins to 100 h in the presence of corrosive melt from volcanic ash. The microstructure, composition, and phase analysis are performed using Field emission scanning electron microscopy, including Energy dispersive spectroscopy and X-ray diffraction. After the heat treatment, hematite (Fe2O3-TiO2) and monoclinic YSZ phases are found in TBCs. Furthermore the interface area between the molten volcanic ash layers and YSZ coatings becomes porous with increases in the heat treatment time as the YSZ coatings dissolved into molten volcanic ash. The maximum thickness of this a porous reaction zone is 25 μm after 100 h of heat treatment. © 2013 The Korean Ceramic Society.

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  • Microstructure and oxidation behavior of Ti-6Al-2Zr-1Mo-1V-based alloys with Sc addition

    W. L. Xiao, D. H. Ping, H. Murakami, Y. Yamabe-Mitarai

    Materials Science and Engineering A   580   266 - 272  2013.09

     View Summary

    The microstructural evolution, mechanical properties and oxidation behavior of Ti-6Al-2Zr-1Mo-1V-based alloys with 0-3.8wt% Sc additions have been investigated. The Sc addition decreased the β-trans temperature and refined the microstructure of the alloy. Sc partitioned into the α-phase and was depleted in the β-phase with increasing Sc content. This resulted in the formation of a small amount of Sc2O3 particles in the high Sc-containing alloys. The Sc addition improved the yield strength at both room temperature and high temperature, whereas refined the oxide products and decreased the oxidation resistance of the base alloy. The oxidation behavior with Sc addition was discussed. © 2013 Elsevier B.V.

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  • Effect of Gd<inf>2</inf>O<inf>3</inf> on the thermal conductivity of ZrO<inf>2</inf>-4 mol.% Y<inf>2</inf>O<inf>3</inf> ceramics fabricated by spark plasma sintering

    Byung Koog Jang, Seongwon Kim, Yoon Suk Oh, Hyung Tae Kim, Yoshio Sakka, Hideyuki Murakami

    Scripta Materialia   69 ( 2 ) 165 - 170  2013.07

     View Summary

    Porous and dense ZrO2-4 mol.% Y2O3 ceramics were fabricated by spark plasma sintering at 1100 and 1500 °C. The porosity of the ceramics sintered at 1100 °C increased with the addition of ∼0-5 mol.% Gd2O3. In contrast, the ceramics sintered at 1500 °C showed full density regardless of the amount of Gd 2O3 addition. The fluorite-type Gd2Zr 2O7 phase was formed as the second phase in the tetragonal ZrO2 matrix. The thermal diffusivity and thermal conductivity of sintered ZrO2-4 mol.% Y2O3 decreased with increasing Gd2O3 addition. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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  • Microstructure and oxidation behaviors of near-α Ti-6.5Al-4Sn-4Zr-0. 5Mo-based alloys with Ir addition

    W. L. Xiao, H. Murakami, D. H. Ping, Y. Yamabe-Mitarai

    Journal of Materials Science   48 ( 9 ) 3363 - 3369  2013.05

     View Summary

    The microstructure and oxidation behaviors of near α-Ti-based alloys with small amount of iridium (Ir) additions were investigated. The microstructure of both Ir-free and Ir-containing alloys was observed to consist of α + β Widmanstätten colonies. The β lamellae gradually became continuous with increasing Ir additions since Ir acted as a β-stabilizer in the alloys. Isothermal oxidation test indicated that Ir addition reduced the oxidation resistance at 650 C; while at 750 C, the adherence of thermally grown oxides was enhanced, and a thin Al 2O3-enriched layer on the oxide scale was promoted in the Ir-containing alloy, which suggests that Ir addition was effective in improving oxidation resistance of near-α-based alloys at 750 C. © 2013 Springer Science+Business Media New York.

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  • Effects of initial oxidation on β phase depletion and oxidation of CoNiCrAlY bond coatings fabricated by warm spray and HVOF processes

    Takeru Mori, Seiji Kuroda, Hideyuki Murakami, Hiroshi Katanoda, Yukihiro Sakamoto, Scott Newman

    Surface and Coatings Technology   221   59 - 69  2013.04

     View Summary

    Thermal barrier coatings (TBC) have been applied extensively onto the high-temperature components in turbine engines to prolong their service life in extremely harsh environments. TBCs are typically composed of a ceramic top coating for thermal insulation and a metallic bond coating (BC) for oxidation resistance and providing adhesion to the top coating. MCrAlY, where M stands for Co, Ni or an alloy of these elements, is a widely used material for BC and usually produced by low pressure plasma spraying (LPPS) in industry. Recently high velocity oxy-fuel (HVOF) spraying is attracting significant attention as a more economical alternative procedure to LPPS. In terms of the quality of sprayed coatings, however, LPPS is still superior in terms of oxygen pick-up during coating preparation, which should affect the performance as a bond coating in service. In this study, a modified HVOF process, so called 2-stage HVOF or warm spray (WS) was applied to deposit a CoNiCrAlY alloy. Comparisons between BCs fabricated by HVOF and WS were made in terms of microstructure, surface morphology, and cyclic and isothermal oxidation behaviors in air at 1423. K up to 100. cycles and 100. h respectively. The results showed that rougher and less oxidized BCs were deposited by the WS process, which exhibited slower kinetics of β-phase depletion during oxidation. A simple Al diffusion model revealed that apparently a small difference in the initial oxidation between the two spraying processes had significant influence on the β-depletion phenomena, which may influence the life time as a bond coating. © 2013 Elsevier B.V.

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  • Anisotropic creep behavior of aluminized Ni-based single crystal superalloy TMS-75

    Fahamsyah H. Latief, Koji Kakehi, Hideyuki Murakami

    Materials Science and Engineering A   567   65 - 71  2013.04  [Refereed]

     View Summary

    We examined the creep behavior of aluminized Ni-based single crystal superalloy TMS-75 with {100} and {110} side-surfaces. The specimens were aluminized by pack aluminizing treatment at 1000 degrees C for 5 h under argon flow. The creep rupture tests were performed at 900 degrees C under a stress of 392 MPa. The stress orientation of all of the specimens was within 4 degrees of &lt; 001 &gt;. It was evident that the {100} specimens exhibited higher creep rupture lives compared to the {110} specimens, indicating that anisotropy of the secondary orientation-which is normal in the primary orientation-occurred. The anisotropic creep behavior of the aluminized specimens was induced primarily by the different arrangements of {111} &lt; 101 &gt; slip systems between the two side-surface orientations. The gradient of hardness values in the coating-to-substrate regions and depth of TCP penetration were additional degradation factors. (C) 2013 Elsevier B.V. All rights reserved.

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    9
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  • Formation of superhard Ti-Hf-Si-N/NbN/Al<inf>2</inf>O<inf>3</inf> multilayer coatings for highly effective protection of steel

    A. D. Pogrebnyak, V. M. Beresnev, A. Sh Kaverina, A. P. Shypylenko, O. V. Kolisnichenko, K. Oyoshi, Y. Takeda, H. Murakami, D. A. Kolesnikov, M. S. Prozorova

    Technical Physics Letters   39 ( 2 ) 189 - 192  2013.02

     View Summary

    Hard micro- and nanostructured Ti-Hf-Si-N/NbN/Al2O3 multilayer coatings on steel substrates have been obtained for the first time using various deposition technologies and characterized by a combination of methods. It is established that the proposed coatings possess, in addition to high hardness (H = 47-56 GPa), high elastic modulus (E = 435-570 GPa), and good plasticity index (We = 0. 08-0. 11), a rather low friction coefficient that varies within μ = 0. 02-0. 001 depending on the deposition conditions. The coatings remain stable at temperatures above 1000°C. © 2013 Pleiades Publishing, Ltd.

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    12
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  • Recent global trends in structural materials research

    Hideyuki Murakami, Takahito Ohmura, Toshiyuki Nishimura

    Science and Technology of Advanced Materials   14 ( 1 )  2013.02

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  • Anisotropic creep properties of aluminized Ni-based single-crystal superalloy at intermediate and high temperatures

    F. H. Latief, K. Kakehi, H. Murakami

    Scripta Materialia   68 ( 2 ) 126 - 129  2013.01

     View Summary

    An aluminized Ni-based single-crystal superalloy was examined to investigate the anisotropic creep properties at intermediate and high temperatures. It was found that the specimens with a (1 0 0) side surface showed longer creep rupture life than the specimens with a (1 0 0) side surface. This means that anisotropy of the secondary orientation, which is normal to the primary orientation, occurred. The anisotropic creep behavior of the aluminized specimen was induced by a different arrangement of {1 1 1}〈1 0 1〉 slip systems between the two side-surface specimens. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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    11
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  • Numerical analysis of oxygen transport in alpha titanium during isothermal oxidation

    T. Kitashima, L. J. Liu, H. Murakami

    Journal of the Electrochemical Society   160 ( 9 )  2013

     View Summary

    Oxygen dissolution in titanium during oxidation is simulated with oxide/metal interface migration using a finite volume method. In this simulation, the oxidation rate resulting from both oxide growth and oxygen penetration into a metal are taken into account. The results show the temperature dependency of oxygen concentration at the oxide/metal interface in the metal, which is about 21 at% up to 600°C, and increases drastically to the oxygen solubility limit at temperatures above 600°C. This suggests that high stress due to the oxygen penetration may develop in the metal near the interface. This stress distribution could be dependent on temperature according to the relation between oxygen content and lattice parameters. © 2013 The Electrochemical Society. All rights reserved.

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    16
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  • Effects of Sc addition on the microstructure and tensile properties of Ti-6.6Al-5.5Sn-1.8Zr alloy

    W. L. Xiao, S. Q. Wu, D. H. Ping, H. Murakami, Y. Yamabe-Mitarai

    Materials Chemistry and Physics   136 ( 2-3 ) 1015 - 1021  2012.10

     View Summary

    The effects of Sc additions on the microstructure and tensile properties of Ti-6.6Al-5.5Sn-1.8Zr-based alloys have been investigated. The results indicated that Sc acts as α-stabilizer in the studied alloy system. The grain sizes of as-cast and forged alloys were refined significantly by Sc additions. With increasing Sc addition, a large number of Sc 5Sn 3 and oxides are formed. The formation of brittle α 2-Ti 3Al precipitates during long-term aging treatment is suppressed by high content of Sc addition. The Sc addition increases the solid solution strengthening; meanwhile, the formation of Sc 5Sn 3 and oxides plays a role in grain refinement and dispersion strengthening. Therefore, it has been found that both room temperature and high temperature strengths are gradually improved, whereas the ductility is decreased with increasing Sc addition. © 2012 Elsevier B.V. All rights reserved.

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    18
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  • Influence of Crystallographic Orientation on Creep Behavior of Aluminized Ni-Base Single Crystal Superalloys

    F. H. Latief, K. Kakehi, H. Murakami, K. Kasai

    Superalloys 2012     311 - 320  2012.10

     View Summary

    The effect of chemical compositions and secondary crystallographic orientation dependence on the formation of diffusion layer and creep behavior were investigated. Three alloys of different generations: PWA 1480 (1 st generation), CMSX-4 (2nd generation) and TMS-75 (3 rd generation), were studied. After aluminized coating process, creep rupture test were performed at 900 °C and 392 MPa. The creep rupture properties of the alloys with and without the aluminized coating treatment were compared. The coating treatment resulted in a significant decrease in rupture lives for three alloys. No reductions in rupture elongation were observed. Creep strength was significantly degraded with aluminide coating due to the diffusion layers formed under coating layer. Creep fractures of the coated materials were essentially controlled by the crack nuclei in the coated layer. It was also found that the specimens with (100) side-surface showed longer creep rupture life than the specimens with (110) side-surface, which means the results were anisotropic with respect to the secondary orientation which is normal to the primary orientation. The anisotropic creep behavior was caused by the different arrangement of {111}<101> slip systems between the two side-surface specimens. The results obtained in this study suggest the importance of secondary orientation normal to the component surface on designing the single-crystalline turbine blades.

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  • Nanoindentation and Nano-Compresion Testing of Ni<inf>3</inf>Al Precipitates

    B. Gan, H. Murakami, R. Maaß, L. Meza, J. R. Greer, T. Ohmura, S. Tin

    Superalloys 2012     83 - 91  2012.10

     View Summary

    Using an AFM-based instrumented nanoindentation system, the nanoindentation response of γ′ particles (Ni3Al) in a Ni-base single crystal superalloy CMSX-4 was characterized to demonstrate the influence of the softer γ matrix on the measurement of nano-mechanical properties of γ′ particles. The properties of the γ′ particles were measured after both a standard and a coarsening heat treatment, in which the initially sub-micron sized cuboidal γ′ particles transformed to large, irregularly shaped γ′ precipitates with dimensions in excess of 30 urn. The measured nano-hardness of the coarsened γ′ precipitates in CMSX-4 appears to increase slightly with the increasing load. Nano-compression testing of cuboidal γ′ particles, electrolytically extracted from a CMSX-4 after a standard solution and age heat treatment, was also performed. With a maximum value of 10 GPa, the measured yield stress of these dislocation-free γ′ particles approaches the ideal strength and is equivalent to -G/17. Compared to the measured yield strengths of γ′ phase in its bulk crystal form, these nano-compression results are more than a factor of 33 higher. Additionally, the 'softening' effect of Ga+ ion implantation on the strength of dislocation-free nanocrystals was also assessed. The resulting yield strengths of cuboidal γ′ precipitates imaged with a focused ion beam were at least 5 GPa lower than those that had not been subjected to Ga+ ion implantation.

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  • Novel processing in inert atmosphere and in air to manufacture high-activity slurry aluminide coatings modified by Pt and Pt/Ir

    M. C. Galetz, X. Montero, H. Murakami

    Materials and Corrosion   63 ( 10 ) 921 - 928  2012.10

     View Summary

    Slurry-derived coatings are an interesting alternative method to pack aluminization of nickel-base superalloys, which provide similar properties and protection at high temperatures. For highest performance, these aluminide coatings are modified by the addition of Pt or, as recent research suggests, with Pt/Ir. While the combination of Pt and Pt/Ir with an out-of-pack process is state of the art, slurry coatings are of special interest as a repair method for turbine blades. In this study, the microstructural evolution of slurry-derived coatings manufactured on CM 247 in inert atmosphere as well as in air was investigated. Layers of Ni, Pt, and Pt/Ir mixtures were electrodeposited. After annealing, a diffusion heat-treatment with a slurry containing aluminum or aluminum-silicon powder was applied on the samples. The addition of silicon is well known to be beneficial for hot corrosion environments. The reaction and interdiffusion behavior of aluminum/aluminum- silicon determines the microstructural evolution of the coatings. Depending on the initial electroplated layer on the surface, different microstructures can be obtained, such as the Pt/Ir-modified beta phase (Ni,Pt)Al or two-phase layers of PtAl2 and NiAl. Additionally, the reactivity between the elements at the surface and those from the slurry was shown to determine homogeneity and surface roughness of the diffusion coating, also depending on the atmosphere used during slurry aluminization. Finally, it was demonstrated that iridium has a high influence on the diffusion behavior and especially the distribution of platinum in the coatings. Such new coatings have the potential to overcome some disadvantages of conventionally manufactured high-activity aluminide coatings, as the combination of Pt/Ir-electroplating with the slurry process results in less detrimental substrate elements like molybdenum or tungsten close to the surface. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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  • Ultrafast production of silicon via aluminothermic reduction of tetrachlorosilane in a thermal plasma jet

    Kentaro Shinoda, Hideyuki Murakami, Yoshinari Sawabe, Kunio Saegusa

    Chemical Engineering Journal   198-199   61 - 64  2012.08

     View Summary

    This paper proposed a new route for producing silicon at a high rate via the reduction of tetrachlorosilane with aluminum molten particles by utilizing a thermal plasma jet. High-purity aluminum particles melted and sprayed by an atmospheric dc plasma jet were reacted with tetrachlorosilane vapor. Silicon generation was confirmed in the reacted particles. The residence time was calculated to be a few milliseconds. The obtained results demonstrate the feasibility of the aluminothermic reduction process in a thermal plasma jet for rapid silicon production. © 2012 Elsevier B.V.

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    7
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  • Development of Pt-base two layered coating for oxidation protection

    Yoshitaka Matsumura, Hideyuki Murakami, Kazuki Kasai, Hiromichi Murakami, Kazuhiko Noda

    Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals   76 ( 5 ) 321 - 326  2012.05

     View Summary

    Oxidation resistance and microstructural changes of Pt/Ni + Al two layered coatings were investigated in comparison to those of Pt diffusion coatings. The two layered coatings were successfully prepared by a two-step electroplating process, which involves Pt electroplating followed by Ni + Al-particle composite electroplating. The cross-sectional observations of specimens after the cyclic oxidation test confirmed that the two-layered coatings retarded the formation of voids, suggesting that the top Ni + Al-particle composite layer played a role as the diffusion retarder between the coated layer and the substrate. In addition, the Ni + Al-particle composite coatings preferentially accelerated the formation of the Al2O3 scale on the surface. These results confirmed the beneficial effect of the designed two layered coatings. © 2012 The Japan Institute of Metals.

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  • Wet chemical synthesis of Ni-Al nanoparticles at ambient condition

    Nor Akmal Fadil, Saravanan Govindachetty, Hideki Yoshikawa, Yoshiyuki Yamashita, Shigenori Ueda, Keisuke Kobayashi, Toyokazu Tanabe, Toru Hara, Venkata Ramesh Gubbala, Hideyuki Murakami, Kazuhiko Noda, Hideki Abe

    Advanced Materials Research   557-559   442 - 447  2012

     View Summary

    The synthesis of intermetallic Ni-Al nanoparticles by co-reduction approach of several organometallic precursors with sodium naphthelide in non-aqueous solution was studied. The state of the art in nanoparticles synthesisation is the selection of suitable precursors and the adaption of colloid chemistry to non-aqueous media at the room temperature under inert atmosphere. The reduction of an organometallic precursor, nickel (II) acetylacetonate, Ni(Acac) 2 as a source of Ni element of the intermetallic, and aluminum trichloride, AlCl 3 in tetrahydrofuran (THF) solution gave a black particles. The powder X-ray diffraction spectroscopy (pXRD) result shows an expansion of lattice parameter for FCC-Ni indicating the cooperation of Al atoms in Ni structures. The estimation value of Al concentration using Scherrer's equation is 10 at%. The particles were investigated in more detail by hard X-ray photoemission spectroscopy (HX-PES). The HX-PES spectrums confirmed that the black particles has binding energy consistent to standard materials of Ni 3Al. The absence of organic residues shown by the Fourier-transform infrared, FTIR spectrometer indicates that the as prepared Ni-Al nanoparticles are free from by-products. © (2012) Trans Tech Publications, Switzerland.

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  • Changes in mechanical properties of heat resisting alloy for a satellite propulsion system after a nitriding process

    Hideshi Kagawa, Go Fujii, Kenichi Kajiwara, Daisuke Kuroda, Takuya Suzuki, Yoko Yamabe-Mitarai, Hideyuki Murakami, Yoshinori Ono

    European Space Agency, (Special Publication) ESA SP   691 SP  2012

     View Summary

    Haynes25 (L-605) is a common heat resistant alloy used in mono-propellant structures and screen materials for catalyst beds. The lifetime requirements for thrusters have expanded dramatically after studies conducted in the 1970s on mono-propellant materials used to extend the service life. The material design had long remained unchanged, and the L-605 was still used as thruster material due to its good heritage. However, some important incidents involving degradation were found during the test-unit break-up inspection following the thruster life tests. The Japanese research team focused on the L-605 degradations found on the catalyst bed screen mesh used for mono-propellant thruster and analysed the surface of the wire material and the cross-section of the wire screen mesh used in the life tests. The investigation showed that the degradation was caused by nitriding L-605 component elements. The team suggested that the brittle fracture was attributable to tungsten (W) carbides, which formed primarily in the grain boundaries, and chromium (Cr) nitride, which formed mainly in the parts in contact with the hot firing gas. The team also suggested the installation of a platinum coating on the material surface as a countermeasure L-605 nitric degradation. Inconel 625 is now selected as a mono-propellant structure material due to its marginal raw material characters and cost. The team believes that Inconel 625 does not form W carbides since it contains no tungsten component, but does contain Cr and Fe, which form nitrides easily. Therefore, the team agreed that for the Inconel 625, there was a need to evaluate changes in the microstructure and mechanical properties following exposure to hot nitrogen gases. This paper will describe these changes of Inconel 625.

  • Influence of La<inf>2</inf>O<inf>3</inf> addition on thermophysical properties of ZrO<inf>2</inf>-4 mol%Y2O3 ceramics fabricated by spark plasma sintering

    Byung Koog Jang, Seongwon Kim, Yoon Suk Oh, Hyung Tae Kim, Yoshio Sakka, Hideyuki Murakami

    Journal of the Ceramic Society of Japan   119 ( 1396 ) 929 - 932  2011.12

     View Summary

    The influence of La2O3 addition on the thermal diffusivity and thermal conductivity of ZrO2 ceramics has been investigated. Porous and dense ZrO2 ceramics were fabricated by spark plasma sintering at 1100 and 1500°C. The porosity of ZrO2 ceramics sintered at 1100°C increased with increasing 05 mol% La 2O3 addition. In contrast, ZrO2 ceramics sintered at 1500°C showed full density regardless of the amount of La 2O3 addition. The La2Zr2O 7 of crystalline pyrochlore phase was formed by reaction between La2O3 and ZrO2. The XRD peak of the La 2Zr2O7 phase increased with increasing La 2O3 addition. The thermal diffusivity and thermal conductivity of sintered ZrO2 decreased with increasing La 2O3 addition. In addition, the thermal conductivity of the present samples decreased remarkably with increasing the porosity. © 2011 The Ceramic Society of Japan.

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    8
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  • Surface geometry of pure iridium oxidized at 1373 K in air

    Z. B. Bao, H. Murakami, Y. Yamabe-Mitarai

    Applied Surface Science   258 ( 4 ) 1514 - 1518  2011.12

     View Summary

    The surface microstructure of a polished Ir sample during isothermal heat treatment at 1373 K in air was characterized. Various surface morphologies including triangular pits and terraces, "pyramid"-like plateaus and striated edges were observed. Changes in surface geometry were highly dependent on the original grain orientation. Most grains were confirmed to possess or partly exhibit a geometric configuration of {1 1 1} faceting habit, while periodic bond chain (PBC) vectors played an important role in determining the ultimate surface morphology. The mechanism and process of how these distinct surface morphologies formed are discussed. © 2011 Elsevier B.V. All rights reserved.

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  • Secondary reaction zone formations in pt-aluminised fourth generation ni-base single crystal superalloys

    A. S. Suzuki, C. M.F. Rae, R. A. Hobbs, H. Murakami

    Advanced Materials Research   278   78 - 83  2011.07

     View Summary

    Fourth generation superalloys are characterised by the addition of Ru which contributes to improved creep resistance whilst improving the microstructural stability. However, Ru additions have a negative effect on coated Ni-base superalloys, promoting Secondary Reaction Zone (SRZ) formation. Formation of a layer of SRZ beneath an aluminised or Pt-aluminised coating has the potential to reduce the effective cross section of a blade by in excess of 100 ?m or 10% of the wall thickness. In this paper the effects of alloy composition on the formation of the SRZ in Pt-Aluminised fourth generation alloys were investigated systematically. A series of experimental fourth generation alloys was used having two distinct compositions of Co, Mo, W and Ru and conforming to a four factorial 'Design of Experiments' model. These alloys showed significant and consistent changes in the SRZ depending on alloy composition. These were in distinct contrast to the effects of these elements on stability in the bulk. Mo was demonstrated to be by far the most effective element suppressing SRZ formation, followed by Co. In contrast, both W and Ru enhance. © (2011) Trans Tech Publications, Switzerland.

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  • Microstructural investigation of PGM-based alloy coatings for oxidation protection

    Y. Song, C. Zhou, H. Murakami

    Materials and Corrosion   62 ( 7 ) 674 - 680  2011.07

     View Summary

    It is well known that Pt addition significantly improves the resistance of aluminide coatings to high-temperature oxidation and hot corrosion, which has led to the widespread application of Pt modified aluminide coatings on the superalloy components of advanced gas turbine engines. Other platinum group metals (PGMs) such as Ir and Ru attract researchers for high temperature applications. In this study, oxidation properties of Pt-Ir and Pt-Ru based alloy coatings were investigated. Pt, Ru, and Ir were electroplated on a directionally solidified Ni-base superalloy DZ125. The cyclic oxidation test revealed that both Pt-Ir and Pt-Ru alloys exhibited good oxidation performance. The effect of substrate alloy and coating compositions on microstructural changes during cyclic oxidation tests were discussed. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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  • Effects of thermal exposure on Ir-based alloys with and without Pt coating

    Z. B. Bao, H. Murakami, Y. Yamabe-Mitarai

    Corrosion Science   53 ( 4 ) 1224 - 1229  2011

     View Summary

    Isothermal oxidation tests were performed on Ir, Ir-3Hf, Ir/Pt, and Ir-3Hf/Pt in static air at 1100°C for 200h. The microstructural evolution and changes in the composition of the alloys were characterisation by performing scanning electron microscopy (SEM) and field-emission electron probe microanalysis (FE-EPMA). The results indicated that the specimens of all the Ir-based alloys exhibited linear mass loss due to the formation/evaporation of gaseous Ir oxide species (mainly IrO3). The oxidation behaviour, including mass change kinetics, evaporation of gaseous IrO3, and the role of Hf and Pt are discussed. © 2010 Elsevier Ltd.

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  • Cyclic-Oxidation Behavior of Multilayered Pt/Ru-Modified Aluminide Coating

    Yingxue Song, Hideyuki Murakami, Chungen Zhou

    Journal of Materials Science and Technology   27 ( 3 ) 280 - 288  2011

     View Summary

    Multilayered Pt/Ru modified aluminide coating for thermal barrier coating (TBC) systems has been investigated. 2 μm Pt+2 μm Ru+2 μm Pt was first deposited on nickel-base superalloy DZ125 by electrodeposition, and then the coating was treated by annealing and a conventional pack-cementation aluminizing process. The cyclic oxidation tests were carried out at 1423 K in air. It was found that the thermal cyclic oxidation resistance of Pt/Ru-modified aluminide coating was comparable to that of Pt-modified aluminide coating, which was much better than simply aluminized DZ125. The addition of Ru to Pt-modified aluminide coating increased the resistance to rumpling. The microstructures and phase constitutions of the coating before and after oxidation were investigated. © 2011 The Chinese Society for Metals.

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  • Effect of surface treatment and crystal orientation on microstructural changes in aluminized Ni-based single-crystal superalloy

    Kazuki Kasai, Hideyuki Murakami, Seiji Kuroda, Hachiro Imai

    Materials Transactions   52 ( 9 ) 1768 - 1772  2011

     View Summary

    The effect of pre-surface treatment and crystal orientation on microstructural changes in the aluminized 4th generation Ni-based single- crystal superalloy TMS-138 was investigated. The substrate superalloy was cut along the {100} and {110} planes on which three kinds of surface finishing, such as grit blasting, mechanical polishing and electro-polishing were conducted prior to the conventional high-activity aluminizing process. A thermal cycling test at 1373 K revealed that heavy deformation of the substrate surface by grit blasting gave rise to the formation of a secondary reaction zone (SRZ) in the vicinity of the interdiffusion zone/substrate interfaces. When the surfaces were finished by electropolishing, voids were formed in the vicinity of the interdiffusion zone/substrate interfaces. It was also found that accelerated formation of SRZ and voids was observed along the h110i directions rather than the h100i directions during the thermal cycling test. The difference in morphological changes of substrates can be related to the residual stress introduced by the surface finishing. © 2011 The Japan Institute of Metals.

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  • Ab initio and thermodynamic study of the CrRe system

    M. Palumbo, T. Abe, C. Kocer, H. Murakami, H. Onodera

    Calphad: Computer Coupling of Phase Diagrams and Thermochemistry   34 ( 4 ) 495 - 503  2010.12

     View Summary

    In this work the assessment of the CrRe system, using a combined ab initio and CALPHAD (CALculation of Phase Diagrams) approach, is presented. To model the sigma phase, a five-sublattice combined CEF model was applied and the present description reproduces reasonably well the previously published experimental phase diagram. Formation enthalpies of the stable/metastable configurations of the sigma phase were estimated from ab initio calculations. A comparison of results using simplified two- and three-sublattice models and the five-sublattice model for the sigma phase is also presented, which highlights differences in calculated properties as site occupancies. According to present outcomes, the five-sublattice model (Cr,Re)2(Cr,Re) 4(Cr,Re)8(Cr,Re)8(Cr,Re)8, based on Wyckoff positions, is necessary to reproduce the atomic ordering in the sigma phase. Certain areas of the phase diagram still remain uncertain and deserve further experimental investigation. © 2010 Elsevier Ltd. All rights reserved.

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  • Nanostructured LaB<inf>6</inf> field emitter with lowest apical work function

    Han Zhang, Jie Tang, Jinshi Yuan, Jun Ma, Norio Shinya, Kiyomi Nakajima, Hideyuki Murakami, Tadakatsu Ohkubo, Lu Chang Qin

    Nano Letters   10 ( 9 ) 3539 - 3544  2010.09

     View Summary

    LaB6 nanowires are ideal for applications as an electrical field-induced ion and electron point source due to their miniature dimensions, low work function, as well as excellent electrical, thermal, and mechanical properties. We present here a reliable method to fabricate and assemble single LaB6 nanowire-based field emitters of different crystal orientations. The atomic arrangement, emission brightness from each crystal plane, and field emission stability have been characterized using field ion microscopy (FIM) and field emission microscopy (FEM). It is found that the 〈001〉 oriented LaB6 nanowire emitter has the highest field emission symmetry while the 〈012〉 oriented LaB6 nanowire has the lowest apical work function. The field emission stability from the single LaB6 nanowire emitter is significantly better than either the LaB6 needle-type emitter or W cold field emitters. © 2010 American Chemical Society.

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  • Characterization of crystallographic texture in plasma-sprayed splats by electron-backscattered diffraction

    Kentaro Shinoda, Masahiko Demura, Hideyuki Murakami, Seiji Kuroda, Sanjay Sampath

    Surface and Coatings Technology   204 ( 21-22 ) 3614 - 3618  2010.08

     View Summary

    The surface microtexture of splats deposited by atmospheric dc plasma spraying was studied by the electron-backscattered diffraction method. The examined splats were yttria-stabilized zirconia (YSZ) and nickel deposited onto a mirror-polished stainless steel substrate preheated to 500. K. The YSZ splats exhibited a disk-shaped morphology and had a peculiar < 111> fiber texture in their peripheral region; the fiber axes were perpendicular to the substrate at the periphery but inclined toward the center as their radial positions shifted toward the center. This implies that the microtexture of an YSZ splat cannot be regarded as a single fiber texture even when it exhibits a disk-shaped morphology. It was in contrast to nickel splats, which consisted of a single < 100> fiber texture. © 2010 Elsevier B.V.

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  • Cyclic corrosion behavior of Pt/Ru-modified bond coatings exposed to NaCl plus water vapor at 1050°C

    Yingxue Song, H. Murakami, Chungen Zhou

    Journal of Materials Science and Technology   26 ( 3 ) 217 - 222  2010.03

     View Summary

    In the present investigation, Pt/Ru-modified bond coating consisted of 2 μm Pt+2 μm Ru was deposited on a nickel-based superalloy by electroplating method and followed by conventional Al pack cementation. The cyclic corrosion behavior of Pt/Ru-modified bond coating exposed to NaCl plus water vapor has been investigated under atmospheric pressure at 1050°C. The result shows that the cyclic corrosion life of Pt/Ru-modified bond coating is longer than that of the conventional Pt-modified aluminide coating in the presence of NaCl plus water vapor. The addition of Ru makes the coating possess the increased strength and suppress the rumpling behavior. The absence of rumpling may be responsible for the improved corrosion resistance of Pt/Ru-modified aluminide coating. © 2010 by the Editorial Board of Journal of Materials Science and Technology. Published by Elsevier Limited. All rights reserved.

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  • Splat morphology of yttria-stabilized zirconia droplet deposited via hybrid plasma spraying

    Kentaro Shinoda, Hideyuki Murakami

    Journal of Thermal Spray Technology   19 ( 3 ) 602 - 610  2010.03

     View Summary

    The splat morphology of yttria-stabilized zirconia (YSZ) droplets deposited by dc-rf hybrid plasma spraying (HYPS) was studied. Two types of YSZ powder were used, namely fused and crushed powder (FC) and hollow spherical powder (HOSP). The three-dimensional shape of more than 600 disk-shaped splats on preheated substrates was evaluated using a laser-scanning microscope to determine the splat diameter, thickness, and their dimensionless forms. The HOSP showed a higher degree of flattening than the FC. Both the FC with a powder size distribution of 45-75 μm and the HOSP of 30-120 μm can be used as spray materials in the HYPS to achieve a coating design based on fully molten particles. The effect of the substrate temperature on the splat morphology was similar to that of atmospheric dc plasma spraying; however, the transition from a splashed shape to a disk shape gradually occurred at higher substrate temperatures. © ASM International.

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  • Effect of surface treatment and crystal orientation on microstructural changes in aluminized Ni-based single-crystal superalloy

    Kazuki Kasai, Hideyuki Murakami, Seiji Kuroda, Hachiro Imai

    Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals   74 ( 1 ) 19 - 23  2010.01

     View Summary

    The effect of pre-surface treatment and crystal orientation on microstructural changes in the aluminized 4th generation Nibased single-crystal superalloy TMS-138 was investigated. The substrate superalloy was cut along the {100} and {110} planes on which three kinds of surface finishing, such as grit-blasting, mechanically polishing and electro-polishing were conducted prior to the conventional high-activity aluminizing process. A thermal cycling test at 1373 K revealed that heavy deformation of substrate surface by grit blasting gave rise to the formation of a secondary reaction zone (SRZ) in the vicinity of interdiffusion zone/substrate interfaces. While when the surfaces were finished by electro-polishing, voids were formed in the vicinity of interdiffusion zone/substrate interfaces. It is also found that accelerated formation of SRZ and voids was observed along <110> directions rather than <100> directions during the thermal cycling test. The difference in morphological changes of substrates can be related to the residual stress introduced by the surface finishing. © 2010 The Japan Institute of Metals.

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  • Simulation of reaction-diffusion phenomena occurring between Ir coating and Ni-Al alloy substrate using phase-field model

    Machiko Ode, Taichi Abe, Hideyuki Murakami, Yoko Yamabe-Mitarai, Hidehiro Onodera

    International Journal of Materials Research   101 ( 4 ) 523 - 526  2010

     View Summary

    It is very important to understand the development of the microstructure in the interdiffusion zone between coatings and superalloy substrates for designing bond coat materials. In this study, the reaction-diffusion phenomena in the Ir-coating/Ni -Al binary substrate system are simulated using the phase-field model. The thermodynamic database developed based on the CALPHAD method is directly incorporated in the simulation. The effect of coating thickness on the growth of the secondary precipitated β-phase is discussed. In the case of a semi-infinite system, the thickness of the β-phase increases proportionally with the square root of time. In the case of a thin (1 μm thick) coating, inconstant β-phase growth is observed. It is suggested that the phase growth is related to the change in the β-phase fraction, which is derived from a thermodynamic line calculation from the substrate composition to pure Ir. © 2010 Carl Hanser Verlag.

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  • Experimental investigation and computer simulation of diffusion in Pt-coated bulk Ni

    W. Gong, L. Zhang, M. Ode, H. Murakami, C. Zhou

    Journal of Mining and Metallurgy, Section B: Metallurgy   46 ( 2 ) 153 - 160  2010

     View Summary

    The concentration profiles of thin-film Pt/bulk Ni coatings annealed at 1150, 1250 and 1300 °C for different time were measured by means of electron probe microanalysis. The corresponding interdiffusion coefficients were then determined using the thin-film solution. The calculated concentration profiles based on the presently obtained interdiffusion coefficients agree well with the experimental ones, but better at a higher temperature or a longer time. The comparison between the presently measured concentration profiles and the DICTRA simulated ones indicates that it is promising to apply the well-established atomic mobility databases due to bulk diffusion information in coating systems with some simple modifications for diffusivities.

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    4
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  • Structure dependence of thermal conductivity of plasma-sprayed zirconia coatings

    Y. Xu, H. B. Guo, H. Murakami, S. Kuroda, M. Yamazaki

    Proceedings of the 30th International Thermal Conductivity Conference and the 18th International Thermal Expansion Symposium, Thermal Conductivity 30/Thermal Expansion 18     393 - 402  2010

     View Summary

    Pores included in plasma-sprayed zirconia coatings have been classified into four types: inter-splat cracks, globular pores, segment cracks and branch cracks, according to the dimension, geometry and formation mechanism of the pores. Thermal conductivities of three samples produced under different atmospheric plasma spraying conditions with similar total porosity but different volume fractions of each type of pores were measured. Different thermal conductivities were observed. Dependences of thermal conductivity on volume fraction of each type of pores and the geometry of the pores were simulated by a multi-stepped finite element method, which shows that effect of pores on thermal conductivity depends on the shape and dimension of the pores.

  • Oxidation resistance of electroplated Pt-Ir alloy coatings on Ni-based single crystal superalloys followed by diffusion annealing treatment

    Yoshito Yasui, Hideyuki Murakami, Mikiyo Yoshida, Akihiro Sato, Akira Tateno, Seiji Kuroda, Hachiro Imai

    Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals   73 ( 12 ) 913 - 918  2009.12

     View Summary

    Oxidation resistance of electroplated Pt and Pt-Ir alloy coatings followed by simple annealing heat treatments was investigated. The so-called 2nd generation Ni-based single crystal superalloy TMS-82 + and 4th generation TMS-138 were used as substrate materials. The cyclic oxidation tests and corresponding surface observations suggested the comparable oxidation resistance of Pt-coated TMS-82 + and Pt-Ir-coated TMS-138. However, cross-sectional microstructural analysis revealed that Pt-coated TMS-82 + exhibited the accelerated formation of voids, indicating the beneficial effect of Ir addition in terms of suppressed void formation. This study also revealed the importance of cross sectional analysis for thoroughly evaluating specimens subjected to the cyclic oxidation tests, and confirmed that oxidation resistance of the electroplated coatings was dependent on the quality of films deposited, which was drastically affected by the substrate surface finishing and substrate composition. © 2009 The Japan Institute of Metals.

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    4
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  • Interdiffusion in L1 <inf>2</inf>-Ni <inf>3</inf>Al Alloyed with Re

    N. Garimella, M. Ode, M. Ikeda, H. Murakami, Y. H. Sohn

    Journal of Phase Equilibria and Diffusion   30 ( 3 ) 246 - 253  2009.06

     View Summary

    Ternary interdiffusion in L1 2-Ni 3Al with ternary alloying addition of Re was investigated at 1473 K using solid-to-solid diffusion couples. Interdiffusion flux of Ni, Al, and Re were directly calculated from experimental concentration profiles and integrated for the determination of average ternary interdiffusion coefficients. The magnitude of main interdiffusion coefficients D̄̃ NiNiAl and D̄̃ AlAlNi was determined to be much larger than that of the main interdiffusion coefficient D̄̃ ReReAl(orNi) A moderate tendency for Re to substitute for Al sites was reflected by its influence on interdiffusion of Al, quantified by large and positive D̄̃ AlReNi coefficients. Similar trends were observed from ternary interdiffusion coefficients determined by Boltzmann-Matano analysis. Profiles of concentrations and interdiffusion fluxes were also examined to estimate binary interdiffusion coefficients in Ni 3Al, and tracer diffusion coefficients of Re (5.4 × 10 -16 ± 2.3 × 10 -16 m 2/s) in Ni 3Al. © 2009 ASM International.

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  • Phase equilibria and oxidation behavior of Ir-rich Ir-Y binary alloys

    Nobuaki Sekido, Hideyuki Murakami, Yoko Yamabe-Mitarai

    Journal of Alloys and Compounds   476 ( 1-2 ) 107 - 112  2009.05

     View Summary

    The phase equilibria and oxidation behavior of Ir-Y binary alloys have been investigated. A phase diagram at the Ir-rich end in the Ir-Y binary system has been proposed on the basis of experimental results. Iridium is found to dissolve little Y in the FCC solid solution. The pure-Ir and Ir-Y binary alloys exhibit severe weight loss upon air-exposure at 1373 K with elapsed time. Protective oxide layers are not formed on the surface of the alloys with Y up to, at least, 20 at.%. The kinetics of the oxidation is controlled by the formation and sublimation of Ir oxides at the surface. © 2008 Elsevier B.V. All rights reserved.

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  • Degradation on L-605 heat resistant alloy used for mono-propellant thrusters

    Kenichi Kajiwara, Hideshi Kagawa, Daisuke Goto, Yoko Yamabe-Mitarai, Hideyuki Murakami, Yoshinori Ono, Daisuke Kuroda

    45th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit    2009

     View Summary

    Haynes25 (L-605) is a common heat resistant alloy used in mono-propellant structures and in screen materials for catalyst beds. The lifetime requirements for thrusters have expanded dramatically after studies conducted in the 1970s on mono-propellant materials used for achieving a long life. The material design had not changed for a long time, and L-605 was still used as thruster material due to its good heritage. Some important degradation incidents were found in the test-unit break-up inspection after the thruster life tests. The Japanese research team focused on the L-605 degradations found on the catalyst bed screen mesh used for mono-propellant thruster. The team analyzed the surface of the wire material and the cross-section of the wire screen mesh used in the life tests. The investigation showed that the degradation was caused by nitriding L-605 component elements. The team also suggested the installation of a platinum coating on the material surface as a L-605 nitric degradation measure. © 2009 by the American Institute of Aeronautics and Astronautics, Inc.

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  • Thermodynamic re-assessment of the Al-Ir system

    T. Abe, C. Kocer, M. Ode, H. Murakami, Y. Yamabe-Mitarai, K. Hashimoto, H. Onodera

    Calphad: Computer Coupling of Phase Diagrams and Thermochemistry   32 ( 4 ) 686 - 692  2008.12

     View Summary

    The thermodynamic assessment of the Al-Ir binary system was performed using the CALPHAD technique. The B2-AlIr phase was described, using the two sublattice model with the formula (Al,Ir,V a)1/2(Al,Ir,V a)1/2, while Al9Ir2, Al3Ir, Al13Ir4, Al45Ir13, Al28Ir9, and Al2.7Ir compounds were treated as stoichiometric compounds. The fcc-based phases (L10-AlIr, L12-Al3Ir, L12-AlIr3 and A1) were described using the four sublattice model with the formula, (Al,Ir)1/4(Al,Ir)1/4(Al,Ir)1/4(Al,Ir)1/4. From ab initio calculations (VASP) the formation enthalpies of the stable/metastable intermetallic phases involved in the Al-Ir system were estimated. The thermodynamic quantities, such as the phase equilibria, invariant reactions, and formation enthalpies of the intermetallic phases, were calculated using the obtained parameter set, and agree well with experimental data. © 2008 Elsevier Ltd. All rights reserved.

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  • An investigation of the phase diagram of the Al-Ir binary system

    Machiko Ode, Taichi Abe, Hideyuki Murakami, Yoko Yamabe-Mitarai, Toru Hara, Kosuke Nagashio, Cenk Kocer, Hidehiro Onodera

    Intermetallics   16 ( 10 ) 1171 - 1178  2008.10

     View Summary

    Al-rich portion of the Ir-Al phase diagram has been investigated with 10 alloys by means of differential thermal analysis, X-ray diffraction, scanning electron microscopy, inductivity coupled plasma spectrometry and electron probe microanalysis. Invariant reactions are observed: L ↔ AlIr + Al2.7Ir, L + Al2.7Ir ↔ Al3Ir, L + Al3Ir ↔ Al13Ir4, L + Al13Ir4 ↔ Al9Ir2. Although the eutectic reaction, L ↔ AlIr + Al2.7Ir, cannot be regarded as definitely established by previous research, it is confirmed in the present research. Liquidus curve has been determined for the first time for Al-rich alloy (Ir ≤ 33.0 at%). The Ir-Al binary phase diagram has been modified based on these experimental observations. © 2008 Elsevier Ltd. All rights reserved.

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    17
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  • Effects of Ir or Ta Alloying Addition on Interdiffusion of L1<inf>2</inf>-Ni<inf>3</inf>Al

    N. Garimella, M. Ode, M. Ikeda, H. Murakami, Y. H. Sohn

    Intermetallics   16 ( 9 ) 1095 - 1103  2008.09

     View Summary

    Ternary interdiffusion in L12-Ni3Al with ternary alloying additions of Ir and Ta was investigated at 1473 K using solid-to-solid diffusion couples. Average ternary interdiffusion coefficients were determined from an integration of interdiffusion fluxes calculated directly from experimental concentration profiles. The magnitude of over(over(D, ̃), -)NiNi and over(over(D, ̃), -)AlAl coefficients was determined to be much larger than that of over(over(D, ̃), -)IrIr and over(over(D, ̃), -)TaTa coefficients. Ir substituting in Ni-site influenced the interdiffusion of Ni significantly, and over(over(D, ̃), -)NiIrAl coefficients were determined to be large and positive. On the other hand, Ta substituting for Al influenced the interdiffusion of Al significantly, and over(over(D, ̃), -)AlTaNi coefficients were determined to be large and positive. An excellent agreement was found with ternary interdiffusion coefficients determined by Boltzmann-Matano analysis. Profiles of concentrations and interdiffusion fluxes were also examined to estimate binary interdiffusion coefficients in Ni3Al, and tracer diffusion coefficients of Ir (14.7 × 10-16 ± 1.4 × 10-16 m2/s) and Ta (2.6 × 10-16 ± 2.4 × 10-16 m2/s) in Ni3Al. © 2008 Elsevier Ltd. All rights reserved.

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  • Anisotropy of secondary reaction zone formation in aluminized Ni-based single-crystal superalloys

    Hideyuki Murakami, Takashi Sakai

    Scripta Materialia   59 ( 4 ) 428 - 431  2008.08

     View Summary

    Distribution of the secondary reaction zone (SRZ) on the (1 0 0) plane was investigated in the aluminized Ni-based single-crystal superalloy TMS-75. Preferential formation of the SRZ was observed along the {0 0 1}〈1 1 0〉 directions, whereas the SRZ was not formed along the {0 0 1}〈1 0 0〉 directions. This result can be explained by the anisotropy of recrystallization in the single-crystalline substrate. © 2008 Acta Materialia Inc.

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  • Reaction kinetics in the combustion synthesis of Al-Ir-Ni intermetallic compounds

    Kai Cai, Machiko Ode, Hideyuki Murakami

    Journal of Materials Research   23 ( 7 ) 1953 - 1960  2008.07

     View Summary

    The combustion synthesis of Al50 Ir48Ni2 (at.%) was conducted at different heating rates in both a differential scanning calorimetry (DSC) chamber and a vacuum furnace. It was found that a higher heating rate, a sufficient amount of reactant powder, and effective control of the heat loss facilitated the complete reaction and resulted in combusted single IrAl phase products. Otherwise, multiphase products containing IrAl, unreacted Ir, and Al3Ir were synthesized. The reactions involved in different processes were discussed in terms of the thermal competition between heat generation and loss during the reaction. All ignition temperatures were below 773 K, indicating that the combustion reaction occurs at the solid-solid state. With increasing heating rate, the ignition temperature increased while the product density decreased. © 2008 Materials Research Society.

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  • Ternary interdiffusion in L1<inf>2</inf>-Ni<inf>3</inf>Al with Ir alloying addition

    M. Ode, N. Garimella, M. Ikeda, H. Murakami, Y. H. Sohn

    Defect and Diffusion Forum   273-276   637 - 642  2008

     View Summary

    Average ternary interdiffusion coefficients in Ni3Al with Ir additions have been determined using solid-to-solid diffusion couples annealed at 1200°C for 5 hours. Disc shaped alloy specimens were prepared by the vacuum arc melting at compositions of Ni-24Al, Ni-25Al, Ni-26Al, Ni-23.5Al-1Ir, Ni-24.5Al-1Ir, Ni-23Al-2Ir, Ni-23Al-2Ir, Ni-24Al-2Ir, Ni-23Al-3Ir (at.%). Surfaces of alloys were polished down to 1200 grit and diffusion couples were assembled in Si3N4 jig for initial bonding heat treatment at 1200°C for 0.5 hours. Additional diffusion anneal was carried out at 1200°C for 4.5 hours outside of Si3N4 jig so that diffusion couples can be water quenched. Concentration profiles of individual components were measured by electron probe microanalysis using pure standard of Ni, Al and Ir. Interdiffusion flux of individual component was determined directly from the experimental concentration profiles, and the moments of interdiffusion flux were examined to calculate the average ternary interdiffusion coefficients, D̃̄ijk either with Al or Ni as dependent component. Calculated interdiffusion coefficients suggest that Ir-alloyed Al2O3-forming oxidation resistant coatings would be beneficial to reduce the interdiffusion flux of Ni from superalloy substrates to the coating, and reduce the interdiffusion flux of Al from the coating to the superalloy substrate.

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    3
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  • Secondary reaction zones in coated 4th generation Ni-based blade alloys

    Aya Suzuki, C. M.F. Rae, M. Yoshida, Y. Matsubara, H. Murakami

    Proceedings of the International Symposium on Superalloys     777 - 786  2008

     View Summary

    The morphologies and growth kinetics of the Secondary Reaction Zone (SRZ) formed at the interface between the Pt-Aluminised coating and 4th generation Ni-base superalloy substrates have been investigated. Three alloys with systematically varying Ru content in the range of 2-5wt% were studied. Each alloy was tested using four different surface finishes prior to Pt coating. At the lowest Ru content sporadic formation of SRZ was observed, whilst in the higher Ru-containing alloys a continuous SRZ was formed. These alloys also showed more rapid SRZ growth, regardless of the original surface finish. The precipitation of TCPs in the substrate inhibited the growth of the SRZ towards the end of the exposure reversing the advantage of early nucleation. The volume fraction of the SRZ formation followed that in the substrate, reducing with Ru content. EBSD analysis revealed that the higher Ru-containing alloys nucleate many more grains of the SRZ but the number of grains adjusts with growth towards a constant grain size. On balance Ru leads to a deeper penetration depth into the substrate.

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  • In situ visualization of impacting phenomena of plasma-sprayed zirconia: From single splat to coating formation

    Kentaro Shinoda, Hideyuki Murakami, Seiji Kuroda, Kohsei Takehara, Sachio Oki

    Journal of Thermal Spray Technology   17 ( 5-6 ) 623 - 630  2008

     View Summary

    The authors have developed an in situ monitoring system for particle impacts under atmospheric dc plasma spraying conditions. This system utilized a high-speed video camera coupled with a long-distance microscope, and was capable of capturing the particle-impinging phenomena at one million frames per second. To understand the coating formation mechanism, two approaches were attempted, i.e., observation of the single splat formation and the subsequent coating formation. In the former case, the deformation and cooling processes of yttria-stabilized zirconia (YSZ) droplets impinging on substrates were successfully captured. In the latter case, multiple-droplet-impacting phenomena were observed as an ensemble treatment. Representing the coating process, the tower formation (0-dimensional) and bead formation (1-dimensional) were observed under typical plasma spray conditions for thermal barrier coatings using a triggering system coupled with the motion of a robot. The obtained images clearly showed the coating formation resulting from the integration of single splats. © ASM International 2008.

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    16
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  • Effect of Hf addition on oxidation properties of Pt-Ir modified aluminide coating

    Akihiro Yamaguchi, Hideyuki Murakami, Seiji Kuroda, Hachiro Imai

    Materials Transactions   48 ( 9 ) 2422 - 2426  2007.09

     View Summary

    In the present study, the effect of Hf addition on oxidation kinetics of Pt-Ir modified aluminide coatings was investigated. Pt-15 at%Ir and Pt-15 at%Ir with trace amount of Hf films with 7-8 μm thick were deposited on a Ni-based single crystal superalloy TMS-82+ using magnetron sputtering, followed by a diffusion treatment and conventional Al pack cementation. While there were no significant microstructual differences observed in as aluminized specimens, cyclic oxidation test at 1423 K revealed clear advantages of Hf addition. Hf containing coatings showed smaller mass change with retarded surface rumpling during cyclic oxidation test. These results confirmed the beneficial effects of Hf addition reported for other alloy systems such as Pt modified γ-γ′ coatings and Pt modified aluminide coatings. © 2007 The Japan Institute of Metals.

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  • Oxidation behavior of Pt-Ir modified aluminized coatings on Ni-base single crystal superalloy TMS-82+

    Aya Suzuki, Yingna Wu, Akihiro Yamaguchi, Hideyuki Murakami, Catherine M.F. Rae

    Oxidation of Metals   68 ( 1-2 ) 53 - 64  2007.08

     View Summary

    The oxidation resistance of Pt-Ir modified aluminized coatings, prepared by magnetron sputtering, was investigated. Cyclic oxidation tests revealed that Pt-30 at%Ir and Pt-50 at%Ir modified aluminide coatings demonstrated a smaller mass change compared with Pt, Pt-80 at%Ir and Ir modified aluminide coatings. Cross-sectional analyses following cyclic oxidation tests showed that the TGO layer formed on the Pt modified aluminide coating surface is almost twice as thick as those on the Pt-30 at%Ir and Pt-50 at%Ir coatings. In addition, the Pt-30 at%Ir and Pt-50 at%Ir samples had a much smoother surface than the Pt modified coatings after cyclic oxidation, and the latter suffered from severe surface rumpling. However, when the Ir content exceeded 80 at% in Pt-Ir modified coatings, internal voids formed during cyclic oxidation. These results show that the addition of 30-50 at%Ir to Pt-modified aluminized coatings is most effective in enhancing oxidation resistance. © Springer Science+Business Media, LLC 2007.

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  • Phase equilibria between the B2, L1<inf>2</inf>, and fcc phases in the Ir-Ni-Al system

    Y. Yamabe-Mitarai, T. Aoyagi, K. Nishida, H. Aoki, T. Abe, H. Murakami

    Intermetallics   15 ( 4 ) 479 - 488  2007.04

     View Summary

    The isotherms of the Ir-Ni-Al in the composition range up to 50 at% Al are presented at 1573 K. The phase constitution and microstructure of the Ir-Ni-Al alloys were examined using X-ray diffractometry (XRD), scanning electron microscopy (SEM) with an electron probe microanalyzer (EPMA), and transmission electron microscopy (TEM) after heat treatment at 1573 K for 168 h. The B2-NiAl and B2-IrAl phases connected with each other at 1573 K. The highest solubility limit of Ir into Ni3Al was about 3.5 at% in the tested alloys. Then, a wide fcc + B2 and a narrow fcc + L12 and B2 + L12 two-phase region appeared in the isothermal section. In part of the B2 phase, a martensitic transformation from the B2 to the L10 phase was observed. © 2006 Elsevier Ltd. All rights reserved.

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  • Effect of Ta on microstructure and phase distribution in cyclic oxidized Ir-Ta modified aluminide coatings on nickel base single crystal superalloy

    P. Kuppusami, H. Murakami

    Materials Science and Engineering A   452-453   254 - 261  2007.04

     View Summary

    The effect of Ir-Ta coating (with varying amounts of Ta) on microstructure, composition variation and phase distribution in the modified aluminides on Ni-base single crystal superalloy, TMS-75 subjected to cyclic oxidation has been investigated. Ir-Ta coatings with Ta = 14.1, 23.1, 40.7 and 82.9 at.% deposited on 〈1 0 0〉 oriented nickel base single crystal superalloy, TMS-75 were aluminized and subjected to cyclic oxidation at 1373 K, for 600 h. The mass gain as a function of number of cycles data demonstrates an improved oxidation resistance with decreasing Ta content. In contrary to Ta-rich aluminides, Ir-rich aluminides show a large volume fraction of the β-phase, a less amount of γ′-Ni3Al phase near the surface of the coatings. Rapid decrease in the concentration of Al in the diffusion zone and formation of interconnected γ′-Ni3Al has been found to be responsible for the lower oxidation resistance of Ta-rich aluminides. © 2006 Elsevier B.V. All rights reserved.

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  • Hot corrosion behavior of Pt-Ir modified aluminide coatings on the nickel-base single crystal superalloy TMS-82+

    Ying Na Wu, A. Yamaguchi, H. Murakami, S. Kuroda

    Journal of Materials Research   22 ( 1 ) 206 - 216  2007.01

     View Summary

    Platinum-iridium films (Ir = 0, 32, 46, 83, 100 at.%) were deposited on the nickel-base single crystal superalloy through magnetron sputtering. After annealing and aluminizing, the Pt-Ir modified aluminide coatings mainly consisted of PtAl2 and β-(Ni,Pt,Ir)Al phases. Hot corrosion resistance of Pt-Ir modified aluminide coatings with the different Ir contents were evaluated by exposure at 1173 K in the presence of the 90%Na2SO4 + 10%NaCl (wt%) salt deposits. The corrosion kinetics curves of the specimens were plotted up to 100 h heating time. The phase constitution, morphology of corrosion products, and element concentrations along the cross section were also measured. The lowest mass gain (0.299 mg/cm2, after 100 h) was observed for Pt-46Ir aluminide coating because the dense and continuous protective Al2O3 scale formed. Phase transformation from β-(Ni,Pt)Al to γ′- (Ni,Pt)3Al, characteristics of the scale, and protection by P t/Ir enriched layer had the important effects on the hot corrosion behavior of modified aluminide coatings. © 2007 Materials Research Society.

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  • A thermodynamic description of the Al-Ir system

    T. Abe, M. Ode, H. Murakami, C. S. Oh, C. Kocer, Y. Yamabe-Mitarai, H. Onodera

    Materials Science Forum   539-543 ( PART 3 ) 2389 - 2394  2007

     View Summary

    The thermodynamic assessment of the Al-Ir binary system, one of the key sub-systems of the Ir-based alloys, was performed using the CALPHAD technique. The AlIr(B2) phase was described using the two sublattice model with the formula (Al,Ir)0.5(Ir,Va)0.5, while other intermetallic phases were treated as stoichiometric compounds. The calculated data of the phases in the Al-Ir system can be used to accurately reproduce experimental data, such as phase equilibria, invariant reactions, and formation enthalpies of the intermetallic phases.

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  • Thermal cycling and hot-corrosion behaviors of plasma sprayed segmented thermal barrier coatings

    Hongbo Guo, Hideyuki Murakami, Seiji Kuroda

    Materials Science Forum   546-549 ( PART 3 ) 1713 - 1716  2007

     View Summary

    Modified zirconia thermal barrier coatings (TBCs) with segmentation cracks were sprayed onto a TMS 82+ single crystalline substrate. The thermal cycling lifetime of the modified TBC was improved by 10 times compared to that of the traditional non-segmented TBC. Also, the modified coating showed much better resistance to high temperature cyclic hot-corrosion.

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    6
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  • Effect of Hf addition on high temperature properties of Ir-containing alloy coatings

    H. Murakami, K. Kamiya, A. Yamaguchi, Y. N. Wu, S. Kuroda

    Materials Science Forum   546-549 ( PART 3 ) 1689 - 1694  2007

     View Summary

    In the present study, high temperature properties of Ir-modified and Ir-Hf-modified aluminide coatings on Ni-based single crystal superalloy TMS-82+ were discussed. They were prepared by depositing pure Ir and Ir-Hf alloys on TMS-82+ using magnetron sputtering and EB-PVD, followed by a conventional Al-pack cementation process. The effects of Hf addition on the oxidation resistance and top-coat spallation resistance were investigated. Cyclic oxidation test at 1423K for 1h as one heating cycle revealed that while there is a small difference in oxidation kinetics and spallation lives between Ir and Ir-Hf coatings, drastic difference in surface morphology was observed. After 50 oxidation cycles the Ir-modified aluminide coating showed surface rumpling whereas the Ir-Hf modified aluminide coatings kept the flat surface. It was also revealed that excessive addition of Hf promoted the internal oxidation, resulting in the deterioration of substrates. These results agree with the conventional Pt-modified aluminide coatings and Ni-Al-Hf alloys.

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  • High-speed thermal imaging of yttria-stabilized zirconia droplet impinging on substrate in plasma spraying

    Kentaro Shinoda, Hideyuki Murakami, Seiji Kuroda, Sachio Oki, Kohsei Takehara, Takeharu Goji Etoh

    Applied Physics Letters   90 ( 19 )  2007

     View Summary

    The authors have developed an in situ monitoring system that captures the impacting phenomena of plasma-sprayed particles at 1× 106 framess. The system clearly captured deformation and cooling processes of an yttria-stabilized zirconia droplet of 50 μm in diameter impinging at 170 ms on a smooth quartz glass substrate kept at room temperature. The images show that the liquid sheet jetting out sideways from the droplet detached from the substrate and kept on spreading without disintegration until its maximum extent. While the sheet was spreading, the center region of the flattened droplet cooled down much more rapidly. © 2007 American Institute of Physics.

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  • Effect of hollow spherical powder size distribution on porosity and segmentation cracks in thermal barrier coatings

    H. B. Guo, H. Murakami, S. Kuroda

    Journal of the American Ceramic Society   89 ( 12 ) 3797 - 3804  2006.12

     View Summary

    The effect of characteristics of hollow spherical (HOSP) powders on porosity and development of segmentation cracks in plasma-sprayed thick thermal barrier coatings (TBCs) was investigated. Three powders with particle size ranges of 20-45, 53-75, and 90-120 μm were selected from a commercial HOSP powder feedstock for spraying the TBCs. The 20-45 μm powder has a higher deposition efficiency and a greater capability of producing segmented coatings than the other larger powders. Diagnostics of in-flight particles show that the average surface temperature and velocity of the particles sprayed from the fine powder is higher by 250°C and 50 m/s compared with those sprayed from the 90 to 120 μm powder, respectively, due to its greater ratio of surface area to mass. The lower porosity of the coating sprayed from the fine powder is mainly attributed to the decreased volume of intersplat gaps and voids. © 2006 The American Ceramic Society.

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  • Numerical prediction of fraction of eutectic phase in Sn-Ag-Cu soldering using the phase-field method

    Machiko Ode, Minoru Ueshima, Taichi Abe, Hideyuki Murakami, Hidehiro Onodera

    Journal of Electronic Materials   35 ( 11 ) 1969 - 1974  2006.11

     View Summary

    A combination of macroscale solidification simulation and phase-field calculation is employed to predict the volume fraction of the eutectic phase in Sn-4.0 mass%Ag-XCu solder alloys (X = 0.5-1.1 mass%). The solidification simulation incorporates the cooling rate in the phase-field simulation. We assume the residual liquid solidifies as eutectic phase when the driving force for the nucleation of Cu 6Sn 5 amounts to a critical value, which is determined based on the experimental data. Though the calculation results depend on the experimental data, the obtained fractions are about 40% for 0.5 mass%Cu and more than 90% for 1.1 mass%Cu alloy, which shows good agreement with the experimental data.

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  • Influence of polyelectrolyte dispersants on the surface chemical properties of aluminum in aqueous suspension

    Kai Cai, Machiko Ode, Hideyuki Murakami

    Colloids and Surfaces A: Physicochemical and Engineering Aspects   284-285   458 - 463  2006.08

     View Summary

    Colloidal processing is widely used for forming ceramics. However, its application to metal was rarely reported. To apply colloidal processing to form IrAl intermetallic compound, detailed knowledge about the surface properties of Al and its interaction with the solvent and dispersants is indispensable. In this study, the hydration process, the surface potential and particle dispersion of Al in aqueous suspension with and without dispersants were investigated. The effects of two polyelectrolyte dispersants, an anionic PAA- and a cationic PEI-H+, were studied. Time dependence of pH value indicated that both dispersants promoted the hydration of Al, while the slow hydration stopped in several hours due to the formation of the bayerite layer. Addition of the dispersants greatly increased the absolute zeta potential values, indicating strong interactions with the Al particle. Comparison of particle size suggested that PEI-H+ was better than PAA- for the dispersion of Al, although the latter could induce a much higher absolute zeta potential. FTIR spectroscopy indicated that the interaction in the Al-PAA- system was mainly between the -COO- group of PAA- with the Al surface groups, while hydrogen bonding was the main interaction in the Al-PEI-H+ system. The results also implied that not the electrostatic effect, but the steric effect was dominant for the dispersion of Al. © 2006 Elsevier B.V. All rights reserved.

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  • Role of iridium in hot corrosion resistance of Pt-Ir modified aluminide coatings with Na<inf>2</inf>so<inf>4</inf>-NaCl salt at 1173 K

    Ying Na Wu, Akihiro Yamaguchi, Hideyuki Murakami, Seiji Kuroda

    Materials Transactions   47 ( 8 ) 1918 - 1921  2006.08

     View Summary

    Platinum-iridium films (Ir = 0,32,46,83,100 at%) were deposited on the nickel-base single crystal superalloy TMS-82+ through magnetron sputtering. After annealing and aluminizing, the Pt-Ir modified aluminide coatings mainly consisted of PtAl2 and β-(Ni,Pt,Ir)Al phases. Hot corrosion resistance of the different Pt-Ir modified aluminide coatings was evaluated through exposure at 1173 K with the Na2SO4+10 mass%NaCl salt coatings. The lowest mass gain (2.99 × 10-3 kg/m 2, after 100 h) was observed for the Pt-46Ir aluminide coating, which formed the dense and continuous protective Al2O3 scale on the surface. The effect of Ir on the corrosion resistance of Pt-Ir modified aluminide coatings was discussed from three aspects - phase transformation, protective scale formation and the role of a Pt-Ir enriched layer. © 2006 The Japan Institute of Metals.

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  • Comparison of microstructure and oxidation behavior of CoNiCrAlY bond coatings prepared by different thermal spray processes

    Mitsuhiro Shibata, Seiji Kuroda, Hideyuki Murakami, Machiko Ode, Makoto Watanabe, Yukihiro Sakamoto

    Materials Transactions   47 ( 7 ) 1638 - 1642  2006.07

     View Summary

    To protect various gas turbine components against high temperature in the hot sections of power generation plants and aircraft engines, thermal barrier coatings (TBCs) have been developed and widely used. Conventional TBCs consist of a MCrAlY bond coating for oxidation resistance and a ceramic top coating for thermal insulation. High quality coatings of MCrAlYs have been produced mostly by low pressure plasma spraying but other more economical processes are also used depending on the operating conditions of the component to be coated. In this study, CoNiCrAlY powders were deposited on Inconel 718 substrate with 3 types spraying system, i.e., low pressure plasma spraying, high velocity oxy-fuel spraying, and atmosphere plasma spraying. The specimens without top ceramic coating were isothermally tested for up to 100hrs in air at 1373 K and mass gain of the coatings was measured. Microstructure of the coating cross sections and the surface oxides were observed with SEM. Moreover, phase changes during the oxidation test were investigated with calculated phase diagrams for the CoNiCrAlY alloy. © 2006 Japan Thermal Spraying Society.

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  • Microstructural investigation of CoNiCrAlY coated Ni-based single crystal superalloy prepared by LPPS

    Takashi Sakai, Mitsuhiro Shibata, Hideyuki Murakami, Seiji Kuroda

    Materials Transactions   47 ( 7 ) 1665 - 1670  2006.07

     View Summary

    Recent demand in increasing the efficiency of gas turbines has led the component materials to be exposed at much higher operating temperatures, which accordingly accelerates the microstructural deterioration of the coated materials, mainly caused by the interaction between the coated layer and the substrate. Therefore, controlling the chemistry of interfaces by the coating process is one of the most important keys to minimizing the microstructural changes during service. In this study, effects of surface treatments and coating conditions on microstructure changes of Ni-based superalloy substrates were investigated. CoNiCrAlY (AMDRY 9954) powder was coated on Ni-based single crystal superalloy TMS-82+ by low pressure plasma spraying (LPPS). It was found that grit-blasted treatment drastically distorted the coherent γ/γ′ microstructure of substrates, which accordingly promoted the uniform and accelerated precipitation of topologically close-packed (TCP) phases by the post heat treatment at 1273 K for 30 min. On the other hand, specimens without the grit-blast treatment had less amount of TCP precipitates, but showed preferred precipitation orientation along 〈011〉{100} direction. Surface preheating and surface sputter cleaning seemed to have less influences on microstructure change compared to the grit-blast treatment. © 2006 Japan Thermal Spraying Society.

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  • Segmented thermal barrier coatings produced by atmospheric plasma spraying hollow powders

    H. B. Guo, S. Kuroda, H. Murakami

    Thin Solid Films   506-507   136 - 139  2006.05

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    Segmented thermal barrier coatings were produced by plasma spraying hollow and solid ZrO2-8 wt.% Y2O3 powders. The solid powder (SP) had greater capability of producing segmentation cracks in coating compared with the hollow powder (HP). High substrate temperature (T s) gave rise to an increased segmentation crack density (D s). The segmentation crack network was still stable even after sintering at 1300 °C for 15 h. The microcracks that are mainly attributed to delaminations between splats had a negative effect on the origin and propagation of segmentation cracks. © 2005 Elsevier B.V. All rights reserved.

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  • Microstructures and properties of plasma-sprayed segmented thermal barrier coatings

    Hongbo Guo, Seiji Kuroda, Hideyuki Murakami

    Journal of the American Ceramic Society   89 ( 4 ) 1432 - 1439  2006.04

     View Summary

    Thermal barrier coatings (TBCs) with different levels of segmentation crack density were produced by spraying two types of ZrO2-8Y 2O3 powders. The fused and crushed powder has a greater capability of forming segmented coatings than the hollow sphere (HOSP) one. The highly segmented coatings reveal much lower porosity compared with traditionally sprayed coatings, thereby compromising the property of thermal insulation of TBCs. Microstructure and thermal conductivity of the HOSP coatings are more sensitive to the changes in spray conditions. Segmentation cracks had a strong influence in decreasing Young's modulus of coatings. Fifty hours heat treatment at 1250°C had little effect on the mechanical property of the highly segmented coatings. Copyright © 2006 by The American Ceramic Society.

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  • Thermal cycling behavior of plasma sprayed segmented thermal barrier coatings

    Hongbo Guo, Hideyuki Murakami, Seiji Kuroda

    Materials Transactions   47 ( 2 ) 306 - 309  2006.02

     View Summary

    Thermal barrier coatings (TBCs) with different levels of segmentation crack densities (Ds) were sprayed at different substrate temperatures (Ts). Thermal cycling resistance of the TBC specimen was examined. The segmented coating significantly improved the thermal cycling resistance as compared to the traditional non-segmented coating. Maximum thermal cycling lifetimes were achieved in the coating with a crack density level of 2.2 mm -1. New segmentation cracks were hardly generated during thermal cycling testing. Spallation of segments within the segmented coating occurred, which is different from spallation of the whole coating from substrate in the case of traditional coating. Oxidation of bond coat and limited phase decomposition of YSZ topcoat were considered as not responsible for the failure of TBCs. © 2006 The Japan Institute of Metals.

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  • Interdiffusion behavior of electroplated platinum-iridium alloys on nickel-base single crystal superalloy TMS-82+

    Y. N. Wu, A. Suzuki, H. Murakami, S. Kuroda

    Materials Science Forum   522-523   293 - 300  2006

     View Summary

    In the present study, platinum-iridium alloys (Ir = 15.8, 27.3, 36.1, 100at.%) were electroplated on a nickel-base single crystal superalloy TMS-82+ followed by a diffusion treatment at 1373K for 1 h. Interdiffusion behavior between the Pt-Ir films and substrates was investigated in terms of chemical composition, phase constitution and morphology. X-ray analysis revealed that annealed specimens consisted of several fee solid solutioned phases with various lattice parameters, together with ordered intermetallic compounds (Ll 2-(Pt,Ni)3Al and B2-(Ir,Ni)Al), due to the inward diffusion of Pt and Ir from the electrodeposited films to the superalloy substrates, and the outward diffusion of solute elements (Ni, Al, Cr, Co) in the superalloy substrates into the films during annealing. The depth concentration analysis indicated that the Pt-36.1Ir film effectively retarded the outward diffusion of solute elements, especially nickel, from the substrate.

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  • Segmentation cracks in plasma sprayed thin thermal barrier coatings

    Hongbo Guo, Hideyuki Murakami, Seiji Kuroda

    Ceramic Engineering and Science Proceedings   27 ( 3 ) 17 - 27  2006

     View Summary

    Thick thermal barrier coatings (TBCs) containing segmentation cracks have seen a successful application in combustion chamber parts. In this work, rather thin TBCs were produced by spraying yttria stabilized zirconia (YSZ) coatings onto two kinds of bond coats: CoNiCrAlY and Pt-Ir. The effects of process parameters including substrate temperature on the density of segmentation cracks were studied. The thermal cycling performance and failure mechanisms of the segmented thin TBCs were also investigated. The coating sprayed at 1073 K contains a segmentation crack density of approximately 5 mm-1. The segmented coatings significantly improved the thermal cycling lifetime as compared with the non-segmented coatings. The segmented TBC with Pt-Ir bond coat attained a lifetime of more than 5000 cycles (each cycle includes 3 minute heating up to the maximum temperature and 7 minute holding at the temperature), revealing an excellent thermal cycling performance.

  • Development of Re-based diffusion barrier coatings on nickel based superalloys

    T. Narita, K. Z. Thosin, L. Fengqun, S. Hayashi, H. Murakami, B. Gleeson, D. Young

    Materials and Corrosion   56 ( 12 ) 923 - 929  2005.12

     View Summary

    A diffusion barrier type coating with a duplex layer structure, an inner σ-(Re, W, Cr, Ni) as a diffusion barrier and outer Ni-aluminide as an Al reservoir, was formed on a Nickel based, single crystal, superalloy (TMS-82 +) and on Hastelloy X. Oxidation properties of both the alloys with or without the diffusion barrier coating were investigated in air under thermal cycling between room temperature and 1423 K for up to 360 ks. The inner σ layer with a composition (at%) of (35-40) Re, (15-20) W, (15-25) Cr and (15-25) Ni was produced by electrodeposition of Ni-70Re and NJ-20W films from aqueous solutions followed by Cr-pack cementation at temperatures between 1473 and 1573 K, and the outer Ni-aluminides of β-(Ni,Cr)Al + γ′-(Ni,Cr) 3Al was formed by electrodeposition of a Ni film, followed by Al pack cementation. After the 360 ks oxidation it was found that the structure and composition of both σ layer and alloy substrate were retained with little change. Furthermore, there was little Al in the σ layer. It could be concluded that the Re-based alloys such as σ (Re(W),Cr,Ni) are very promising candidates as a diffusion barrier between the outer Al-reservoir layer and alloy substrate at temperature of 1423 K. It was found that the Re(W)-Cr-Ni acts as a diffusion barrier for both inward diffusion of Al and outward diffusion of alloying elements in the alloy substrate. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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  • Characterization of electroplated platinum-iridium alloys on the nickel-base single crystal superalloy

    Yingna Wu, Aya Suzuki, Hideyuki Murakami, Seiji Kuroda

    Materials Transactions   46 ( 10 ) 2176 - 2179  2005.10

     View Summary

    As a high temperature protective layer, platinum-iridium alloys were electroplated on the nickel-base single crystal superalloy TMS-82+ from amidosulfuric acid solutions by the direct current method. It was found that Ir content in the films was always lower than the ratio of concentration ([Ir 3+]/([PtCl62-] + [Ir3+])) in the electrolytes, indicating that preferential deposition of Pt occurred in this study. The asdeposited Pt-Ir films formed an fee single phase structure with a granular surface. When Ir content exceeded 3 at%, Pt-Ir films exhibited the 〈111〉 preferred growth orientation. It was also found that pre-deposition of Ni and Pt promoted the deposition rate and changed composition of overlaying Pt-Ir films. © 2005 The Japan Institute of Metals.

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  • Effects of heat treatment on microstructures and physical properties of segmented thermal barrier coatings

    Hongbo Guo, Hideyuki Murakami, Seiji Kuroda

    Materials Transactions   46 ( 8 ) 1775 - 1778  2005.08

     View Summary

    Highly segmented thermal barrier coatings (TBCs) were produced by "hot" plasma spraying. The effects of heat treatment on the microstructures, mechanical and thermophysical properties were studied. The segmented coatings are denser than traditionally plasma-sprayed TBCs due to its good insplat bonding at high substrate temperature. The segmentation cracks and associating branching cracks hardly propagated or closed during sintering process, indicative of a good stability of crack network. Due to its low porosity, the segmented coatings compromised the property of thermal insulation of TBCs. For the coatings after 24 h heat treatment at 1523 K, the thermal conductivity was improved by around 35%. The segmentation cracks had a strong impact on decreasing the Young's modulus. Heat treatment could not effectively promote the increase of the Young's modulus, especially for the case of highly segmented coatings. © 2005 The Japan Institute of Metals.

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  • Microstructual characterization of sputtered Pt-Ir coatings on Ni-base single crystal superalloy

    Aya Suzuki, Masanori Harada, Yingna Wu, Hideyuki Murakami

    Materials Transactions   46 ( 8 ) 1760 - 1763  2005.08

     View Summary

    Characterization of Pt-Ir alloy coatings on a Ni-base single crystal superalloy was carried out. Pt-Ir alloys with varied composition were sputter-deposited on TMS-82+, which were then subjected to annealing and Al-pack cementation treatments. It was found that all the as-deposited films consisted of Pt-Ir solid solutioned fee single phase, while the annealing treatment at 1423 K for 1 h drastically changed the microstructure, depending on the composition of coated layers. On the other hand, concentration profiles of alloying elements were not drastically changed by the Al-pack cementation process. It was also revealed that Ir addition increases the surface hardness of all the as-deposited, annealed, and aluminized specimens. © 2005 The Japan Institute of Metals.

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  • Self-propagating high-temperature synthesis of IrAl and its application to coating process

    M. Ode, H. Murakami, H. Onodera

    Scripta Materialia   52 ( 10 ) 1057 - 1062  2005.05

     View Summary

    An IrAl intermetallic compound was fabricated by a self-propagating high-temperature synthesis (SHS) process. Single phase IrAl pellets were produced; however, the IrAl coatings produced, with the aid of numerical simulations, contained unreacted Ir and was found to have poor adhesion. Nevertheless, this study confirmed that IrAl coatings can be obtained using the SHS process. © 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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  • Behaviour of Ir-24 at.-%Ta films on Ni based single crystal superalloys

    P. Kuppusami, H. Murakami, T. Ohmura

    Surface Engineering   21 ( 1 ) 53 - 59  2005.02

     View Summary

    The properties of Ir-24Ta (at.-%) films prepared by dc magnetron cosputtering from a composite target on a 〈100〉 oriented Ni based single crystal superalloy TMS-75 were investigated. Ir-Ta was proposed as a novel oxidation resistant bond coat material for thermal barrier coatings. XRD showed the formation of a Ir3Ta fcc phase with a fibre texture with 〈111〉 orientation tilted by ∼ 10° with respect to the plane normal to the substrate surface. The Ir-Ta film subjected to post-heat treatment was found to be very stable, with no secondary phase formation up to 1073 K, and there seems to be interdiffusion at 1273 K. Nanoindentation measurements indicated that the addition of tantalum decreased Young's modulus and increased the hardness obtained in the as deposited films compared with the hardness of iridium films. The Ir-Ta films heat treated up to 1273 K also exhibited nanomechanical properties comparable with those of as deposited films at 573 K. © 2005 Institute of Materials, Minerals and Mining.

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  • Characterization of an Ir-Hf alloy coating as a bond coat material

    Koichiro Kamiya, Hideyuki Murakami

    Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals   69 ( 1 ) 73 - 79  2005.01

     View Summary

    High temperature oxidation resistance and the topcoat spallation lives of Ir-Hf coated and aluminized Ni-base superalloys are investigated in comparison to Pt coated and aluminized ones. An Ir-Hf binary alloy, proposed as a novel metallic bond coat material, was coated on a Ni-base single crystal superalloy TMS-82 + using electron beam physical vapor deposition followed by a conventional Al pack cementation process. Although cyclic oxidation tests revealed that these Ir-Hf coated and aluminized specimens did not exhibit as good oxidation resistance as the Pt coated and aluminized specimens, formation of TCP phases in the substrate is suppressed by the presence of the Ir-Hf enriched layer. On the other hand, thermal cyclic tests for YSZ coated specimens revealed that Ir-Hf coated and aluminized specimens showed better adhesion to the ceramic top coat layer and demonstrated a longer spallation life than Pt-coated and aluminized specimens, which can be explained by the formation of Al 2O 3 and HfO 2 two-phase structure in the TGO layer. These results indicated that the Ir-Hf alloy system is promising as a new metallic bond coat material for high temperature structural materials. © 2005 The Japan Institute of Metals.

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  • Cyclic oxidation behaviour of Ir-Ta modified aluminide coatings on nickel base single crystal superalloy

    P. Kuppusami, H. Murakami

    Transactions of the Indian Institute of Metals   57 ( 5 ) 491 - 499  2004.10

     View Summary

    An investigation on microstructure, composition variation and phase distribution in Ir-Ta modified aluminides on Ni-base single crystal superalloy, TMS-75 subjected to cyclic oxidation at 1373K is reported. Ir-Ta coatings with Ta = 14.1, 23.1, 40.7 and 82.9 at.% were deposited on <100> oriented nickel base single crystal superalloy, TMS-75 by DC magnetron sputtering and then subjected to aluminizing and cyclic oxidation. For Ir-rich aluminides (Ta ≤ 40.7 at. %) in the as aluminized condition, Al content in the Ir-rich base layer is about 60 at. % and the diffusion zone containing β-NiAl phase is seen distinctly from the other zones. The mass gain as a function of number of cycles data demonstrates an improved oxidation resistance with decreasing Ta content. In contrary to Ta-rich aluminides, Ir-rich aluminides show the absence of γ'-Ni3Al and topologically close packed phases ahead of the diffusion zone, suggesting the fact that Ir-rich aluminide works as a diffusion barrier, distribut ion of Al after the aluminizing process dominantly affects the microstructure and the oxidation resistance of the modified aluminides, and internal microstructural evolution.

  • Process dependence of Ir-based bond coatings

    Hideyuki Murakami, Tetsuro Yano, Satoshi Sodeoka

    Materials Transactions   45 ( 9 ) 2886 - 2890  2004.09

     View Summary

    The process dependence of Ir-based-alloy-coated Ni-based single crystal superalloys was investigated. An Ir-4 at%Ta alloy, 10 μm thick, was coated on a Ni-based single crystal superalloy TMS-82+, by means of magnetron sputtering deposition (SD) and electron beam physical vapor deposition (EB-PVD). X-ray analysis revealed that the grain size of the as-SD-coated layer (∼20 nm) has five times smaller than that of the as-EB-PVD coated layer (∼100nm), and the SD-coated specimens showed more rapid outer diffusion of Ni and Al from the substrate after annealing, than the EB-PVD-coated ones. On the other hand, the aluminizing treatment reduces the differences between the SD coated and EB-PVD coated specimens in terms of compositional distribution and oxidation resistance. This study also revealed that even without aluminizing, the simple annealing treatment enhances the oxidation resistance of the Ir-4%Ta-coated TMS-82+.

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  • A comparative study of cyclic oxidized Ir aluminide and aluminized nickel base single crystal superalloy

    P. Kuppusami, H. Murakami

    Surface and Coatings Technology   186 ( 3 ) 377 - 388  2004.09

     View Summary

    A comparative study on microstructure, composition variation and phase distribution in an Ir-aluminide and an aluminized Ni-base single crystal superalloy, TMS-75 subjected to cyclic oxidation test at 1373 K is reported. The weight change as a function of number of oxidation cycles has demonstrated an improved oxidation resistance for iridium aluminide. While aluminized TMS-75 has an enrichment of aluminium to approximately 50 at.%, an addition of iridium increases the average aluminium concentration of the surface to approximately 70 at.% in the as aluminized condition. Unlike the aluminized TMS-75, the cyclic oxidized Ir aluminide reveals a precipitate free surface layer, followed by a significantly reduced precipitation of γ′-Ni3Al and TCP phases in the diffusion zone. © 2004 Elsevier B.V. All rights reserved.

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  • High temperature characteristics of Ir-Ta coated and aluminized Ni-base single crystal superalloys

    A. Suzuki, F. Wu, H. Murakami, H. Imai

    Science and Technology of Advanced Materials   5 ( 5-6 ) 555 - 564  2004.09

     View Summary

    High temperature oxidation and .hot corrosion properties of Ir-Ta coated and aluminized Ni-base superalloys are presented. An Ir-Ta binary alloy, proposed as a novel metallic bond coat material, was coated on a Ni-base single crystal superalloy TMS-75 using electron beam physical vapor deposition, followed by a conventional low activity Al pack cementation process. Cyclic oxidation tests and hot corrosion tests revealed that these Ir-Ta coated and aluminized specimens showed reasonably good oxidation and hot corrosion resistance. In addition, it was found that the formation of TCP phases is suppressed by the presence of the Ir-Ta enriched layer. These results indicated that the Ir-Ta alloy system is promising as a new metallic bond coat material for high temperature structural materials. © 2004 Elsevier Ltd. All rights reserved.

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  • Microstructure and mechanical properties of Ir-Ta coatings on nickel-base single-crystal superalloy TMS-75

    P. Kuppusami, H. Murakami, T. Ohmura

    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films   22 ( 4 ) 1208 - 1217  2004.07

     View Summary

    The iridium-alloy coatings with varying contents of tantalum, prepared by dc magnetron sputtering from a composite target, were investigated. It was observed that pure coatings of iridium and tantalum form polycrystalline films with rms roughness of ∼5 nm. The Ir-Ta coatings formed a nanocrystalline structure whose crystallite size and rms roughness decrease with increase in tantalum content. It was shown that the Young's modulus and hardness of the coatings generally decrease with increase in the tantalum content.

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  • Electrodeposition of Pt-Ir alloys on nickel-base single crystal superalloy TMS-75

    Feng Wu, Hideyuki Murakami, Yoko Yamabe-Mitarai, Hiroshi Harada, Hideki Katayama, Yoshinobu Yamamoto

    Surface and Coatings Technology   184 ( 1 ) 24 - 30  2004.06

     View Summary

    Platinum-iridium alloy coatings from amidosulfuric acid solutions have successfully been electrodeposited on nickel-base single crystal superalloy TMS-75 by direct current method. The effects of electrolyte temperature, current density and mole concentration ratios of [Ir3+]/([PtCl62-]+ [Ir3+]), on the deposition rate, composition and crystallographic structures of Pt-Ir alloy coatings are investigated. It is found that with increasing electrolyte temperature, deposition rate and Ir content increase, whereas the grain size of Pt-Ir alloy coatings decreases. Smooth and dense Pt-Ir alloy coatings can be obtained at 1 A/dm2 and 353 K. Pt-Ir alloy coatings with expected compositions can be readily fabricated by controlling the mole concentration ratios of [Ir3+]/([PtCl62-] +[Ir3+]) in the electrolyte. A detailed investigation of the structure and morphology of electrodeposited Pt-Ir alloy coatings is also presented. XRD analysis revealed that all the coated Pt-Ir alloys have a single phase with f.c.c structure, and the lattice parameters of the coatings decrease linearly with increasing Ir content, suggesting that the coated Pt-Ir alloy system follows the Vegard's law. © 2003 Elsevier B.V. All rights reserved.

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  • Application of Ir-base alloys to novel oxidation resistant bond-coatings

    H. Murakami, A. Suzuki, F. Wu, P. Kuppusami, H. Harada

    Proceedings of the International Symposium on Superalloys     589 - 596  2004

     View Summary

    High temperature oxidation and hot corrosion properties of Ir-Ta, Ir-Pt coated and aluminized coatings are presented. An Ir-Ta binary alloy, proposed as a novel metallic bond coat material, was coated on a Ni-base single crystal superalloy TMS-75 using electron beam physical vapor deposition (EB-PVD), followed by a conventional Al pack cementation process. Cyclic oxidation tests and hot corrosion tests revealed that these Ir-Ta coated and aluminized specimens showed reasonably good oxidation and hot corrosion resistance. In addition, the formation of TCP phases is suppressed by the presence of the Ir-Ta enriched layer. These results indicated that the Ir-Ta alloy system is promising as a new metallic bond coat for high temperature structural materials. On the other hand, substitution of Pt with Ir in Pt-Al coatings is of interest for cost reduction and strengtheneing purposes. Cyclic oxidation properties of (Ir, Pt)-coatings, which were successfully electrodeposited on TMS-75, followed by aluminization, were also investigated. Cyclic oxidation tests at 1373 K in air revealed that materials having better thermal cyclic oxidation resistance can be arranged in the following order: Pt-Al = (36at%Ir, Pt)-Al > Ir-Al ≫ simply aluminized. The presence of Ir and/or Pt may promote the formation of dense and adherent oxide scale and thus retard the growth of thermally grown oxides. Ir addition is thus promising, not just in terms of cost reduction, but also expected solid solution strengthening.

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  • Cyclic Oxidation Behavior of Iridium-Modified Aluminide Coatings for Nickel-Base Single Crystal Superalloy TMS-75

    Feng Wu, Hideyuki Murakami, Hiroshi Harada

    Materials Transactions   44 ( 9 ) 1675 - 1678  2003.09

     View Summary

    The oxidation behavior of iridium-modified aluminide (Ir-Al) coating obtained by a two-step process was investigated. A pure Ir layer was first electrodeposited on the nickel-base single crystal superalloy TMS-75, and then the Ir-coated TMS-75 was treated by a conventional low activity pack-cementation aluminizing process. The oxidation resistance of the Ir-Al coated TMS-75 and the simply aluminized TMS-75 was evaluated by a cyclic oxidation test at 1373 K in air. The results showed that the Ir-Al coated TMS-75 had better thermal cyclic oxidation resistance than the simply aluminized TMS-75. The existence of Ir in the Ir-Al coatings may promote the formation of dense and adherent Al2O3 scale and thus retard the degradation of β-(Ir, Ni)Al phase during oxidation process.

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  • Development of an iridium-tantalum modified aluminide coating as a diffusion barrier on nickel-base single crystal superalloy TMS-75

    F. Wu, H. Murakami, A. Suzuki

    Surface and Coatings Technology   168 ( 1 ) 62 - 69  2003.05

     View Summary

    An iridium-tantalum modified aluminized (Ir-Ta-Al) coating obtained by a two-step process is investigated. Ir-71 at.% Ta alloy was first coated on a nickel-base single-crystal superalloy TMS-75 by an electron beam physical vapor deposition (EB-PVD), and the resultant Ir-Ta alloy coating on the TMS-75 was treated by a conventional low-activity pack-cementation aluminizing process. It is found that a triplex layer structure, Ir-Ta alloy layer/β-NiAl/Al-diffusion zone, is formed in the order from the surface. In order to assess the oxidation resistance of the coated material, cyclic oxidation tests were carried out at 1373 K in air. It is found that Ir-Ta-Al coated TMS-75 had better thermal cyclic oxidation resistance than simply aluminized TMS-75. The microstructural evolution caused by the interdiffusion of solute elements shows remarkable difference on the precipitation of detrimental phases such as topologically close-packed (TCP) phase. The precipitation of TCP phase in the Ir-Ta-Al-coated TMS-75 was limited only within 180 μm in depth from the surface, whereas in the simply aluminized TMS-75, such detrimental phases were precipitated to the depth of more than 300 μm from the surface, indicating that the Ir-Ta-Al coating functions as an effective diffusion barrier for the alloying elements. © 2003 Elsevier Science B.V. All rights reserved.

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  • Influence of Ta content on microstructure and mechanical properties of Ir-Ta as diffusion barrier coatings on nickel base single crystal superalloy TMS-75

    P. Kuppusami, H. Murakami, T. Ohmura

    TMS Annual Meeting     309 - 318  2003

     View Summary

    A methodology for the controlled addition of tantalum in iridium alloy coatings prepared by DC magnetron sputtering from a composite target has been proposed in the present investigation. Ir-Ta coatings with 16.2, 23.9, 40.7 and 65.1 at% Ta were deposited at 573 K on 〈100〉 oriented nickel base single crystal superalloy, TMS-75 by suitably selecting the ratio of surface area of tantalum to surface area iridium on the basis of reported values of the sputtering yield of the respective elements. The microstructural analysis indicates that pure coatings of iridium and tantalum form polycrystalline films with RMS roughness of ∼5 nm, while the Ir-Ta coatings form nanocrystalline structure whose crystallite size and RMS roughness decrease with increase in tantalum content. Measurement of mechanical properties using nanoindentation technique shows that the Young's modulus and hardness of the coatings generally decrease with increase in the tantalum content. However, there is a peaking of hardness in the composition range 16.2-23.9at % tantalum because of the formation of If3Ta phase with Ll2 structure. The data on the mechanical properties are presented to explore the possibility to use Ir-Ta as a post-coat diffusion barrier material on nickel base single crystal superalloy.

  • Molecular dynamics simulations for a nickel metal with an edge dislocation and some impurities

    Takayuki Sakai, Hideyuki Murakami, Hiroshi Harada

    Proceedings of Asian Simulation Conference; System Simulation and Scientific Computing (Shanghai)     239 - 243  2002

     View Summary

    Examination of dislocation behavior in the level of atomic scale is one of effective approaches for understanding inelastic deformation behavior of engineering materials. In this study, a molecular dynamics code incorporated with a modified EAM (MEAM) potential proposed by Baskes was developed to investigate the dislocation behavior in nickel alloys. As verification of the code, molecular dynamics simulations for a nickel metal containing with an edge dislocation and an impurity element were carried out. Consequently, it was shown that the dislocation behavior had some temperature dependences in terms of dislocation velocity and Peiers stress. Then, it was demonstrated that the edge dislocation was made transferred under the influence of some kinds of element that seemed to be effective for solid-solution strengthening of nickel alloys. Therefore, it was proved that the MEAM potential was effective for at least qualitative evaluation of dislocation behavior.

  • The precipitate morphology and lattice mismatch of ternary (Pt)/Pt<inf>3</inf>Al alloys

    P. J. Hill, Y. Yamabe-Mitarai, H. Murakami, L. A. Cornish, M. J. Witcomb, I. M. Wolff, H. Harada

    Proceedings of the International Symposium on Structural Intermetallics     527 - 533  2001

     View Summary

    The effects of some ternary alloying additions (Ti, Cr, Ru, Ta and Ir) on the phase relations and properties of two-phase (Pt)/∼Pt3Al alloys were investigated. The crystal structure and morphology of the ∼Pt3Al phase were examined, and the compressive strengths and melting temperatures of the alloys were determined. It was found that Ti, Cr and Ta partitioned to ∼Pt3Al and stabilised the Ll2 form of this phase. The ∼Pt3Al phase in these alloys had cuboidal morphologies and a small lattice misfit with the matrix (about -0.7%). The tetragonal D0′C form of ∼Pt3Al was observed in the alloys containing Ru and Ir. The ∼Pt3Al in these alloys formed in groupings shaped somewhat like a Maltese cross, which may result from the higher lattice misfit in these alloys (-1 to -1.2%) or the D0′C crystal structure of the ∼Pt3Al. The compressive strengths of the alloys were found to depend on the proportion of ∼Pt3Al in the microstructures; higher strengths being recorded for alloys containing larger proportions of ∼Pt3Al.

  • Thermodynamic evaluation of the Mo-Ru system

    Chang Seok Oh, Hideyuki Murakami, Hiroshi Harada

    Journal of Alloys and Compounds   313 ( 1-2 ) 115 - 120  2000.12

     View Summary

    A thermodynamic assessment of the Mo-Ru binary system has been attempted by using the Calphad method. Solution and intermediate σ phases were described by regular-type and the compound energy model, respectively, and their Gibbs energy parameters were evaluated through a computerized optimization procedure with available experimental information. The phase diagrams calculated with the obtained thermodynamic parameters are in very good agreements with the reported literature values.

    DOI

    Scopus

    10
    Citation
    (Scopus)
  • Atom probe microanalysis of Ir-bearing Ni-based superalloys

    H. Murakami, Y. Koizumi, T. Yokokawa, Y. Yamabe-Mitarai, T. Yamagata, H. Harada

    Materials Science and Engineering A   250 ( 1 ) 109 - 114  1998.07

     View Summary

    The effects of Ir addition on the microstructural characteristics of Ni-based single crystal superalloys have been investigated using atom probe field ion microscopy (APFIM) and the results were compared with numerical estimations obtained from the Monte Carlo simulations (MCS) and the cluster variation method (CVM). The alloy investigated was TMS-79 (Ni-12.6 at% Al-7.7 at% Cr-4.6 at% Mo-2.7 at% Ta-1.8 at% Ir). APFIM analysis revealed that Ir atoms have a slight preference to partition into the γ-phase and to substitute for the Al site in the γ′ precipitates, which showed agreement with numerical estimations by CVM and MCS. The possible effect of Ir additions on the mechanical properties is briefly discussed.

    DOI

    Scopus

    38
    Citation
    (Scopus)
  • Rh-base refractory superalloys for ultra-high temperature use

    Y. Yamabe-Mitarai, Y. Koizumi, H. Murakami, Y. Ro, T. Maruko, H. Harada

    Scripta Materialia   36 ( 4 ) 393 - 398  1997.02

    DOI

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    90
    Citation
    (Scopus)
  • Al partitioning in MA 956, an ods ferritic stainless steel

    H. G. Read, H. Murakami, K. Hono

    Scripta Materialia   36 ( 3 ) 355 - 361  1997.02

    DOI

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    27
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  • Platinum group metals base refractory superalloys

    Y. Yamabe-Mitarai, Y. Koizumi, H. Murakami, Y. Ro, T. Maruko, H. Harada

    Materials Research Society Symposium - Proceedings   460   701 - 706  1997

     View Summary

    Ir- and Rh-base refractory superalloys with an fcc and Ll2 two phase structure similar to Ni-base superalloys, yet with considerably higher melting temperatures have been proposed. Fcc and Ll2 two phases were observed in these alloys by transmission electron microscopy and X-ray powder diffractometry. The compression tests of these alloys showed that the strengths of several alloys were about 200 MPa at 1800°C and these alloys have potential to become ultra-high temperature materials for use in power engineering field.

  • Development of Ir-base refractory superalloys

    Y. Yamabe, Y. Koizumi, H. Murakami, Y. Ro, T. Maruko, H. Harada

    Scripta Materialia   35 ( 2 ) 211 - 215  1996.07

    DOI

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    143
    Citation
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  • Microstructural influences on the decomposition of an Al-containing ferritic stainless steel

    H. G. Read, H. Murakami

    Applied Surface Science   94-95   334 - 342  1996.03

     View Summary

    The behaviour of as-received and recrystallised (homogenised) MA 956, a Cr-rich ferritic stainless steel, aged at 475 °C for up to 588 h has been investigated. The convolution of recovery and phase separation results in aged as-received material contrasting with hardening due to phase separation only in homogenised material. Atom probe microanalysis of the decomposition products reveals that Al does not partition significantly to the Fe-rich phase, as calculated by phase diagram modelling. The wavelength of the decomposition is found to be significantly larger in aged as-received material compared to aged homogenised material. The spinodal decomposition of the MA 956 supersaturated solid solution is possibly a paraequilibrium transformation with respect to Al. The behaviour of Al is discussed in terms of competing chemical and stress gradients in the nanophase material and the influence of a such a structure on its thermodynamic description.

    DOI

    Scopus

    28
    Citation
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  • Super high rate thermal plasma CVD of ceramics

    Hideyuki Murakami, Hifumi Nagai, Tohru Irokawa, Toyonobu Yoshida, Kazuo Akashi

    Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi/Journal of the Ceramic Society of Japan   97 ( 1 ) 49 - 55  1989

    DOI

    Scopus

    10
    Citation
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  • High‐Rate Thermal Plasma CVD of SiC

    HIDEYUKI MURAKAMI, TOYONOBU YOSHIDA, KAZUO AKASHI

    Advanced Ceramic Materials   3 ( 4 ) 423 - 426  1988.07

     View Summary

    Using a hybrid thermal plasma reactor for a new CVD process, thick SiC layers were successfully deposited at a rate of ≅500 μm/h on a graphite substrate from SiCl4 and CH4 under soft vacuum (≅2.7times104 Pa). The process is performed typically under flow rates of SiCl4=1.5 g/min and CH4=300 cm3/min and deposition temperatures of 1000° to 1100°C. The effects of deposition conditions on certain characteristics of the deposited layers were investigated. SEM revealed that the appearance of the deposited layers strongly depended on the substrate position. The prepared layers were dense and nearly stoichiometric β‐SiC with (100) preferred orientation. A new method for measuring substrate temperature is also reported, and growth characteristics are discussed. 1988 The American Ceramic Society

    DOI

    Scopus

    30
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Misc

  • Effect of thermal history on microstructural changes in aluminized nickel-based single-crystal superalloy

    Kazuki Kasai, Hideyuki Murakami, Kazuhiko Noda

    Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals   76 ( 6 ) 402 - 406  2012.06

    Book review, literature introduction, etc.  

     View Summary

    The effect of thermal history on microstructural changes in the aluminized and Pt-aluminized Ni-based single-crystal superalloy was investigated. The substrate superalloys were firstly electro-polished in order to get rid of the surface residual strain. Then the electrodeposition of Pt followed by vacuum annealing was conducted for some of the substrates, and the aluminized and Pt-aluminized specimens were prepared by the conventional aluminizing process. It was found that in the case of the aluminized specimens, voids were formed in the vicinity of substrate/coating interfaces, by thermal cyclic heating, whereas secondary diffusion zones (SDZ) were formed by isothermal heating. These differences in microstructural changes of the aluminized specimens can be explained by the diffusion kinetics between the coating layer and the substrate during heating/cooling processes. In the case of the Pt-aluminized specimen, on the other hand, secondary reaction zone (SRZ) formation was observed by both the thermal cyclic and isothermal treatments. These results can be explained by the poly-crystallization of the substrate surface by the annealing process that could promote the interdiffusion, resulting in the formation of SRZ. © 2012 The Japan Institute of Metals.

    DOI

  • Effective use of platinum group metals

    Yamabe Mitarai Yoko, Zebin Bao, Murakami Hideyuki, Abe Hideki, Matsumoto Toru

    Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals   75 ( 1 ) 10 - 20  2011.01

    Book review, literature introduction, etc.  

     View Summary

    Platinum group metals (PGMs) are used in a wide range of application fields such as catalysts, electrodes, medical and electrical devices, and high-temperature materials. The supply of platinum is only about 200t per year and its price drastically fluctuates depending on the economic situation. Even so, PGMs are used in industry because of their excellent properties which are difficult to obtain in alternative materials. Thus, an understanding of the mechanism that results in these properties is necessary in order to optimize the properties of PGMs, increase their lifetimes, and find alternative materials. In this paper, some attempts to increase the lifetimes of PGMs and to search for alternative materials for use as high-temperature materials, coating materials, catalyst materials, and electrode materials are reported. © 2011 The Japan Institute of Metals.

    DOI

  • Effects of detrimental diffusion zone and heat treatment on creep strength of aluminized nickel-based single crystal superalloys

    Shunsuke Shimabayashi, Koji Kakehi, Hideyuki Murakami, Shinya Uehara

    Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals   74 ( 8 ) 508 - 514  2010.08

     View Summary

    The detrimental effect of the diffusion zones between coating and the substrate had been investigated by performing creep tests on aluminized Ni-based single crystal superalloys. In present study, creep test-pieces that had two kinds of sub-surface crystallographic orientations, 〈100〉 and 〈110〉, were aluminized by the pack-cementation process at 1000°C for 5 h. To exclude the influence of the coating heat treatment on the creep strength of the bulk material, a set of specimens without coating was heat treated in vacuum at 1000°C for 5 h, which was equivalent to the condition during aluminizing process, for comparisons the creep rupture test was carried out at 900°C and 392 MPa. The cross-sectional microstructures of the as-aluminized and creep rupture specimens were observed by SEM using the backscattered electron image mode (BSE) and TEM. Analysis indicated that creep strength of the aluminized samples was decreased by the formation of the inter-diffusion zone (IDZ) and the substrate diffusion zone (SDZ) ; both of them reduced the load bearing thickness, thus resulting an increase in the effective stress. Furthermore, it was found that the disappearance of the secondary precipitate during coating treatment also decreased the creep strength. The IDZ, which formed at the coating/substrate interface grew more rapidly on the {110} plane than that on the {100} plane because of the preferential growth of the Topological-Close Paced (TCP) phase. Ru addition on the substrate could enhance the creep strength and the SDZ formation between the IDZ/substrate interface. © 2010 The Japan Institute of Metals.

    DOI CiNii

  • Effect of substrate concave pattern on splat formation of yttria-stabilized zirconia in atmospheric plasma spraying

    Kentaro Shinoda, Mehdi Raessi, Javad Mostaghimi, Toyonobu Yoshida, Hideyuki Murakami

    Journal of Thermal Spray Technology   18 ( 4 ) 609 - 618  2009.12

    Book review, literature introduction, etc.  

     View Summary

    Splat morphology of yttria-stabilized zirconia (YSZ) on a microconcave-patterned substrate was investigated by both numerical and experimental approaches under a dc-rf hybrid-plasma spray condition. The spreading behavior of molten droplets on a microdimple pattern was numerically simulated in a three-dimensional form. For comparison, impact of a YSZ droplet onto a microdimple pattern of a quartz glass substrate was studied in situ utilizing thermal emissions from the droplet. Concave aspects of a substrate surface play an important role in fingering/splashing of a spreading droplet as well as convex patterns. The main mechanism that causes splashing is likely due to the slipping of a spreading droplet at the edge of concave patterns. The viscosity decrease of the spreading droplet enhances the droplet splash. © 2009 ASM International.

    DOI

  • Design and analysis of Ni-base superalloys on an atomistic basis

    Hideyuki Murakami, Hiroshi Harada, Yoshiyuki Saito

    Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals   63 ( 6 ) 723 - 732  1999

     View Summary

    This study focused on the application of Monte Carlo Simulation (MCS) in the prediction of atomic locations of alloying elements in some Ni-base single crystal superalloys. MCS based on the direct exchange of nearest neighbouring atoms was conducted by employing the Lennard-Jones pair potential, and a new algorithm optimised for vector computers was developed. For Re-containing Ni-base superalloys TMS-71 and CMSX-4, the γ and γ′ phase compositions and the site occupancy of alloying elements in the γ′ phase under equilibrium conditions were predicted by MCS, and the predictions were then compared with the estimates obtained by the Cluster Variation Method (CVM) and the experimental results obtained by atom-probe field ion microscopy (APFIM), so as to verify the applicability of MCS. It was found that the MCS estimations were generally in good agreement with both CVM predictions and APFIM results. It was also revealed that the changes in numbers of nearest neighbouring atom pairs as a function of Monte Carlo steps well describe the characteristics of alloying elements, such as partitioning and site occupancy behaviour. In addition, the drastic increase in Mo-Re nearest neighbouring pairs in TMS-71 and Cr-W pairs in CMSX-4 suggested the formation of Mo-Re and Cr-W clusters, respectively. These clusters may be a sign of precipitation of a third phase such as Topologically Close-Packed (TCP) phase. It was thus concluded that MCS is useful not only in predicting the atomic locations of alloying elements in the γ and γ′ phases, but also in discussing the possibilities of third phase precipitation and the clustering of elements, etc.

    DOI

  • The influence of Co on creep deformation anisotropy in Ni-base single crystal superalloys at intermediate temperatures

    H. Murakami, T. Yamagata, H. Harada, M. Yamazaki

    Materials Science and Engineering A   223 ( 1-2 ) 54 - 58  1997.02

     View Summary

    In order to investigate the influence of Co on creep deformation anisotropy, some Ni-base single crystal superalloys have been creep tested at 800°C and 735 MPa. It is found that, although Co-containing alloys have higher creep resistance when the tensile direction is within 5° of 〈100〉, they are much more sensitive to tensile orientation than Co-free alloys, which have similar creep rupture lives for tensile orientations within 25° of 〈100〉. It is revealed that for Co-containing alloys the {111}〈112〉 slip system is operative during first-stage creep, when heterogeneous deformation occurs. Also, the degree of heterogeneous deformation correlates very well with creep rupture lives. It may thus be concluded that Co is not essential in improving creep rupture properties of Ni-base superalloys. © 1997 Elsevier Science S.A.

    DOI

  • The location of atoms in Re- and V-containing multicomponent nickel-base single-crystal superalloys

    H. Murakami, H. Harada, H. K.D.H. Bhadeshia

    Applied Surface Science   76-77 ( C ) 177 - 183  1994.03

     View Summary

    The atomic structure of some Ni-base single-crystal superalloys has been investigated using atom-probe field ion microscopy (APFIM) and the results compared with numerical estimations obtained from the alloy design program (ADP) and the cluster variation method (CVM). Re-containing CMSX4 and V-containing RR2000 alloys, together with TMS63 are the subjects of this study. It is found that Ti, Ta and Hf have a strong preference for the γ′ phase, whereas Cr, Co, W, Re and Mo tend to dissolve in the γ phase. V has a very weak preference for the γ phase. The results agree very well with both ADP and CVM. The APFIM analyses have also revealed that Re atoms in CMSX4 tend to preferentially occupy the sites normally designated for Al atoms in the γ′ phase. It is demonstrated that the site occupancy of other alloying elements such as Ta, W, Mo and Co can be estimated accurately using CVM, although there are some discrepancies for the distributions of Cr, Ti and Ni. The effect of solute atom distribution on creep properties is discussed briefly. © 1994.

    DOI

  • Distribution of Platinum Group Metals in Ni-base Single-Crystal Superalloys

    Superalloys, 2000, 747-756  

  • The Location of Atoms in Re- and V- containing Multicomponent Nickel-base Single Crystal Superalloys

    Applied Surface Science 76/77 (1994) 177-183  

    DOI

  • Determination of Atomic Structure of Ni-base Single Crystal Superalloys Using Monte Carlo Simulations and Atom-probe Microanalysis

    Superalloys 1996 pp.249-257  

  • Atom-Probe Microanalysis of Some Ni-base Single Crystal Superalloys

    Proceedings of the 3rd International Charles Parsons Turbine Conference (1995), pp.211-216  

  • Design of Ni-Base Super alloys(Chap 2. pp.39-70 in Springer Series in Materials Science Volume 34 「Computational Materials Design」 )

    T. Saito ed. Springer (1999)  

  • Distribution of Platinum Group Metals in Ni-base Single-Crystal Superalloys

    Superalloys, 2000, 747-756  

  • The Influence of Co on Creep Deformation Anisotropy in Ni-base Single Crystal Superalloys at Intermediate Temperatures

    Material Science and Engineering A223 (1997) pp.393-398  

  • Determination of Atomic Structure of Ni-base Single Crystal superalloys Using Monte Carlo Simulations and Atom-probe Microanalysis

    Superalloys 1996 pp.249-257  

  • Atom-Probe Microanalysis of Some Ni-base Single Crystal Superalloys

    Proceedings of the 3rd International Charles Parsons Turbine Conference (1995), pp.211-216  

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Awards

  • (英国金属学会主催)にて最優秀論文賞

  • 1995年 第3回チャールズパーソンズ卿タービン会議

  • 1996年 日本金属学会奨励賞

  • Metals

  • The Young Researchers'Award for the Japan Institute of

  • 3rd International Charles Parsons turbine Conference

  • The Conference Prize for the Most Outstanding Paper at the

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Research Projects

  • 耐熱合金へのコーティング技術開発

    Project Year :

    2001
    -
     
     

  • モンテカルロシミュレーション、クラスタ変分法及びアトムプローブ 解析を用いたニッケル基単結晶超合金のナノ構造研究

  • Nano-Structural Investigation of Nickel-base Single Crystal Superalloys Using Monte-Carlo Simulations, Cluster Variation Method and Atom-Probe Analyses

 

Syllabus

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