The effect of Nb and W alloying additions to the thermal expansion anisotropy and elastic properties of Mo 5 Si 3
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. INTRODUCTION
THE Mo5Si3 has great potential for ultra-high-temperature applications, since B-doped Mo5Si3 exhibits excellent oxidation resistance.[1] However, its coefficient of thermal expansion (CTE) is highly anisotropic and the ratio of CTE(c)/ CTE(a) is 2.0 to 2.2.[2,3] Such a large CTE anisotropy causes thermal microcracking, resulting in many cracks in polycrystalline Mo5Si3. In the crystal structure of Mo5Si3 (the tetragonal W5Si3 type crystal structure, shown in Figure 1), there are two types of Mo sites: 4b (Wyckoff notation) chain sites consisting of Mo-Mo chains along the c-axis and 16 k sites in the basal plane. It was pointed out by Fu and Wang[4] that the highly directional Mo-Mo chain structure along the c-axis and the strong covalent bonding in the basal plane cause the anisotropic thermal expansion. According to Fu and Wang’s calculations, these Mo-Mo chains along the c-axis exhibit a high degree of anharmonicity and therefore the thermal expansion in the c direction is relatively high.[4] Efforts to reduce the CTE anisotropy have included substitution of Nb,[2,5] V,[2,5] and W[2] for Mo and substitution of B for Si resulting in phase transformation to the tetragonal Cr5Si3 type structure.[6] The Nb and V substitution were found to decrease CTE anisotropy, while W substitution was found to increase it.[2] The larger Nb atoms tend to occupy the larger 16 k sites, while the smaller V atoms tend to occupy the smaller 4b sites. The site preference of W atoms (similar in size to Mo) is not known. In this study, the effect of ternary alloying additions on CTE anisotropy and elastic properties has been studied through the use of single crystalline (Mo0.8Nb0.2)5Si3, TAISUKE HAYASHI and MISAKO TAKAMOTO, Students, and KAZUHIRO ITO, Associate Professor, are with the Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan. Contact e-mail: [email protected] KATSUSHI TANAKA, Associate Professor, is with the Department of Advanced Materials Science Faculty of Engineering, Kagawa University, Takamatsu 761-0396, Japan. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
(Mo0.85W0.15)5Si3, and Mo5Si3. Niobium and tungsten have opposite effects on the CTE anisotropy of Mo5Si3, and so these two elements were chosen as representative alloying elements. The elastic anisotropy E[unw]/E[100] and estimated polycrystalline Young’s (E), bulk (K), and shear (G) moduli were calculated from the measured single-crystal elastic constants. The elastic and thermal expansion anisotropies are compared between the three compositions. II. EXPERIMENTAL PROCEDURE Rods of (Mo0.8Nb0.2)5Si3 and (Mo0
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