Phase Equilibria, Microstructure, and High-Temperature Strength of TiC-Added Mo-Si-B Alloys
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DEMANDS for energy efficiency improvements in heat engines such as gas turbines and jet engines have prompted calls for ultrahigh-temperature materials available above the operating temperature of Ni-base superalloys. The leading candidates for this purpose are Mo-Si-B alloys consisting of Mo solid solution (Moss) and Mo5SiB2 (T2) because of their high melting point, excellent high-temperature (creep) strength, and moderate oxidation resistance in the ultrahigh-temperature range.[1–14] However, the low fracture toughness is a drawback, but this can be improved by optimal control SHIMPEI MIYAMOTO, Graduate Student, formerly with Graduate School of Environmental Studies, Tohoku University, 6-6-02 Aramaki Aza Aoba, Aobaku, Sendai, Miyagi 980-8579, Japan, is now with IHI Corporation, 5292 Aioi, Aioi, Hyogo 678-0041, Japan. KYOSUKE YOSHIMI and KOUICHI MARUYAMA, Professors, are with Graduate School of Engineering, Tohoku University. Contact e-mail: [email protected] SEONG-HO HA, Post Doctoral Fellow, formerly with Graduate School of Engineering, Tohoku University, is now with Korea Institute of Industrial Technology, 7-47 Songdo-dong, Yeonsu-ku, Inchon, Korea. TAKAHIRO KANEKO, Graduate Student, JUNYA NAKAMURA, Assistant Professor, are with Graduate School of Environmental Studies, Tohoku University. TETSUYA SATO, Graduate Student, formerly with Graduate School of Environmental Studies, Tohoku University, is now with Central Japan Railway Company, 2-1-85 Konan, Minato-ku, Tokyo, 1088204, Japan. RONG TU, formerly Associate Professor with Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi, Japan, is now Professor with Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei 430070, P.R. China. TAKASHI GOTO, Professor, is with Institute for Materials Research, Tohoku University. Manuscript submitted December 21, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
of the microstructure and Moss volume fraction.[15] The high density is also problematic for structural applications: For example, in turbine rotors, an increase in the weight results in an increase in the centrifugal force, which shortens the creep lifetime of the material. The density of Ni-base superalloys ranges from 8.6 to 9.2 g/cm3[16] and the density of T2 is of a similar value: approximately 8.9 g/cm3.[2,8] However, in the case that T2 is incorporated with Moss (a density of approximately 10.2 g/cm3[17]), it is unavoidable that the density of a Mo-Si-B alloy becomes higher than that of Ni-base superalloys. Furthermore, the introduction of a large amount of Moss into T2 results in a lowering of the creep strength in the ultrahigh-temperature region.[11,12] It is well known that Mo and TiC have a eutectic reaction.[18] The eutectic point is at about Mo-23TiC (at. pct) at 2448 K (2175 C), and the two solid phases stably coexist below the eutectic temperature without any phase transformation. As shown in Figure 1, each phase is soluble in another one at a certain amount, resulting in solid-solution
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