Oxidation protective silicide coating on Mo-Si-B alloys
- PDF / 2,255,781 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 43 Downloads / 192 Views
TRODUCTION
DEVELOPMENT of ultra-high-temperature structural materials based on Mo and Nb and coatings for these structural materials is proceeding. In terms of oxidation resistance at high temperatures, disilicides such as MoSi2 have been considered better candidates than Mo-rich silicides such as Mo5Si3 and Mo3Si. In 1996, Meyer and Akinc found that boron additions in the range 1 to 2 wt pct improve the oxidation resistance of Mo5Si3 by about five orders of magnitude at 1200 °C.[1] Later studies of ultra-high-temperature materials tended to move toward the Mo-rich silicides in the Mo-Si-B ternary system. Oxidation behavior of Mo-Si-B alloys has been studied mainly by the Ames Group[2] and the United States Air Force Research Laboratory.[3,4] The Mo-Si-B alloys are vulnerable to rapid oxidation at 600 °C to 750 °C due to volatilization of MoO3. After an initial transient oxidation period, oxidation proceeded at a much slower rate. At and above 1000 °C, there is high degree of oxidation protection due to passivation provided by formation of borosilicate glass scale. The oxidation rate was found to vary inversely with the ratio of B/Si in the Mo-Si-B alloys. Mechanical properties of Mo-Si-B alloys have also been studied by the Oak Ridge Group[5] and the Lawrence Berkeley KAZUHIRO ITO and HIROSHI NUMAKURA, Associate Professors, and TAISUKE HAYASHI and MASATO YOKOBAYASHI, Students, are with the Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan. Contact e-mail: [email protected]. media.kyoto-u.ac.jp TAKASHI MURAKAMI, Researcher, is with the Institute of Mechanical Systems Engineering, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8564, 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 SMDCorrosion 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
Group.[6] We also reported that the ternary compound Mo5SiB2, referred to as the T2 phase, has been found to have a higher creep resistance than the MoSi2 and Mo5Si3 (T1) phases.[7,8] The Mo-9Si-18B (at. pct) alloy has a eutectic microstructure consisting of T2 and 28 vol pct Mo solid solution (Moss) phases, and shows an attractive combination of high strength at elevated temperatures and high fracture toughness at room temperature (RT).[9] Our choice of the Mo-9Si-18B alloy for study is not only due to its good balance of mechanical properties at elevated temperatures and RT, but also due to a simpler microstructure than the Mo-Si-B alloys, which others have studied. When the Mo-9Si-18B alloys are exposed to air at 1300 °C, significant oxidation occurred, even though some Mo-Si-B alloys have a high degree of oxidation protection at 1300 °C. Thus, the Mo-9Si-18B a
Data Loading...
