Phase stability and structural defects in high-temperature Mo-Si-B alloys

For high-temperature application beyond the range of Ni-base superalloys, Mo-Si-B alloys with composition that yield the tertiary intermetallic, Mo5SiB2, T-2, phase as a key microstructure constituent, offer an attractive property balance of high-melting temperature, oxidation resistance, and useful high-temperature mechanical properties. With the T-2 tertiary phase as the focal point of the microstructure designs, the fundamental basis of the alloying behavior in T-2 including the mutual solid solution with a wide range of transition metals has been established in terms of the governing geometric and electronic factors. For non-stoichiometric compositions, constitutional defects such as vacancies for Mo-rich compositions and anti-site defects for Mo-lean compositions control the homogeneity range. Moreover, the aggregation of constitutional vacancies has been discovered to play a key role in the development of dislocation and precipitation reactions in the T-2 phase that directly impact high-temperature structural performance. The characteristically sluggish diffusion rates within the T-2 phase have also been quantified and applied to the materials processing strategies. The materials design based on the phase stability, diffusion and defect structure analysis in the Mo-Si-B system can also be applied to the design of new multiphase high-temperature alloys with balanced environmental and mechanical properties. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.