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 ternary intermetallic, Mo5SiB2, T2, 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 T2 ternary phase as the focal point of the microstructure designs, the fundamental basis of the alloying behavior in T2 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 T2 phase that directly impact high-temperature structural performance. The characteristically sluggish diffusion rates within the T2 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.