For applications at ultrahigh temperatures the multiphase microstructural options that can be developed in the Mo-Si-B system have demonstrated an effective and attractive balance of essential characteristics. The coexistence of the high melting point (>2100°C) ternary intermetallic Mo5SiB2 (T2) phase with Mo provides a useful option for in-situ toughening. A further enhancement is available from a precipitation reaction of Mo within the T2 phase that develops due to the temperature dependence of the solubility behavior of the T2 phase. However, direct access to Mo+T2 microstructures is not possible in ingot castings due to solidification segregation reactions that yield nonequilibrium boride and silicide phases with sluggish dissolution. Alternate routes involving rapid solidification of powders are effective in suppressing the segregation induced phases. The processing and microstructure options can also be augmented by selected refractory metal substitutional alloying, such as the incorporation of Nb, that alters the solubility of the T2 phase and the relative phase stability to yield solidification of two phase refractory solid solution + T2 structures directly. The observed alloying trends highlight the role of atomic size in influencing the relative stability of the T2 phase. A key component of the overall microstructural control and long term microstructural stability is determined by the kinetics of diffusional processes. The analysis of selected diffusion couples involving binary boride and silicide phases has been used to assess the relative diffusivities in the T2 phase and coexisting phases over the range of solubility and to provide a basis for the examination of the kinetics of reactions involved in coatings and oxidation.
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This work is sponsored by the Air Force Office of Scientific Research, USAF under grant number F49620-00-1-0077 which is most gratefully acknowledged. We thank Dr. J. Fournelle for expert guidance with EPMA measurements.
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Perepezko, J., Sakidja, R. & Kim, S. Phase Stability in Processing and Microstructure Control in High Temperature Mo-Si-B Alloys. MRS Online Proceedings Library 646, 20–31 (2000). https://doi.org/10.1557/PROC-646-N4.5.1