Multiphase microstructures and stability in high temperature Mo-Si-B alloys


The isothermal phase diagram section for the Mo-Si-B system for compositions that include the ternary intermetallic Mo5SiB2 (T2) phase has been established at 1600°C. The composition variation of the c and a lattice parameters of the tetragonal T2 structure indicates that while the substitution of B by Si is limited, a large degree of substitution of Si by B is possible. An initial thermodynamic model provides an analysis of the measured phase stability around the T2 phase region. In ternary Mo-Si-B alloys direct formation of Mo+T2 structures is prevented by severe solidification segregation. It has been established that selected refractory metal alloying can alter the solubility and stability of the T2 phase to yield solidification of two phase refractory solid solution + T2 structures directly. The kinetics of T2 phase development as a reaction product between Mo5Si3 and Mo2B indicates sluggish diffusion (D<10-16 m2/s at 1600°C) in the T2 phase. Upon oxidation of two-phase (Mo(ss) + T2) alloys an outer layer of SiO2 that contains some B2O3 develops as an amorphous, adherent and oxidation resistant coating. The established behavior in the Mo-Si-B system allows for an effective balance of microstructural stability, oxidation resistance and structural performance.

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Conference Proceeding

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Journal Title

Proceedings of the International Symposium on Structural Intermetallics