Phase stability of MoSi2 in the C11b and C40 structures at high temperatures

The phase equilibrium of molybdenum disilicide in arc melt and plasma spray processed samples has been investigated in order to define the phase relationship between α-MoSi2 (C11b structure) and β-MoSi2 (C40 structure) phases. The analysis of solidification microstructures and X-ray diffraction patterns of arc melt processed MoSi2-based alloys indicates solidification takes place by primary α-MoSi2. Retained β-MoSi2 phase in plasma spray processed powders exhibits an irreversible transformation to α-MoSi2, at T≈1110 K, with an exothermic enthalpy of transformation of ΔHβ–α=0.9±0.1 kJ g-atom−1 atom during heating from room temperature at 0.5 K s−1. The Mo–Si and MoSi2–TiSi2 equilibria phase diagrams have been used to derive the upper bound of the metastable melting point of the β-MoSi2 phase Tβm≈2299 K, which is below the experimentally measured melting point of MoSi2 (Tm=2303 K). The phase stability analysis indicates that the β-MoSi2 phase is a metastable phase relative to α-MoSi2 at temperatures below the congruent melting point.