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

The phase equilibrium of molybdenum disilicide in are melt and plasma spray processed samples has been investigated in order to define the phase relationship between alpha-MoSi2 (C11(b) structure) and beta-MoSi2 (C40 structure) phases. The analysis of solidification microstructures and X-ray diffraction patterns of are melt processed MoSi2-based alloys indicates solidification takes place by primary alpha-MoSi2. Retained Rho-MoSi2 phase in plasma spray processed powders exhibits an irreversible transformation to alpha-MoSi2, at T approximate to 1110 K, with an exothermic enthalpy of transformation of Delta Hbeta-alpha = 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 beta-MoSi2 phase T-m(beta) approximate to 2299 K, which is below the experimentally measured melting point of MoSi2 (T-m = 2303 K). The phase stability analysis indicates that the beta-MoSi2 phase is a metastable phase relative to alpha-MoSi2 at temperatures below the congruent melting point. (C) 1998 Elsevier Science S.A. All rights reserved.