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PHASE STABILITY OF MoSi2 WITH Cr ADDITIONS

P. S. Frankwicz, J. H. Perepezko, and D. L. Anton*. Dept. of Materials Science and Engineering, University of Wisconsin-Madison, 1509 University Ave., Madison WI. 53706 USA; *United Technologies Research Center, East Hartford CT. 06108 USA. ABSTRACT The phase stability of MoSi 2 with Cr additions has been investigated in order to explore the issues of ternary solubility and structural stability of MoSi 2 . The solidification microstructure of MoSi 2 -rich alloys, along the MoSi 2 -CrSi 2 ternary section, displays a two phase mixture of primary MoSi 2 (C IIb) and intercellular ternary CrSi 2 (C40). The development of the phase equilbria between the C II b and C40 disilicides, as observed in this system, is characteristic of a broad class of intersilicide reactions involving MoSi 2 . The issues of chemical reactivity and structural stability of MoSi 2 composite designs underscores the importance of phase equilibria investigations. The solubility of Cr in annealed MoSi2 was observed to be on the order of 3 atomic percent. Past studies demonstrated that Ti and Ta have limited solution in MoSi 2 ; the

minor solubility of Cr inMoSi 2 corroborates the trend of limited solubility of transition metals

in the MoSi 2 (Cl lb) structure. The relatively small changes in the lattice parameters of MoSi 2 with Cr additions point to an inability of the C1 1b disilicide structure to accommodate the lattice perturbation resulting from solute atoms. The observations of this investigation suggest that the phase stability of MoSi 2 is primarily controlled by geometrical factors. INTRODUCTION Current investigations of high temperature MoSi 2 based intermetallic matrix composites are directed toward the development of multiphase designs with enhanced mechanical properties while maintaining the superior oxidation resistance of MoSi2 [1]. The viability of MoSi 2 based composites during high temperature service will depend on the critical understanding of interfacial reactions and intersilicide compound formation. To achieve this goal, the ternary phase equilibria and miscibility relationships, which govern the structure and chemical reactivity of MoSi2 in a multiphase environment, must be established. The phase reaction sequence of MoSi 2 with several transition metals (i.e. TM = Nb, Cr, Ti and Ta) involves the formation of ternary silicide complexes (Mo,TM)Si 2 and (Mo,TM) 5 Si 3 [2]. The ternary alloying of MoSi 2 with transition metal additions may add some degree of solid solution strengthening and effect microstructural toughing via methods for second phase precipitation within the MoSi 2 matrix. In the present study, the Mo-Cr-Si system has been chosen to investigate the solubility of Cr in MoSi 2 and the associated ternary phase equilibria in the vicinity of the MoSi2 -CrSi 2 section. The Mo-Cr Si ternary system has been investigated by Nowotny [3] and Ageev [4] and an isothermal section at 1300*C is shown in figure 1. Nowotny el at. has reported, based on the analysis of annealed (1300°C