ON THE NATURE OF FAULTS IN MOSI2

Experimental evidence of stacking faults in MoSi2, processed over a range of solidification velocities, is presented. Shear-type faults are observed on {110) planes in plasma-sprayed polycrystalline specimens (grown at a rate of 10(6) cm h (-1)) that are thermally unstable and disappear upon heat treatment. Condensation faults on (001) planes, probably associated with silicon loss during processing, are observed in single crystals (grown at rates of 1-30 cm h (-1)) and are thermally stable. Faults (other than twins) are nominally present in materials undergoing a melting operation, and the possible role of these faults in C40 --> C11(b) and MoSi2 --> Mo5Si3 phase transformations is elucidated. Prior experimental evidence for the transformation of C40-MoSi2 (high temperature phase) to C11(b)-MoSi2 (low temperature phase) has been re-evaluated such that the fault character, and in particular the existence of the C40 structure, is questioned. Thus, transformation-induced stacking faults, originally described as 1/4[111] lying on {110), have been reinterpreted as 1/6[001] condensation faults on (001). The possibility of these faults contributing to strain (under the action of diffusion-assisted mechanisms) and strain-hardening (by crystallographic glide processes) is elucidated.