On the nature of faults in MoSi2

Abstract

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 10⁶cmh ⁻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 → C1 1b 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 ¼〈111〉 lying on (110), have been reinterpreted as ⅙[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.