Role of stacking faults in solid state transformations

This article illustrates the two different roles played by stacking faults in solid state transformations viz. (i) in accommodating part of the transformation strains as observed in the noble metal-based alloys undergoing martensitic transformations, and (ii) in providing a mechanism for changing the stacking sequence of layers in a variety of materials like SiC, ZnS, Co and its alloys, and certain steels. Diffraction patterns taken from the martensitic phases of noble-metal-based alloys as well as from SiC and ZnS crystals undergoing transformation from one close-packed modification to another reveal the presence of characteristic diffuse streaks. It is shown that from a theoretical analysis of the observed intensity distribution along streaked reciprocal lattice rows in terms of physically plausible models for the geometry and distribution of faults, one can make a choice between various possible routes for transformation. From simple computer simulation studies, it is shown that the observed arrest of transformations in SiC is essentially due to the insertion of stacking faults in a random space and time sequence leading to an irregular distribution of solitons.