TWINNING IN DERIVATIVE STRUCTURES OF BCC AND FCC

In this paper, we briefly note some general rules governing the possible twinning modes of both cubic and non-cubic superlattice structures derived from disorded bcc or fcc solid solutions. The treatment is based on the Bilby-Crocker and Bevis-Crocker formulation of the theory of deformation twinning, and we make the simplifying assumption that the twinning shear must reproduce the superlattice structure in a new orientation without any atomic 'shuffles'. As an example, the orthorhombic Pt//2Mo type superlattice is considered which has space group Immm with six atoms in the body-centered orthorhombic unit cell.