Topological model of type II deformation twinning in NiTi martensite

A topological model of the formation of type II twins in NiTi martensite is presented. Disconnection dipoles are generated initially and expand on the rational k1=(011) plane, subsequently accumulating to form a tilt wall perpendicular to k1. Equilibrium occurs when the strain and rotational distortion fields of the constituent defects are equally partitioned between the adjacent crystals. The resultant interface is parallel to the irrational conjugate plane, k2=(0.721,1,1¯), consistent with the classical geometric theory of deformation twinning and previously published experimental observations. It is proposed that the formation of type II twins on k2 occurs in this manner, rather than formation of type I twins on k1, because the disconnections have limited mobility on k1 even though they are readily nucleated on this plane. We support this conjecture by showing that the Burgers vector of the defects has a small magnitude, implying easy nucleation, but their motion along k1 is accompanied by complicated atomic shuffling.