On the habit planes between elastically distorted austenite and martensite in NiTi

The cubic B2 to monoclinic B19' martensitic transformation in NiTi shape memory alloys proceeds by the nucleation and propagation of habit plane interfaces that remain undistorted and unrotated during the transformation. Due to incompatibility of austenite and martensite lattices in NiTi, habit plane interfaces are assumed to form between austenite and twinned martensite as described by Phenomenological Theory of Martensite Crystallography (PTMC). This view is currently widely spread in the literature in spite of the fact that it contradicts experimental observations of interfaces between austenite and single martensite lattice by TEM and EBSD methods frequently reported in the literature. On this account, we propose a different solution for the formation of strain compatible habit planes propagating during stress induced martensitic transformation by considering the effect of elastic deformation of both lattices caused by the external stress. Using modified PTMC theory, we evaluate the magnitudes of the uniaxial stress in tension and compression along a general austenite crystal direction, for which strain compatible habit plane interfaces can be established between austenite and single variant of martensite. The stress calculated in such a way is, however, too high. Nevertheless, considering the pre-transformation softening of austenite, the critical stress decreases to 500 MPa commonly observed in experiments. Three scenarios are simulated assuming softening of elastic constants of austenite. Orientation dependence and tension-compression asymmetry of predicted habit planes of stress induced martensitic transformation is compared with available experimental results.