Pattern formation during cubic to orthorhombic martensitic transformations in shape memory alloys

The superelasticity and shape memory effect of shape memory alloys originate from martensitic transformations (MT) that produce self-accommodated polydomain structures. In order to understand the formation mechanism of various domain patterns, a phase field model of cubic to orthorhombic MT in NiTiPt and NiTiCu is formulated, through which microstructure evolutions dominated by long-range elastic interactions are simulated. A rich variety of morphological patterns are predicted and analyzed. In particular, autocatalytic events leading to multi-variant “pyramidal triangular” and “nested triangular” configurations in NiTiCu are revealed. These morphological patterns are beyond the reach of the classical phenomenological theory of martensitic crystallography (PTMC). The simulation predictions agree well with experimental observations. By distinguishing deformation variant from orientation variant, the theoretical connection and distinction between PTMC and phase field microelasticity theory are discussed, and their predictions are compared with experimental observations.