Mechanisms for phase transformation induced slip in shape memory alloy micro-crystals

Compression of a single crystal, superelastic NiTi shape memory alloy (SMA) micro-pillar and the stress-field around an ellipsoidal twinned martensite (M) plate embedded in an austenite (A) matrix were simulated using a coupled phase transformation and crystal plasticity model. Post-mortem transmission electron microscopy (TEM) analysis of the dislocation structures in a foil extracted from a compressed NiTi micro-pillar was also performed. Based on these modeling and experimental data, we propose mechanisms for phase-transformation-induced defect generation in superelastically stressed NiTi SMA. The geometry of the simulated slip bands shows that dislocations nucleate and grow in the austenite phase adjacent to a growing or receding martensite plate to accommodate local strain gradients. The simulated resolved shear stress on individual slip systems, and Burgers vector analysis of dislocations in the TEM data show that the slip system and amount of slip activity depend on the magnitude of the strain gradients, which are controlled by the martensite crystallography, the dynamics of martensite plate growth, and scale of the twinned structure and A-M interface. In addition to a[0 1 0] (1 0 1 ¯ ) and a [0 0 1] ( 1 ¯ 1 0) slip systems observed in prior experiments, we report the activation of a third slip system: a[0 0 1](1 1 0). We show that the three slip systems are likely to be active at different locations around a martensite plate. The modeling component in this work complements ex-situ TEM characterization by furnishing the resolved shear stress and slip activity on austenite slip systems throughout the cyclic loading.