Influence of crystallographic compatibility on residual strain of TiNi based shape memory alloys during thermo-mechanical cycling

The primary focus of this study was to determine the influence of the inherent microstructure, specifically the crystallographic compatibility between martensite and austenite during the transformation process, on residual strain and strain evolution during isobaric thermal cycling. A series of predominantly single-phase Ti-rich TiNi based shape memory alloys (SMAs), including binary Ti50.1Ni49.9, two ternary TiNiPd25 compositions with slightly different Ni contents, Ti50.5Ni28.5Pt21, and a quaternary Ti50Ni24.5Pd25Sc0.5 alloy, were thermally cycled nominally 100 times at different stress levels. In all cases, the amount of residual strain per cycle was found to decrease asymptotically with increasing number of cycles to some finite value that was related to the compatibility of the transforming phases as determined by the middle eigenvalue of the transformation stretch tensor. In addition, the amount of residual strain generated during the early thermal cycles was also dependent on the resistance of the SMAs against defect propagation, which can be controlled by initial dislocation density and grain size among other factors. In light of these findings, it was concluded that a dimensionally stable SMA actuator for multiple cycle operation should have compatible transforming phases to reduce the generation of defects and a high resistance against defect propagation.