Detwinning of preloaded martensite in shape memory alloys and its effect on the cyclic behavior of NiTi cylindrical actuators

Abstract

A multi-mechanism material model is used to investigate the effect of the degree of martensitic detwinning/reorientation occurring at the end of mechanical preloading, on the performance of shape memory tubular and solid cylindrical actuators. A high-temperature ternary Ni50.3Ti29.7Hf20 alloy, showing almost complete transformation behavior, and an ordinary binary Ni49.9Ti50.1 material, depicting incomplete martensitic transformation, are used. The results indicated a clear correlation between the shape memory cyclic actuation behavior and the martensitic deformation character of the selected alloy. More specifically, while the actuation strokes produced by the Ni50.3Ti29.7Hf20 systems consistently followed a direct pattern for the different geometries and torque magnitudes, the results for the Ni49.9Ti50.1 cases indicated a behavior that is counterintuitive to what may be expected under pure mechanical loading conditions. In particular, when subjected to the same, high preload torques, Ni49.9Ti50.1 solid actuator under lesser stresses generated higher twist strokes than a tubular counterpart experiencing higher stresses.