Some Factors Affecting the Transformation Hysteresis in Shape Memory Alloys, pp. 361-369
Authors: Yong Liu
Abstract: Transformation hysteresis is an important property of shape memory alloys (SMAs) which requires careful consideration during the materials selection for targeted technical applications. When being used for repeated active actuation purposes (for example in MEMS, morphing wing of aerial vehicles, robotics, etc.) a small hysteresis is required since it strongly influences the operational frequency achievable and the power consumption needed for actuation. On the other hand, some applications may require very large transformation hysteresis in order to maintain a stable austenite phase within a large temperature range for retaining the predefined shape (for example in deployable space structures, pipe joining, etc). In order to obtain SMAs with either very low or very high hysteresis, a fundamental understanding on its influencing factors is ultimately important. This short communication highlights several recent results on the factors affecting the transformation hysteresis in shape memory alloys. This aricle is not aimed to provide a complete spectrum on the transformation hysteresis of shape memory alloys. It is hoped to encourage further investigation to understand the fundamental factors controlling the transformation hysteresis, thus enabling an effective alloy design. It is well known that the thermoelastic phase transformation is associated with a local balance between chemical and non-chemical forces [1,2]. Chemical force arises from the difference in Gibbs free energies between austenite and martensite and acts as a driving force promoting the phase with lower energy at each temperature. Non-chemical force arises from two major contributions; one is the elastic energy to accommodate the shape and volume change during transformation and the other is the energy dissipated during phase transformation. The elastic energy stored during forward transformation is released upon reverse transformation acting as driving force, thus does not contribute to the transformation hysteresis. It can be demonstrated that the elastic energy is responsible for the transformation interval. The energy dissipation due to defect formation is irreversible and contributes to the transformation hysteresis. Thus, factors contributing to the frictional forces against structural phase transformation will contribute to the transformation hysteresis. In the following, several observations/investigations on the factors affecting the transformation hysteresis are briefly discussed namely, atomic radius of the alloying elements, lattice mismatch between martensite and austenite, substrate-induced stress in bimorph, and effect of precipitation.