Martensitic Transformation and Response Characteristics in Bias-SMA Spring Actuators with Thermal Cycling

Abstract

The effect of thermal cycling on the martensitic transformation and response characteristics has been studied in bias-SMA spring actuators aged at 350 °C ~ 500 °C. As the number of thermal cycling increases, the Ms temperature of the SMA spring showing only B2→B19' martensitic transformation by aging at 350 or 500 °C goes down. However, SMA spring in which B2→R→B19' martensitic transformation taken place by aging at 400 or 450 °C shows multi-step transformations i.e., M1 and M2 transformations with thermal cycling. The heat flow of M2 transformation increases while the transformation temperature difference between M1 and M2 transformations is enlarged as the number of cycling increases. The recovery displacement and force of bias-SMA spring actuator increase up to the 10th cycle and the extent of increase is the largest at aging temperature of 500 °C. However, both recovery properties decrease after the 103 or 104 cycles, while the decreasing tendency becomes larger with a rise in aging temperature. Such a change in recovery characteristics of bias-SMA spring actuator with thermal cycling is discussed in connection with training effect or lattice defects introduced during thermal cycling.