THE EFFECT OF STRAIN-RATE ON DETWINNING AND SUPERELASTIC BEHAVIOR OF NITI SHAPE-MEMORY ALLOYS

The effect of strain rate on the superelastic and detwinning behavior of shape memory alloys was studied in a series of experiments. The str ain rate was tested in a range from 10(-4) to 10(-2)/s, To ensure a st able and uniform temperature throughout the testing process, fine wire samples were used, and all tests were conducted in a liquid environme nt. Under these conditions, rest results showed that the strain rate h ad no influence on the superelastic behavior and martensite detwinning of the NiTi alloys. In the superelastic deformation process, the aust enite reversible arrow martensite transformation produced rather high latent hear as part of the transformation process within the specimen. As the strain rate increased, and as conditions became more adiabatic , a significant temperature change due to the release and absorption o f latent heat occurred. The temperature rises upon loading and drops u pon unloading. This phenomenon manifests itself, as the diameter of th e specimens becomes progressively larger, or as lest conditions become more adiabatic.