Cyclic Deformation of TiNi Shape Memory Alloys

An experiment with cyclic loading and unloading and an experiment with heating and cooling under the constant stress of TiNi shape memory alloys were carried out. The starting condition and the completing one of the martensitic transformation and the reverse transformation, the cyclic behavior of recovery strain and the conditions for application of recovery deformation were discussed. The results are summarized as follows. During the thermomechanical cycles, the yield stresses of the martensitic transformation and the reverse one decrease, but the transformation temperatures increase. The starting condition and the completing one of both transformations under the cyclic deformation are described by the behavior of the transformation lines in the stresstemperature plane. During the thermal cycles under constant stress, the recovery strain may appear or diminish depending on the relation among the initial strain, the heating temperature and the cooling temperature, thus showing complex cyclic deformation behavior. The cyclic recovery strain is represented by the relation among the heating temperature, the cooling temperature, the transformation lines and the transformation strain. It is important for the shape memory alloy elements to determine the conditions for application based on the cyclic characteristics of the transformation stress, the transformation strain and the transformation temperature.