Issues concerning the measurement of transformation temperatures of NiTi alloys

The transformation between martensite and austenite in shape memory alloy is characterized by four transformation temperatures: martensite start temperature (Ms), martensite finish temperature (Mf ), austenite start temperature (As) and austenite finish temperature (Af ). In actuator designs that rely on the shape memory effect, it is important to obtain an accurate measure of these transformation temperatures, especially As and Af , because they determine the functional temperature range of the actuator. Several methods of determining these temperatures have been reported, but their accuracy and coherence are not clear. The transformation temperatures of NiTi wire under different heat treatment conditions were measured by the three commonly used methods: differential scanning calorimetry (DSC); an electrical resistance method, which uses a sudden change in resistance as an indication of transformation; and an applied loading method, where a macroscopic displacement indicates the transformation. The results show that the transformation temperatures measured by DSC do not correspond to those measured by the other two methods, which are similar. The applied loading method is most effective for providing practical information about the stress-dependent transformation temperatures. The electrical resistance method gives clearly determined points for Ms and Mf in the cooling resistance–temperature curve, but As and Af are not clearly identifiable in the heating process