On the NiTi SMA Thermal Behavior at Various Heat Treatment Conditions

In this paper, the relationship between the heat treatment temperature and the phase transformation behavior in the Ni-rich NiTi Shape Memory Alloy (SMA) was experimentally investigated. The characterization of the thermoelastic martensitic transformation at zero stress was performed based on differential scanning calorimetry (DSC) experiments. It was shown that the R phase behavior is extremely affected by the increase of the heat treatment temperature. For the samples heat treated below 650 °C, the R phase appears in both forward and reverse transformations. However, it completely disappears above this heat treatment temperature. Moreover, it was reported that the NiTi thermal behavior and mainly the transformation temperatures are strongly influenced by the heat treatment temperature change. By increasing this parameter, the austenitic finish temperature (A_f) is considerably decreased while the martensitic one (M_f) is increased which greatly affects the thermal transformation hysteresis. Accordingly, the effect of the this thermal behavior on the NiTi diagram phase was discussed. The obtained findings were largely in line with previously published studies.