Study of the effect of thermal cycling under constant stress on R-phase in NiTi shape memory alloy

Electrical resistivity and Strain recovery measurements have been used for the study of the stability of R-phase in NiTi shape memory alloy upon thermal cycling under a constant tensile stress of 100 MPa. Two samples are chosen for the study of which one sample heat-treated at 560°C exhibits a pure martensitic phase at ambient temperature and the other heat-treated at 380°C consists of a mixture of R-phase and martensitic phase at ambient temperature. In the case of the sample heat-treated at 380°C, under the applied stress of 100 MPa, heating part of the cycle also shows the presence of R-phase unlike the case of stress free condition where only cooling part exhibits the R-phase. For the sample heat-treated at 560°C, the thermal cycling under tensile stress of 100 MPa initiates R-phase in the first cycle itself whereas under stress free condition it requires about 15 cycles to initiate R-phase. The sample heat-treated at 560°C exhibits more recoverable strain in the initial cycles than that of the sample heat-treated at 380°C, but after large number of thermal cycles of the order of 1000 the recoverable strain in both samples is found to be almost the same. The sample heat-treated at 380°C is found to be more stable against plastic deformation with thermal cycling and hence can be preferred over the sample heat-treated at 560°C for the two-way applications of SMA.