Effect of thermal cycling on R-phase stability in a NiTi shape memory alloy

The stability of R-phase during thermal cycling has been studied for the near equiatomic, 40% cold worked, NiTi wire samples using electrical resistivity measurements. Thermal cycles are given to the specimens heat treated in the temperature region of 340–620 °C. In the samples having only M↔A transformations after heat treatment, successive thermal cycles promote and result in the stabilisation of intermediate R-phase. During the initial thermal cycles, Ms is found to decrease facilitating the development of intermediate R-phase as indicated by the emergence of peak in the cooling part of the transformation profile. Once the R-phase is completely developed, the transformation parameters such as Rs, Rf, Ms, Mf and peak resistivity of the transformation profile remains unaffected during thermal cycling. There exists a critical number of thermal cycles for the complete development of R-phase and this critical number is a function of heat treatment temperature.