Effect of Cold Rolling on Phase Transformation Temperatures of NiTi Shape Memory Alloy

The effect of cold rolling and heat treatment on the phase transformation behavior of NiTi shape memory alloy (SMA) heat treated at 660 A degrees C has been investigated. Four sets of samples were cold rolled after heat treatment. The austenite-to-martensite and martensite-to-austenite transformation temperatures for samples without any cold rolling are determined through differential scanning calorimetry (DSC). The austenitic start temperature gets shifted to the higher temperature side with increase in the percentage of the cold rolling up to 12.5%. Austenitic finish temperature could not be detected in cold-rolled samples. Martensitic start temperature increases slightly with increased cold rolling while martensitc finish temperature slightly decreases. Beyond 12.5% cold work, the shape memory effect (SME) is completely lost. The evolution of austenitic phase in SMA subjected to cold rolling was studied through powder x-ray diffraction (XRD) at different temperatures in the range 25 to 160 A degrees C at intervals of 10 A degrees C, during heating and cooling. The XRD results agree with those of DSC. Two sets of cold-rolled samples were again heat treated to 300 and 500 A degrees C and the transformation behavior was studied using DSC. Heat treatment at 300 A degrees C brings back the SME, but with the presence of an intermediate R-Phase due to the additional dislocations present. Even with a heat treatment at 500 A degrees C, the effect of cold work is not completely removed and a single-step transformation is not observed. Another set of samples subjected to cold work were heat treated at 660 A degrees C and the transformation is studied. The effect of cold work even up to 25% is completely removed with this heat treatment as indicated by DSC. The complete regaining of the SME is further confirmed by electrical resistivity measurements also.