Phase Transformation in Ni-Ti Shape Memory and Superelastic Alloys Subjected to High Pressure Torsion

Abstract

A systematic study on the phase transformation of Ni-Ti shape memory and superelastic alloys subjected to Severe Plastic Deformation (SPD) – High Pressure Torsion (HPT) technique has been carried out. Ni-Ti alloys of three compositions were chosen for the study. Specimens of these alloys in as-received (AR) condition and after HPT have been subjected to Differential Scanning Calorimetry (DSC) and X-ray Diffraction (XRD) analyses. In this study, while comparing the results of DSC thermograms and XRD spectra for the same sample conditions, some differences were observed. In the case of NiTi-H alloy after HPT, there appeared one stage phase transformation with DSC both while heating and cooling suggesting Martensite↔Austenite transformation but, with respect to XRD spectra while cooling, at the intermediate temperature of 55°C, the R-phase peaks corresponding to (1 1 2)R and (3 0 0)R planes appeared. In the thermogram obtained for the NiTi-B alloy subjected to HPT, it is observed that, while cooling, the Austenite to R-phase transformation is merged with R-phase to Martensite transformation. The results of the XRD obtained at -180°C show the presence of R-phase along with M-phase. The DSC curve of the NiTi-S alloy subjected to HPT corresponds to one stage phase transformation both while heating and cooling but, the diffractogram of the sample obtained at -180°C corresponds to the presence of both R-phase and M-phase.