Phase transformation and structural study on the severely plastic deformed Ni–Ti alloys

Abstract

In an effort to obtain nanostructures in bulk Nickel-Titanium (Ni–Ti) shape memory alloys (SMAs) and to carryout their characterisation, samples of the alloys with different compositions have been chosen and subjected to severe plastic deformation (SPD). The selected Ti-rich alloy in the as-received (AR) condition has martensite finish temperature (Mf) above room temperature (RT) and Ni-rich has austenite finish temperature (Af) below RT. SPD method is found to be one of the efficient methods to attain nanostructures. SPD of Ni–Ti alloys (Ti-rich and Ni-rich) have been performed by High Pressure Torsion (HPT) at RT. The changes in the phase transformation temperatures before and after SPD are analyzed by studying the Differential Scanning Calorimeter (DSC) plots. Further, the structural evolution of the samples subjected to SPD in the phase transformation temperature region is studied using in situ X-ray diffraction (XRD) from −180 to +180∘C. Heat treated Ti-rich NiTi samples, previously subjected to HPT, show the same transformation characteristics (transformation temperatures and transformation sequence) as those of the as-received ones. But, Ni-rich alloy shows distinct intermediate R-phase transformation, both while heating and cooling, after subjecting it to HPT followed by heat treatments.