Controlling the phase transformation window during stages of hot/cold forging of Ni-rich Ni–Ti alloy

Abstract

A detailed microstructural analysis during the intermediate stages of fabrication of NiTi orthodontic archwire is carried out in this study. These microstructural findings were related to the phase transformation characteristics such as start and finish transformation temperature, thermal hysteresis, etc. The objective was to monitor the transformation window during the different stages of thermomechanical processing. The casted alloy was further subjected to combinations of hot and cold forging steps with intermediate annealing. Four different sample conditions were prepared. The microstructure development in these samples was studied through electron back-scattered diffraction and synchrotron radiation X-ray diffraction (SR-XRD) techniques. The phase transformation temperatures were determined by differential scanning calorimetric measurements. The microstructures showed grain boundary serrations, very large grains of austenite, twin-like features within austenite grains and mixed-phase distribution of austenite and martensite. The differences in microstructures were also clear in terms of local in-grain misorientation and grain boundary fractions. SR-XRD measurements further revealed possible precipitation of Ni4Ti3 and Ni3Ti. The martensite start temperature (Ms) was seen to be a clear function of high angle grain boundary fraction, while the finish temperature (Mf) showed an inverse trend. The transformation interval, Ms–Mf is related to the stored energy of austenite grains that determines the driving force to overcome the frictional work opposing the movement of the habit plane, while and Af –As largely depends on the elastic energy stored of the martensite plates during its growth. The hysteresis during reverse transformation (M → A) was related to the local in-grain misorientation.