Effect of Annealing on the Transformation Behavior of NiTi Shape Memory Alloy

Shape memory alloys (SMAs) have enormous potential for a wide variety of applications. Therefore there has been a wide concern for studying the properties of those alloys. In this study polycrystalline alloys from shape memory alloy have been prepared successfully by thermal plasma spray in a wire form of 3 mm diameter in a graphite cast with three different ratios Ni50Ti50, Ni50.5Ti49.5 and Ni51Ti49. Its structure has been tested by means of X-ray diffraction technique. It has been noticed that annealing at 500° C helps to transform part of the material from an austenite phase into a martensite phase .It has also been noticed that increasing Ni proportion more than 50% to Ti in the alloy increases the martensite phase. The temperature of phase transformation as a function to the temperature of NiTi was calculated by using Differential Scanning Calorimeter (DSC) at a temperature between 100°C and -80.0°C in a scan rate of 10°C/min. The charts show that transformation from an austenite phase into martensite phase is done through an intermediate phase called (R-phase) which means there is no direct transformation, while transformation from martensite into austenite in heating cycle is direct. An exception to that is annealing Ni50Ti50 at a temperature of 400°C where the transformation cycle is in two stages.