X-ray Diffraction Investigations of Shape Memory NiTi Wire

Outstanding properties of nitinol, known as shape memory and superelasticity, make them suitable alternatives in several biomedical, aerospace, and civil applications. For instance, nitinol wires have been used as the actuator components in many innovative medical devices aiming to make surgical tasks less invasive and more efficient. In most of these applications, it is desired to have a consistent strain response of nitinol wires; therefore, it is necessary to investigate the internal phase transformations from microstructural point of view. In this study, the effect of influencing factors such as biased stress during thermal cycle, the maximum temperature wires experienced during heating part of thermal cycle, and also wire diameters on the amount of unrecovered strain occurred between the first and the second thermal cycles has been investigated. The generation of different phase compositions in the same thermomechanical condition for different wire diameters has been discussed using x-ray diffraction (XRD) method. The location and intensity of characteristic peaks were studied prior and after the loading cycles. It was observed that nitinol wires of diameters less than 0.19 mm exhibit unrecovered strain while heated to the range of 70-80 °C in a thermal cycle, whereas no unrecovered strain was found in wires with larger diameter. The observation was supported by the XRD patterns where the formation of R-phase instead of martensite was shown in wire diameters of less than 0.19 mm after cooling back to room temperature.