Phase Transformation Temperature Behaviour of NiTi Alloys with Varied Ni Contents Prepared by Micro Metal Injection Moulding

Abstract

NiTi alloys are often used in biomedical applications. The alloys are required to be tailored for suitable operating temperatures. The phase transformation temperatures (PTT) were optimised for shape memory or super-elasticity properties. Micro metal injection moulding (micro-MIM) is a process that can fabricate millimetre-size metal parts with very complex shapes. It is adapted from MIM with significantly smaller injection volume for improved control of dimensions. NiTi alloys with varied Ni contents (45-55 at.%) were fabricated by micro-MIM and sintered at 1150 °C for 2 hours under high vacuum (< 10-4 Pa). X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDS) and differential scanning calorimeter (DSC) measurements were performed in order to characterise the sintered samples. The results indicated that as Ni contents increased, PTTs were decreased for all transformation temperatures (As, Ap, Af, Ms, Mp and Mf). Moreover, at room temperature (25°C), in the 50Ni-50Ti and 55Ni-45Ti samples, the austenitic B2 NiTi is present, whereas the 45Ni-55Ti sample contains martensitic B19¢ NiTi.