Characteristics of Sputter-Deposited Ti-Ni-Cu Shape Memory Alloy Thin Films

Abstract

Ti-Ni shape memory alloys have been widely used in industrial and medical applications, but are disadvantaged by the large hysteresis resulting from by their low reactivity. While it has been reported that the addition of Cu in Ti-Ni binary alloys will decrease the hysteresis, Ti-Ni-Cu alloys with more than 13 at.% Cu concentration are very brittle and cannot be processed industrially. In this study, Ti-Ni-Cu thin films with more than 13 at.% Cu content were prepared by magnetron sputtering. The Cu content in the prepared films was changed from 15.2 at.% to 22.5 at.%, values that were determined by Energy-dispersive X-ray spectroscopy. X-ray diffraction measurements (XRD) were used to analyze the crystal structure of the thin films, while differential scanning calorimetry was employed to evaluate the hysteresis and phase transformation behavior associated with these films. To confirm the accurate phase transformation behavior, low temperature XRD measurements were performed within the temperature range of 343 K to 280 K. The superelastic characteristics and mechanical properties of the thin films were investigated by Dynamic Mechanical Analysis and a nanoindentation test. The Ti-Ni-Cu thin films exhibited single-stage transformation behavior (B2 (cubic)-B19 (orthorhombic)) during thermal cycling. The phase transformation hysteresis was confirmed to be smaller than that of the Ti-Ni binary thin film. The hardness and the superelasticity energy recovery ratio were found to decrease as the Cu concentration increased in the Ti-Ni-Cu thin films.