The microstructure of Ti-Ni-Cu shape memory thin films: a review

Ti-Ni-Cu shape memory thin films have attracted more attention as a result of the perfect combination of various advantages such as narrower thermal hysteresis, superior thermal cycling stability, higher strength and long-term functional fatigue. The prominent functional properties were closely related to their specific crystal structure and microstructural features. In the present paper, the microstructure in equiatomic, Ti-rich and (Ni,Cu)-rich shape memory thin films were reviewed. In addition, the corresponding results including martensitic transformation, microstructure and strain recovery characteristics of Ti-Ni-Cu thin films without precipitates and containing different types of precipitates were summarised. Besides, the mechanisms for obtaining the outstanding functional properties are discussed and reviewed, which can offer the theoretical basis to the design of Ti-Ni-Cu shape memory alloy thin film micro actuators with excellent performances.