High-entropy shape-memory alloys (HESMAs) exhibit unique properties, including high yield strength, a wide temperature range of phase transformation, and excellent thermal cycling stability, among others. Hence, they have broad prospects for research and application. Currently, various new high-entropy shape-memory alloys have been developed based on TiNi, Fe-based, Ti-based, and NiMn-based alloys. This article focuses on summarizing and analyzing the research status of various types of high-entropy shape-memory alloys, including their microstructure characteristics, martensitic phase transformation, and functional properties. The advantages and disadvantages of high-entropy shape-memory alloys compared to traditional shape-memory alloys are briefly discussed. Finally, based on the analysis of existing problems, future research prospects are proposed, hoping to provide assistance for the development and application of high-entropy shape-memory alloys in the future.

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.

In the present study, various defects such as dislocations were controlled in Ti-V-Al-based shape memory alloy by thermomechanical treatment and introduction of interstitial oxygen (O) atom to optimize the performances. The results revealed that the Ti-V-Al-O shape memory alloys gradually evolved from α” martensite phase to the β parent phase with increasing annealing temperature. Moreover, the degree of lattice distortion can be tailored by changing annealing temperatures. Upon the annealing temperature reached 900 °C, masses of ω precipitates and a nano-sized ordered domain, characteristic of strain glass, can be found. As a result of suppression effect of multiple varieties of defects to the martensitic transformation, no obvious endothermic and exothermic peaks were observed in differential scanning calorimetry curves. The yield strength and maximum tensile fracture strength of the Ti-V-Al-O shape memory alloy increased with the increase of annealing temperatures. Meanwhile, Ti-V-Al-O shape memory alloys annealed at 900 °C possessed superior strain recovery characteristics and corrosion resistance. The excellent performances in Ti-V-Al-O shape memory alloys annealed at 900 °C can be attributed to the formation nanoscale nanodomain.

In the present study, hydrogenation treatment was adopted to tailor the phase constituents of the Ti-V-Al shape memory alloy, further optimizing its performances. It can be found that hydrogenation treatment induced the transition from the α″ martensite phase to the β parent phase. Moreover, large amounts of hydride precipitates can be observed in the hydrogenation treated Ti-V-Al shape memory alloy with longer time of 5h. Meanwhile, the grain size of the Ti-V-Al shape memory alloy was reduced as a result of hydrogenation treatment. The interstitial atom H serving as a β-stabilizing element led to the reduction of martensitic transformation temperature. In proportion, hydrogenation treatment caused the enhancement of yield strength and decrease of elastic modulus, which promoted its application in biomedical fields. Besides, by optimizing the time of hydrogenation treatment, the hydrogenation treated Ti-V-Al shape memory alloy with 1 h possessed the superior corrosion resistance.