Influence of Hf/Zr Ratio on the Phase Transformation and Functional Properties of (NiCu)₅₀(TiHfZr)₅₀ High-Entropy Shape Memory Alloys

Abstract: Enhancing the shape memory behavior of NiTi-based alloys often relies on minimizing plastic deformation through alloying. High-entropy alloys (HEAs), characterized by their high hardness, resistance to plastic deformation, sluggish diffusion, and exceptional thermal stability, offer a promising pathway for developing high-entropy shape memory alloys (HESMAs). These materials have the potential to deliver superior ductility, thermal stability, and actuation performance under extreme conditions compared to conventional high-temperature NiTi-based shape memory alloys (SMAs).

Because the functional properties of SMAs are highly sensitive to composition, particularly the ratio of Ni-equivalent to Ti-equivalent elements, pseudo-equiatomic HESMAs are especially attractive for elevated-temperature applications. In this study, four pseudo-equiatomic (NiCu)₅₀(TiHfZr)₅₀ alloys were systematically investigated to elucidate the influence of the Hf/Zr ratio on their thermomechanical response. The results show that the martensitic start (Mₛ) temperature increases with increasing Hf/Zr ratio in (NiCu)₅₀(TiHfZr)₅₀ alloys. Furthermore, the Hf/Zr ratio markedly affects microstructural evolution and the attainable isobaric transformation strain, underscoring its critical role in tailoring the functional performance of HESMAs for elevated-temperature actuation.