Nanoprecipitates enhanced the yield strength and output work of (TiHfZr)50(NiCu)50 high-entropy shape memory alloys

Abstract

High entropy shape memory alloys (HESMAs) of (Ti40−xHf10Zrx)50(NiCu)50 (x = 0, 1, 5, 10, at%) were prepared by vacuum arc melting, solution treatment (800 °C for 30 min), and aging treatment (300 °C for 120 min for the solution-treated sample and 400 °C for 120 min for the 300 °C aged sample). The effects of Zr content and aging on the microstructure, mechanical performance, and superelasticity properties of HESMAs were investigated. The results show that uniform nanoscale phases are precipitated on the B2 austenite matrix of (TiHfZr)50(NiCu)50 HESMAs after solid solution treatment and aging treatment. Compared with the ternary TiNiCu and quaternary Zr0 alloys, the yield strength and critical strength of the stress-induced martensite transformation of the HESMAs improve significantly with increasing Zr content, and the nanoprecipitates further improve their mechanical performance. Due to the enhanced solid solution strengthening effect caused by the increase in Zr content and the dispersion strengthening effect of the nanoprecipitates, the (TiHfZr)50(NiCu)50 HESMAs show an excellent combination of high strength and large recoverable strain, which endows them with excellent output work. Among them, the Zr5 HESMAs have the best superelasticity properties, and their maximum recoverable strain and superelasticity strain are 8.8–9.7 % and 4.9–5.4 % in ten load cycles with a prestrain of 2–15 %, while the corresponding values of ternary TiNiCu alloys are only 2.6–4.7 % and 1.0–2.1 %, respectively. The output work of Zr5 HESMAs is 163.7–188.5 J/cm3, which is approximately 5–8 times larger than that of ternary TiNiCu alloys.