Achieving high ductility and large recovery strain in the low (Hf,Zr) content of Ti_42.2Ni_49.8(Hf,Zr)₈ shape memory alloy through hot rolling

The effect of adding Zr content on the microstructure, martensitic transformation behavior, mechanical and strain recovery characteristics of Ti_42.2Ni_49.8Hf_8-xZr_x (x = 0, 1, 2, 3, 4 at. %) shape memory alloy prepared by hot rolling was investigated systematically. The results reveal that addition of Zr leads to slightly decrease in the transformation temperature and thermal hysteresis. The maximum recoverable strain of 7.79 % and a work output as high as 19.75 J cm⁻³ were attained during tensile thermal cycling. Moreover, the gradually increased Zr content could increase the yield strength and tensile fracture strain of the Ti-Ni-Hf-Zr alloy. The maximum tensile fracture strength of 1314 MPa and fracture strain of 26 % was obtained in Ti_42.2Ni_49.8Hf₅Zr₃ and Ti_42.2Ni_49.8Hf₄Zr₄ alloys, respectively. These distinctions on martensitic transformation behavior, mechanical as well as functional characteristics of the alloy were mainly related to the structural evolution of twinning mode, grain refinement and fracture mode by increasing of Zr content. In general, the shape memory effect and workability of the Ti-Ni-Hf alloy were significantly improved by combining Zr-doping and hot-rolling.