Effect of Ti/Ni and Hf/Zr ratio on the martensitic transformation behavior and shape memory effect of TiNiHfZr alloys

The functional properties of shape memory alloys are strongly sensitive to the composition of alloys. In this study, the effect of Ti/Ni ratio and Hf/Zr ratio on the martensitic transformation behavior, shape memory effect and microscopic structures of two different quaternary TiNiHfZr alloys (Ti_xHf₁₅Zr₅Ni_80-x and Ti_31.5Hf_yZr_20-yNi_48.5) have been investigated systematically. It is found that increasing Ti/Ni ratio of Ti_xHf₁₅Zr₅Ni_80-x alloys dramatically increases the martensitic transformation temperature and a maximum value (5%) of the recovered strain during shape recovery on heating is obtained for the alloy with x = 30. On the contrary, changing Hf/Zr ratio of Ti_31.5Hf_yZr_20-yNi_48.5 alloys does not result in big change of martensitic transformation temperature or recovered strain. The evolution of macroscopic properties of Ti_xHf₁₅Zr₅Ni_80-x alloys with x can be understood by considering the balanced effect between the Ni content change of the matrix, and the volume fraction of Ti₂Ni-like precipitates, which are often brittle and not desired for SMAs. Our results suggest that the Ti_xHf₁₅Zr₅Ni_80-x (x > 30.5 at. %) alloys are promising shape memory alloys for high temperature applications above 250 °C.