Analysis of Ti–Ni–Hf shape memory alloys by combinatorial nanocalorimetry

Abstract

The martensitic transformation in Ti-Ni-Hf thin films with ultra-fine grain structure has been analyzed as a function of composition using a high-throughput array of nanocalorimeters. The martensite-austenite transformation temperature is significantly lower than in bulk Ti-Ni-Hf, but increases linearly with Hf content at a rate comparable to bulk Ti-Ni-Hf. The response to high- temperature cycling (22oC < T < 850oC) changes with Ni concentration. For Ni ≤ 47 at%, the transformation temperature increases during high-temperature cycling because precipitation of (Ti1-x,Hfx)2Ni enriches the surrounding matrix in Hf; for Ni ≥ 47.7 at%, precipitation of the same phase gradually suppresses the transformation. Low-temperature cycling (22oC < T < 450oC) causes the transformation temperature to initially decrease and then stabilize. Relaxation of internal stresses by dislocations generated during thermal cycling is suggested as the active mechanism. Thermal cycling stability of the films is improved compared to previous studies on bulk Ti-Ni-Hf. This is attributed to the very small grain size (18 ± 5 nm) of the samples. Alloys with superior thermal cycling stability are identified and the ability to control the transformation temperature through multiple thermal cycling is demonstrated.