Phase transformation and precipitation in aged Ti–Ni–Hf high-temperature shape memory alloys

More attention has been paid to ternary Ti–Ni–Hf high-temperature shape memory alloys (SMAs) due to their high phase transformation temperatures, good thermal stability and low cost. However, the Ti–Ni–Hf alloys have been found to have low ductility and only about 3% shape memory effect and these have hampered their applications. It is well known that there are three methods to improve the shape memory properties of high-temperature SMAs: (a) cold rolling + annealing; (b) adding another element to the alloy; (c) aging. These methods are not suitable to improve the properties of Ti–Ni–Hf alloys. In this paper, a method of conditioning Ni-rich Ti–Ni–Hf alloys as high-temperature SMAs by aging is presented. For Ni-rich Ti80−xNixHf20 alloys (numbers indicate at.%) the phase transformation temperatures are on average increased by more than 100 K by aging at 823 K for 2 h. Especially for those alloys with Ni contents less than 50.6 at.%, the martensitic transformation start temperatures (Ms) are higher than 473 K after aging. Transmission electron microscopy shows the presence of (Ti + Hf)3Ni4 precipitates after aging. Compared with the precipitation of Ti3Ni4 particles in Ni-rich Ti–Ni alloys, the precipitation of (Ti + Hf)3Ni4 particles in Ni-rich Ti–Ni–Hf alloys needs higher temperatures and longer times.