Martensitic transformation behavior and shape memory effect of an aged Ni-rich Ti-Ni-Hf high temperature shape memory alloy

The martensitic transformation behavior and shape memory effect (SME) have been investigated in a Ni-rich Ti_29.6Ni_50.4Hf₂₀ high temperature shape memory alloy (SMA) in the present study. After aging, the transformation temperatures of Ti_29.6Ni_50.4Hf₂₀ alloy increase obviously due to the precipitation of (Ti,Hf)₃Ni₄ particles. And the transformation sequence changes from one-step to two-step. When the experimental alloy is aged at different temperatures for 2h, the transformation temperatures increase rapidly with increasing the aging temperature and then change slightly with further increasing the aging temperature. Most of the martensite variants preferentially oriented in the aged Ti_29.6Ni_50.4Hf₂₀ alloy. The aged Ti_29.6Ni_50.4Hf₂₀ alloy shows the better thermal stability of transformation temperatures than the solution-treated one because the precipitates depress the introduction of defects during thermal cycling. In addition, the proper aged Ti_29.6Ni_50.4Hf₂₀ alloy also shows the larger SME than the solution-treated one since the precipitates strengthen the matrix strongly.