Shape-memory behaviors in an aged Ni-rich TiNiHf high temperature shape-memory alloy

The shape-memory behaviors including both one-way and two-way shape-memory effects were investigated in a Ni-rich Ti29.4Ni50.6Hf20 alloy aged at 823 K for different hours. The precipitation of (Ti,Hf)3Ni4 particles strengthens the matrix strongly after the proper aging process, which should be responsible for the improvement of the shape-memory properties. Both one-way and two-way shape-memory strain increase rapidly with prolonging the aging time at the beginning of aging and then decrease with further increasing the aging time. The precipitates strengthen the matrix so that dislocations are hard to be introduced during the thermal cycling, resulting in the increase of the stability of the two-way shape-memory effect in the aged Ni-rich TiNiHf alloy. Therefore, it is feasible to use Ni-rich TiNiHf alloy as high temperature shape-memory alloy (SMA) through aging because the phase transformation temperatures are increased greatly to a high level and the shape-memory properties are also improved.