Martensitic transformation and shape memory effect of Ti–49 at.%Ni alloys

For binary TiNi shape memory alloys, most research work has been devoted to the equiatomic or Ni-rich TiNi alloys, while very few systematical researches are reported on Ti-rich TiNi alloys. In this paper, the martensitic transformation and shape memory effect of Ti-rich Ti–49 at.%Ni alloy were investigated by differential scanning calorimetry, optical microscopy, X-ray diffraction and testing of shape memory in a self-fabricated device. Ti–49 at.%Ni alloy specimens were annealed at 850 °C for 2 h and then cooled in a furnace or in water. The results show that the microstructure of Ti–49 at.%Ni alloys at room temperature consists of three phases, martensite phase B19′ and precipitate phase Ti2Ni, as well as a small amount of retained parent phase B2. The difference of cooling conditions from annealing temperature 850 °C does not affect the martensitic transformation temperatures and microstructure of Ti–49 at.%Ni alloy. However, furnace cooling contributes to better shape recovery of Ti–49 at.%Ni alloy compared with water quenching. With increasing pre-strain, both one-way and two-way shape recovery of Ti–49 at.%Ni alloy decreases gradually. After 0.38% pre-strain deformation at −30 °C, the Ti–49 at.%Ni specimen exhibits 100% one-way shape recovery upon heating to 180 °C.