Multi-stage transformation in annealed Ni-rich Ti49Ni41Cu10 shape memory alloy

Multi-stage transformation (MST) in 500 °C annealed Ni-rich Ti49Ni41Cu10 shape memory alloy (SMA) is investigated by differential scanning calorimetry (DSC), dynamic mechanical analyzer (DMA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The as solution-treated alloy undergoes B2 ↔ B19 ↔ B19′ two-stage transformations. Ti(Ni,Cu)2 precipitates are formed in 500 °C annealed specimens. Alloy annealed at 500 °C for 6–24 h exhibits MST. This MST is confirmed by DMA tests and is composed of B21 ↔ B191 ↔ B19′1 and B22 ↔ B192 ↔ B19′2 transformations corresponding to the regions near and far from Ti(Ni,Cu)2 precipitates, respectively. Experimental results show that the more the annealing time, the more the B21 ↔ B191 ↔ B19′1 transformations and finally only B21 ↔ B191 ↔ B19′1 transformations retain with the transformation temperatures close to those of Ti50Ni40Cu10 SMA.