Annealing-induced evolution of transformation characteristics in TiNi shape memory alloys

The effect of annealing on transformation characteristics of TiNi shape memory alloys (SMAs) was investigated by differential scanning calorimetry (DSC) and the evolution of the microstructure was studied using positron annihilation technology (PAT) and transmission electron microscopy (TEM). The results showed that transformation characteristics depend on the annealing temperature. The R-phase transformation appeared at low annealing temperature. The R-phase disappeared and austensite transformed into martensite directly as the annealing temperature exceeded 550 °C. With increasing annealing temperature, the vacancy cluster and dislocation related positron lifetime decreased. Changes in transformation characteristics can be attributed to the evolution of the microstructrue of the TiNi specimen.