Thermal Cycling Induced Instability of Martensitic Transformation and the Micro-Mechanism in Solution-Treated Ni51Ti49 Alloy

The effect of the maximum temperature of thermal cycling (Tmax) on the instability of martensitic transformation (MT) in the solution-treated Ni51Ti49 alloy was investigated by differential scanning calorimetry (DSC). Results manifest that the peak temperature of martensitic transformation (Mp) decreases linearly with the increase of cycle number, while the transformation hysteresis (H) increases linearly. The instability of MT is promoted by increasing Tmax from 20 to 100 °C, with variation of Mp increasing from 0.9 to 12.3 °C and variation of H increasing from 0.4 to 4.3 °C after 10 thermal cycles. Transmission electron microscopy (TEM) study demonstrates the appearance of transformation-induced dislocations in the NiTi matrix, which are responsible for the instability of MT. Moreover, the dislocation multiplication is obviously enhanced with the increase of Tmax during thermal cycling, as a result of the interaction between dislocations and quenched-in point defects (QIDs) in the solution-treated Ni-rich Ni51Ti49 alloy, which consequently leads to the temperature dependence of MT instability during thermal cycling.