Effects of aging and cooling rate on the transformation of nanostructured Ti-50.8Ni

Cooling rate effect becomes critical for structural applications of Nitinol with thick sections since its influence on the multiple-step martensitic (MST) transformations and hence on the transformation temperatures is significant. In the present work, it is shown that the cooling rates indeed have sufficient impact not only on the transformation temperatures, but also on the transformation sequence of various phases involved in the martensitic transformations. The evolution of the martensitic transformation was accomplished through a series of partial transformation cycles using differential scanning calorimetry (DSC) methods. These studies revealed that the alloy undergoes a transition from Mechanism 1 described as {(A → R) + (R → M1) + (R → M2) || (M2 → R) + (M1 → R) + (R → A)} for aging at T ≤ 500 °C, 1-hr; to Mechanism 2 described as: {(A → R)Volume1 + (A → M2)Volume2 + (R → M1)Volume1 || (M2 → A)Volume2 + (M1 → A)Volume1} for aging at T > 500 °C, 1-hrs. A clear transition in alloy aged at 500 °C for 1 h was captured where both these mechanisms were operative.