Positron annihilation investigation of thermal cycling induced martensitic transformation in NiTi shape memory alloy

Thermal cycling of a Ni-excess NiTi alloy was conducted between 50 °C and liquid nitrogen temperature to induce martensitic transformations and to reverse them after. The starting point was an annealed and slowly cooled state, the end point a sample thermally cycled 1500 times. Positron annihilation lifetime spectra and Coincidence Doppler Broadening profiles were obtained in various states and at various temperatures. It was found that the initial state was low in defects with positron lifetimes close to that of bulk NiTi. Cycling lead to a continuous build-up of a defect structure up to 200−500 cycles after which saturation was reached. Two types of defects created during cycling were identified, namely pure dislocations and vacancies attached to dislocations.