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 degrees 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 tem-peratures. 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 20 0 -50 0 cycles after which saturation was reached. Two types of defects created during cycling were identified, namely pure dislo-cations and vacancies attached to dislocations. (c) 2021 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc.