Microstructural evolution kinetics after plastic deformation of equiatomic Ti–Ni alloy during isothermal annealings

Ti–Ni shape memory alloys are very sensitive to thermomechanical treatments (cold working and annealing). However, these treatments, already used and described in the literature have not been studied systematically. To identify and to understand the phenomena which occur during these treatments in the equiatomic Ti–Ni alloys, a study was carried out for an extensive range of temperatures (538–778 K) and annealing times (10 s–123 days) after cold working at room temperature. Thanks to the sensitivity of the thermoelectric power which has not yet been used for this kind of alloy, quantitative results for the annealing kinetics have been obtained. Two annealing stages have been observed and characterized with activation energies of 2.7 and 3.6 eV. Transmission electron microscopy allowed to identify these stages with phenomena of ‘recovery’ and ‘recrystallization’ characterized by special features due to interaction with the martensitic transformation itself.