Effect of short-time annealing treatment on the superelastic behavior of cold drawn Ni-rich NiTi shape memory wires

The effect of short-time annealing treatment for 3, 9 and 18 s at 700 °C on the superelastic behavior of cold drawn Ni-rich NiTi wires was studied. Superelastic properties of cold drawn and annealed samples, including upper and lower plateau stresses, residual strain and mechanical hysteresis, were specified comparatively. The results show that for all cold drawn wires, annealing at 700 °C for only 3 s leads to the best superelastic behavior. Microstructure investigations were performed using optical microscopy and X-ray diffractometry. It was demonstrated that annealing at 700 °C for 3 s leads to production of an ultra fine grained structure. Also increasing the annealing time, only about a few seconds, results in occurrence of grain growth phenomenon which causes a sharp drop in superelastic behavior. Transformation behavior of the samples was specified by differential scanning calorimetry and R-phase was detected in ultra fine grained materials.

► Highly cold drawn NiTi wires were annealed at 700 °C for 3, 9 and 18 s. ► For all drawn wires, annealing for 3 s leads to the best superelastic behavior. ► Increasing the level of cold work improves superelasticity after annealing for 3 s. ► The rate of microstructure evolutions of cold drawn wires is very high at 700 °C. ► Increasing the annealing time from 3 to 18 s drops recovery strains drastically.