Variation in the shape recovery temperature in NiTi alloys

Abstract

In the near-equiatomic NiTi alloy, certain processing factors significantly affect the shape recovery temperature T f and the extent of shape recovery while others do not. The lowest recovery temperature and the best shape recovery were obtained by annealing between 450 and 500 °C. Varying the annealing temperature resulted in the largest change in these two parameters, causing an increase of 20 °C in T f and a decrease of 13% in the shape recovery (in one sample batch) if the annealing temperature was increased much above 500 °C. Increasing the maximum strain (in the range between 0% and 8%) induced an increase of 10 °C in T f at 500 °C with larger increases at other annealing temperatures but without any effect on the extent of shape recovery. Altering the annealing time had a relatively small effect. A recovery-resistant physical constraint raised the effective T f but if the constraint was maintained at a sufficiently high temperature the transformation occurred, plastic deformation resulted and permanent partial loss of shape memory was suffered. T f extrapolates to nearly the same temperature, at zero strain, for all the annealing temperatures near 500 °C. Similarly, the final temperature of the transformation from matensite to β phase (zero strain) is the same for all the annealing temperatures. This strongly suggests that each annealing temperature yields a high temperature B2 phase with a slightly different structure which interacts differently with stress. Although this study is based on only two batches of NiTi wires of similar composition, it is expected that qualitatively similar variations with processing factors occur in all NiTi alloys which exhibit shape memory.