Effect of surface oxidation on detwinning stress and transformation temperature of Ti-50Ni shape memory alloy

It was reported that the detwinning stress in martensite of Ti-Ni alloys increases with increasing annealing temperature, but the reason for such a strange behavior has remained unclear. In the present study, we made a comparative study between Ti-50Ni samples annealed in air and those in argon atmospheres. We found that samples annealed in air showed a significant increase in the detwinning stress with increasing annealing temperature, but they showed little change in M_s temperature. When the samples were annealed in argon, the detwinning stress and M_s became independent of the annealing temperature. We conclude that the increase of detwinning stress with annealing temperature results from the surface hardening of the sample due to surface oxidation; the insensitivity of M_s to oxidation is due to the fact that surface oxidation does not affect the core of the sample, where transformation takes place first. Our result shows the importance of preventing surface oxidation in obtaining proper properties for this important shape memory alloy.