The effect of ageing treatment on shape-setting and superelasticity of a nitinol stent

Ageing treatments were performed on Ti–50.7 at.% Ni (nitinol) stent wires. The results of shape-setting, austenite finish temperature and the non-linear mechanical properties of nitinol stent wires at body temperature were investigated. The experimental results show that the shape-setting results were stable when wires were constrain-treated at 500 and 550 °C and the ageing time was longer than 10 min. Austenite finish temperatures of the wires increased with ageing time, and decreased with ageing temperature after an initial increase. The peak value of the transition temperature was found to be 400 °C. For the same ageing temperature, both the upper and lower plateau stresses decreased slightly with ageing time. Most of the treated nitinol wires retained good recovery ability at body temperature; permanent sets were less than 0.05% when the ageing treatment was performed at 500 °C for not more than 60 min.