Effect of Repeated Heat-treatment for Expansion on Shape Memory and Mechanical Properties of Ti-50.4at%Ni Shape Memory Alloy for Stent

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In this research, the effect of heat-treatment under constrained condition on shape memory properties of Ti-Ni shape memory alloy was investigated. The chemical composition of the alloy is Ti-50.4 at%Ni. The specimen shape is wire with 1 mm diameter and 20 mm gage length. Tensile strain at single heat-treatment is varied from 4 to 8 %, and the total tensile strain by several times heat-treatment is 40 %. The heat-treatment condition is 773 K for 18 ks. The flexibility and expansibility of stent decrease with increasing tensile strain. In contrast, decrements of flexibility and expansibility during cyclic loading-unloading tests decrease with increasing tensile strain. These tendencies are caused by the difference of dislocation density due to the difference of the condition of the heat-treatment for expansion.
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