The Effect of Applied Strain during Repeated Heat-treatment under Constrained Strain on The Mechanical and Shape Memory Properties of Ti-Ni Shape Memory Alloy

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The repeated heat-treatment under constrained strain is necessary for the manufacturing process of laser-cut Ti-Ni shape memory alloy (SMA) stent. In this research, the effect of applied strain during repeated heat-treatment under constrained strain on the mechanical and shape-memory properties of the Ti-Ni SMA wire was investigated. The chemical composition of the alloy is Ti-50.4at%Ni. At first, the specimen is heat-treated at 673 K for 3.6 ks. And then, heat-treatment under constrained strain, at 773 K for 0.3 ks. The applied strain at a single heat-treatment (εap) was 8 %, 10 % and 13.3 %, and the heat-treatment is repeated so as to achieve a total applied strain of 40 %. The critical stress for inducing martensite (σM) and minimum recovery stress (σmin) increases with increasing εap. However, σM of each specimen becomes constant during amplitude testing. In addition, σmin of εap=8 % and 10 % became 0 MPa during amplitude testing. Meanwhile, the σM of εap=13.3 % does not become 0 MPa during amplitude testing. Thus, the function deterioration is reduced by increasing εap. The reduction of function deterioration is caused by the increase of dislocation density with increasing εap.
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