Superelastic Behavior under Cyclic Loading for Coil Spring of Ti-Ni Shape Memory Alloy

The superelastic behavior of coil springs of a Ti-Ni shape memory alloy was investigated using loading-unloading cycling tests under isothemal temperatures. The effects of the Ni content and shape memory treatment temperatures on the cyclic behavior in the superelastic deformation were investigated. The deformation behavior within the elastic region of the parent phase scarcely changes during tensile cyclic loading. However, when a martensitic phase is induced by the stress, the deformation behavior changes considerably during cyclic loading. The critical shear stress for inducing martensites and the dissipated strain energy per cycle decrease with an increase in the number of cyclic loadings. Such changes in the superelastic behavior under cyclic loading become large with a decrease in the Ni content of the Ti-Ni shape memory alloy and the increase in the shape memory treatment temperature.