Tsunehiko Todoroki1 and Hirokazu Tamura2
1Air-conditioner Division, Matsushita Electric Industrial Co., Ltd., Kusatsu
The effects of the composition (49.7, 50.2 and 50.6 at%Ni) and heat treatment (673 K--933 K) on the shear stress dependence of shear strain vs. temperature relation of Ti-Ni alloy coil springs are investigated with constant bias loads. The resulting shear stress (τ )-transformation temperature (T) diagrams are analyzed in detail. The shear stress dependence of electrical resistance vs. temperature relation is also investigated. The results are summarized as follows.
(1) The maximum shear stress below which the R-phase exists increases as the Ni content increases and the heat treatment temperature decreases.
(2) The τ-T relation of the R → M transformation separates into two independent parts. One is in the low shear stress range, where the R-phase transformation strain increases with increasing shear stress, and the other lying above the former is in the shear stress range, where it remains almost constant. There is also a two-stage variation in the electrical resistance vs. temperature relation in the shear stress range, where the two parts coexist.
(3) From the practical view points, the shear stress dependence of the shear strain vs. temperature relation can be adopted as data sheets for design of coil dimensions and estimation of hysteresis loops. It is also possible to estimate the shear strain vs. temperature relation associated with phase transformations from the τ-T diagrams.
titanium-nickel alloy, shape-memory alloy, heat treatment, phase transformation, hysteresis loop, coil spring, bias load, electrical resistance
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