Effects of Heating and Cooling Cycle on the Shape Memory and Mechanical Characteristics of Tape-shaped Shape Memory Alloy Element

A pulley-type SMA heat-engine using Ti-Ni shape memory alloy (SMA) driven by lowtemperature waste heat energy was investigated. We have proposed and fabricated a pulley-type heat-engine using tape-shaped SMA element for the purpose of improving of the life span of pulley-type SMA heat-engine. However, shape memory and mechanical characteristics of SMA deteriorate with increasing number of thermal cycles. Therefore, effects of thermal cycling on the shape memory and mechanical characteristics of the tape-shaped SMA element are investigated in this study. The chemical composition of the specimen is Ti-45.0Ni-5.0Cu (at%), heat treatment condition is 673K for 3.6ks and the specimen is memorized horizontal shape. The number of thermal cycling (N) is varied from 0 to 105. Transformation temperatures are almost constant as the N increases to 1×103. However, each difference of transformation temperature (Ms-Mf and AsAf) decrease when the N is within a range of 5×103 to 1×104, above which it increases. Mechanical properties improve with increase of N when the N is up to 1×104, above which it deteriorates. Furthermore, 2-stage martensitic transformation appears when N is above 5×104. These tendencies are caused by the variation of dislocation density and residual martensitic phase with the increasing of N.