| Summary: | 碩士 === 國立臺灣大學 === 機械工程學研究所 === 107 === In this study, the effects of thermal/stress cycles on transformation temperature, the effect of stress cycles on elastocaloric effect and the elastocaloric effect at different operating temperatures in Ti49.6Ni50.4 bar, Ti48.9Ni51.1, and Ti50Ni48Fe2 shape memory alloy blocks were investigated. The Ti49.6Ni50.4_275℃×125 hr and TiNi51.1_400 ℃×50 hr aged specimens exhibited the best thermal/stress cycles stability and elastocaloric effect performance, because of the strengthing effect of Ti3Ni4 precipitations formed during aging treatment. The precipitates could effectively hinder the dislocation movement during thermal/stress cycles and improved the functional stability of the material. The Ti49.6Ni50.4_275℃×125 hr and TiNi51.1_400 ℃×50 hr aged specimens maintained a high-temperature change during the stress cycles, indicating that they could contribute better superelasticity after precipitation hardening. However, it is noted from the results of thermal cycles and stress cycles that, although precipitation hardening could effectively improve the thermal cycles stability of TiNi-base shape memory alloys, the phase transition temperatures obviously shifted during the stress cycles. This features indicated that the precipitation hardening could improve the functional stability during thermal cycling but was less effective to resist dislocation motion during stress cycling. As a consequence, the effect of stress on the deterioration of the transformation behavior of shape memory alloy was found greater than the effect of thermal cycling. The Ti49.6Ni50.4_275℃×125 hr and TiNi51.1_400 ℃×50 hr aged specimens also showed better elastocaloric performance. At the temperature range of -40 °C to 150 °C, these two samples showed higher temperature change of cooling during unloading than the other specimens, the temperature change up to 10 ℃~12 ℃. In addition, it was found that the operating temperature range could be expanded by introducing R phase transformation, even if the operating temperature was lower than the Rf, the phase transformation between R↔B19'' could be utilized to provide a temperature change, indicating that the two-stage phase transition of B2↔R↔B19'' can effectively extend the operating temperature range of the elastocaloric effect. In this study, by utilizing digital image correlation(DIC) and infrared image analyses, the strain field distribution and temperature distribution of TiNi-based shape memory alloy superelastic can be directly observed and measured, and all specimens ware found to undergo uniform phase transformation.
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