| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 106 === Ti51.9Ni48Si0.1 and Ti52.6Ni46.8Si0.6 SMAs’ ribbons prepared by rapid solidification process(RSP) are aged at 500℃ for different times to investigate their transformation sequence, microstructure, precipitation behavior, mechanical properties, etc. As-spun ribbons have fine grains at the contact side and cylindrical grains at the free side with lots of Ti2Ni precipitation(ppts) exsisting at grain boundaries. Compared to ribbons containing less Ti content (Ti <51.5 at.%), Ti51.9Ni48Si0.1 and Ti52.6Ni46.8Si0.6 ribbons require longer aging time to make Ti-rich matrix turn into Ni-rich one due to the precipitation and growth of nanoscale Ti2Ni ppts enhanced by the existence of slight Si content in ribbons. Afterward, Ti3Ni4 ppts will precipitate, which can induce more B2-->R transformation and generate the precipitation hardening effect. The precipitation behavior of Ti2Ni and Ti3Ni4 ppts are confirmed by the potential △H value of DSC tests and the ppts peaks’ intensity of XRD tests. These ppts are also found to affect the ribbons’ phase transformation behavior and mechanical properties. For Ti51.9Ni48Si0.1 ribbons, the precipitation hardening of Ti3Ni4 ppts significantly appears when the ribbons aged at 500℃ for 5hrs and 54hrs, and thus can exhibit higher hardness, better pseudoelasticity(PE)/shape memory effect(SME), as revealed by nanoindentation and dynamic mechanical analysis(DMA) tests. For Ti52.6Ni46.8Si0.6 ribbons, the precipitation hardening induced by both Ti2Ni and Ti3Ni4 ppts appears when the ribbons are 500℃-aged at 3hrs and at the time longer than 10hrs individually, in which the ribbons aged at 500℃ for 3hrs possess the better mechanical properties.
|
|---|
