Basic Research on Non-equillibrium Microstructures of Sputter-deposited TiNi-base Shape Memory Alloy Thin Films

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TiNi-base shape memory alloy thin films including Ti-Ni, Ti-Ni-Cu and Ti-Ni-Pd with various alloy compositions have been made by a r. f. magnetron sputtering method. The purpose of the present research is to investigate non-quillibrium microstructures which are characteristic for the thin films. It is expected that the addition of Cu to Ti-Ni is effective to decrease transformation temperature hysteresis, while that of Pd is effective to increase the transformation temperatures. Both the narrow temperature hysteresis and high temperature of the martensitic transformation are expected to be effective to increase the cyclic response of actuation.It is clarified that fine metastable plate precipitates were formed on {100}B2 planes when heat-treated around the crystallization temperature and were effective to achieve stable shape memory behavior, because the critical stress for slip deformation was increased by the precipitation hardening mechanism. The plate precipitates were changed to Ti2Ni equillibrium phase precipitates by increasing heat-treatment temperature, resulting in degradation of the shape memory characteristics. Similar metastable microstructures were also observed both in Ti-Ni-Cu and Ti-Ni-Pd thin films and were effective to achieve stable shape memory behavior. The transformation temperature hysteresis was changed down to about one third of that of Ti-Ni by adding Cu more than 10at%, while the martensitic transformation temperature was increased up to 562K by adding 22at%Pd to the Ti-Ni.