Basic research on the shape memory alloy thin films

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Among several types of performance materials proposed for fabricating microactuators to power micromachines, Ti-Ni shape memory alloy thin films have great advantages such as large deformation and recovery force ; besides, even an intrinsic disadvantage of the shape memory alloy such as a slow response due to the limitation of cooling rate can be greatly improved in thin films. Therefore, the development of Ti-Ni shape memory alloy thin films has been keenly demanded. The present project has been successful in developing a perfect shape memory effect in sputter-deposited Ti-Ni thin films. The transformation temperatures and shape memory characteristics were found to be strongly affected by heat-treatment, alloy composition and sputtering condition. The shape change and recovery force associated with the martensitic transformation in Ti-Ni thin films reached up to 6% and 500Mpa, respectively, which were equivalent to those of bulk specimens.Ti-Ni-Cu and Ti-Ni-Pd ternary shape memory alloy thin films were also made by sputtering. They showed a small temperature hysteresis and high transformation temperatures, respectively ; these characteristics are effective to achieve a high response actuator characteristic. The Ti-Ni-Pd thin film showed a single stage transformation upon cooling and heating similarly to a Ti-Ni binary alloy thin films. Corresponding, they showed a single stage deformation both upon cooling and heating. The Ti-Ni-Cu thin film showed a two-stage transformation and hence a two-stage deformation ; the first stage corresponds to the transformation between B2 and O(Orthorhombic)-phase and the second stage between the O- and M-phases. The first stage transformation was accompanied by a small temperature hysteresis. All these thin films showed perfect shape memory effect.