Shape memory effects associated with the martensitic and R-phase transformations in sputter-deposited Ti-Ni thin films

Ti-Ni shape memory alloy thin films were deposited using RF magnetron sputtering. The lattice constants, the transformation temperatures and shape memory behavior were measured by the X-ray diffractometry, DSC and mechanical tests, respectively. These films showed a perfect shape memory effect which is the same as that of bulk alloys. The shape recovery stress and strain measured in the thin films were large enough for fabricating microactuators. In an age-treated Ni-rich Ti-Ni thin films, the shape recovery stress was more than 600MPa, while the shape memory strain amounted up to 2.6%. The thin films showed both the martensitic and R-phase transformations. Accordingly, they showed a two-stage deformation upon cooling ; the strain induced in each stage recovered perfectly upon heating. The transformation temperatures and shape memory characteristics were strongly affected by age-treatment in Ni-rich thin films, while they were not in Ni-poor thin films.