The transformation temperatures and temperature hysteresis of Ti–Ni–Zr films were found to be very sensitive to grain size. However, films annealed at 1223 K exhibited transformation temperatures and temperature hysteresis nearly the same as those of the corresponding bulk alloys. Annealing of Ti–Ni–Zr–Co films with 10.5–26.7 at.% Zr and 0–12.7 at.% Co at 1223 K for 10 h showed that Co substitution for Ni in the Ti–Ni–Zr films was beneficial to reducing the temperature hysteresis. Correspondingly, a Ti29.3Ni42.7Zr20.9Co7.1 bulk alloy annealed at 1223 K for 10 h exhibited a high martensitic transformation temperature (432 K) and a small temperature hysteresis compared to Ti–Ni–Zr bulk alloys (21 K). The present study verified the usefulness of sputtering for the exploration of new shape-memory alloys.

•Polyimide/Ti–Ni–Cu composite films were fabricated by sputtering.•The composite films can be used as one-way or two-way actuators.•A simulation tool was developed to optimize dimensional and material parameters.•The actuators exhibited a large stroke, a high response speed, and a large force.•Various combinations of actuator force and stroke were obtained using composite films.Polyimide/Ti49Ni33Cu18 composite films were fabricated by the sputtering of an alloy target. The composite films could be used as a simple actuator by cutting an appropriately shaped piece out of an as-sputtered films with scissors and then connecting it to a battery. In order to investigate the design parameters for the composite-film actuators, Ti–Ni–Cu films with various thicknesses (0.8–13 μm) were deposited on two kinds of polyimide films (Kapton EN and H) with various thicknesses (25–175 μm). The force and stroke of the composite-film actuators were found to vary largely with changes in various parameters such as the thickness ratio of Ti–Ni–Cu and polyimide films and the coefficient of thermal expansion of the polyimide films. A simulation tool based on a simple deformation model for a shape memory alloy was developed, and the effects of dimensional parameters (thickness, length, and width) and material parameters (Young's modulus, elastic strain limit, and transformation strain) on the actuation properties (force and stroke) of the composite-film actuators were systematically investigated on the basis of the simulation results. The simulation results revealed that composite-film actuators allow for various combination of force and stroke. Optimized actuators exhibited a long actuation stroke, a high response speed of 3 Hz, and a large force of 0.4 N.

The application of sputtering is interesting for the investigation of shape memory alloys. Alloy films with a wide composition range can be obtained using a multi-target sputtering apparatus. Furthermore, the sample preparation is easy; samples can be cut out of the film with scissors or a wire saw and electropolishing only takes 10 s. Using thin films, the effects of nanostructures on shape memory behaviour, thickness effect, deformation mechanism of martensite, and functionality of composite materials were investigated. On the basis of these results, the feasibility of the sputtering method as a tool for analysis and development was discussed.

•Ti–Ni–Cu films with 55.5 at% Ti were formed by sputtering.•The films annealed at 773–973 K for 1 h had no precipitates within the B2 grains.•The shape memory behavior was sensitive to the B2 grain size.•The B2 grain size was controlled by the grain size of a second phase.•The effects of Cu content and annealing temperature were discussed.The microstructure and shape memory behavior of Ti55.5Ni45.5−xCux (x = 11.8–23.5) thin films annealed at 773, 873, and 973 K for 1 h were investigated. None of the films except the Ti55.4Ni32.8Cu11.8 film annealed at 773 K for 1 h had any precipitates in the B2 grain interiors and their grain sizes were small (less than 1 μm). Increasing the annealing temperature caused grain growth and thus a decrease in the critical stress for slip and an increase in the martensitic transformation start temperature (Ms). The grain size was also controlled by the growth of a second phase. In the three-phase equilibrium region of Ti2Ni, Ti2Cu and TiNi, Ti2Cu grains grew faster than Ti2Ni grains, leading to a decrease in the critical stress for slip and an increase in the Ms temperature with increasing Cu content.

The shape memory behaviours of Ti51.4Ni25.2Cu23.4, Ti51.3Ni21.1Cu27.6, Ti51.2Ni15.7Cu33.1 and Ti51.4Ni11.3Cu37.3 thin films annealed at 773, 873 and 973 K for 1 h were investigated. The Ti51.3Ni21.1Cu27.6 film annealed at 773 K, the Ti51.2Ni15.7Cu33.1 film annealed at 873 K, and the Ti51.4Ni11.3Cu37.3 films annealed at 873 and 973 K showed a perfect shape memory effect at a stress as high as 1 GPa. This improvement in shape memory behaviour was attributed to their fine grain sizes less than 500 nm. Whereas the Ti51.2Ni15.7Cu33.1 and Ti51.4Ni11.3Cu37.3 films annealed at 873 K or higher showed a martensitic transformation start temperature above room temperature, these films annealed at 773 K were in the parent phase at room temperature owing to their very fine grain sizes. The effects of Cu content and annealing temperature on the shape memory behaviour of the Ti51.5Ni48.5−xCux (x > 27) films with submicron grain sizes were discussed in comparison with those of the Ti51.5Ni48.5−xCux (x < 24) films on the basis of their microstructures.Highlights►Ti51.5Ni48.5−xCux (x > 27) thin films show a perfect shape memory effect under 1 GPa. ►This improvement was attributed to their fine grain sizes (<500 nm). ►The films show characteristic shape memory behaviour owing to their fine grains. ►The effect of grain size was discussed on the basis of their martensite structure.

Ti–Ni–Cu shape-memory alloy (SMA) thin films were sputter-deposited on heated polyimide substrates. Ti–Ni–Cu films deposited at substrate temperatures of 543 and 583 K were found to be crystalline. Especially, a Ti48Ni29Cu23 film deposited at 583 K exhibited a high martensitic transformation temperature above room temperature and a narrow transformation temperature range, which enable the film to be used at room temperature. Double-beam cantilevers made of 8 μm thick Ti48Ni29Cu23 films deposited on 12.5 and 25 μm thick polyimide substrates displayed a repeatable shape-memory effect by a battery of 1.5 V and it was verified that the composite film consisting of an 8 μm thick Ti48Ni29Cu23 film and a 25 μm thick polyimide film is capable of moving 0.18 g wings of a dragonfly toy up and down. These results offer the prospect for using an SMA/polyimide actuator as a convenient small actuator, which will find wide-ranging applications.