Thickness Effect on Thermally Induced Phase Transformations in Sputtered Titanium-nickel Shape-memory Films

The effect of the film thickness on the phase transformations encountered in sputtered titanium-nickel (TiNi) shape-memory films due to thermal cycling in the temperature range of −150 to 150 °C was examined in the context of electrical resistivity (ER) measurements. A hysteresis in the ER response was observed for film thickness greater than 300 nm. This phenomenon is characteristic of shape-memory materials and is attributed to the rhombohedral (R) phase produced during cooling from the high-temperature cubic austenite phase to the low-temperature monoclinic martensite phase. The decrease of the TiNi film thickness below 300 nm resulted in a smaller ER hysteresis, leading eventually to its disappearance for film thickness less than ∼50 nm. The results indicate that spatial constraints introduced by the film surface and film/substrate interface generate a resistance force, which prevents lattice distortion and twinning. The inhibition of these mechanisms, which control self-accommodation R-phase transformation, leads to the suppression and eventual disappearance of the shape memory effect for film thickness less than ∼100 nm.