Polycrystalline Ti-rich NiTi thin films and (TiHf)-rich NiTiHf thin films containing 9at% Hf were investigated. These films were deposited from single NiTi and NiTiHf targets using a DC magnetron sputtering system. Free-standing film was obtained by using a single crystal silicon substrate. Thickness of these thin films was controlled at around 10–12 μm. In this investigation, the effect of deposition temperature on the microstructure and transformation temperatures of the thin films was studied. Influence of post heat treatment temperature on the properties of the thin films was also investigated. Transformation temperatures of these thin films were determined by differential scanning calorimetry (DSC). Surface and cross-sectional morphology of the films were studied using a scanning electron microscope (SEM). Crystallography is determined using x-ray diffractometry.; Thin films deposited on a hot substrate are crystalline in anture even when the substrate temperatures are as low as 300°C, while the normal crystallization temperature of NiTi-based shape memory alloy is around 500°C. Microstructure of the films deposited on a hot substrate shows a very fine grain size. The grain size increases with increasing substrate temperature during deposition. The effect of post heat treatment on grain size is minimal. For both NiTi and NiTiHf thin films, martensite and austenite transformation temperatures increase with increasing deposition temperature and increasing post heat treatment temperature. R-phase, which was detected only in NiTi thin films, was found to be much less sensitive to the deposition temperature and post heat treatment temperature. No R-phase was observed in the NiTiHf thin films. Miller indices and lattice parameters of different crystal structures were calculated. Thermal hysteresis of above alloys was also analyzed. Thermal hysteresis of NiTi decreases with increasing deposition temperature and post heat treatment temperature. On the contrary, thermal hysteresis of NiTiHf increases. The observed behaviors of NiTi and NiTiHf thin films are a complex influence from internal stresses, internal defects, crystallography and second phases. For example, with elevated deposition temperature and post heat treatment temperature, internal defects are either rearranged or removed and internal stresses decrease. As a result, both martensite and austenite transformation temperatures increase. Besides, with increasing deposition temperature and post heat treatment temperature, more and more second phases precipitate out. This makes the matrix of the films more thermodynamically stable and strengthens the structure of martensite. Consequently, transformation temperature of martensite and austenite increase.
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