The Study Of Transformation Behavior Of NiTi Shape Memory Alloy Thin Films

NiTi shape memory alloy thin film has the characteristics of large stress, strain and high work density. It is a perfect micro-actuator candidate for the micro-electrical mechanical system (MEMS) in recent years. In order to NiTi shape memory alloy thin film in micro-actuator system, the phase transformation behavior of NiTi thin film is important. Since the heat treatment temperature of NiTi thin film has a great influence on the phase transformation behavior. The crystallization of NiTi shape memory alloy thin film is studied in this work. The effects of substrate temperature on crystallization, surface and interface oxidation behavior are discussed. The phase transformation is detected during thermal cycling by DSC. X-ray diffraction (XRD) and SEM which shows the stress induced precipitation. Systematic observations had been made on the microstructure of NiTi thin film after thermal treatment by TEM. The conclusions are summarized as follows:The average dislocation density in NiTi thin films decrease with the increase of solution temperature which the average dislocation distribution parameter unchanged. The precipitation made the microhardness values is greater. The results show that the microhardness values are not in a good agreement between calculated and measured values.The crystalline temperature of the thin film at room temperature substrate is about 774K, the starting temperature is about 756K, the finishing temperature is about 823K. The crystalline temperature of the thin film at 573K substrate is about 750K, the starting temperature is about 670K, the finishing temperature is about 810K. The temperature range of phase transformation is greater. With the increase of thermal cycling times, the average deformation storage energy density dropped.The NiTi thin films deposited on Cu substrate at different substrate temperatures have been studied by X-ray photoelectron spectroscopy (XPS). The effects of substrate temperature on the surface and interface oxidation of NiTi thin films are discussed. After oxidation the samples are covered by an oxide layer with thickness of a few nanometers. This oxide layer is composed of TiO2. The Ni atom has not been detected in surface. In the film /substrate interface there is an oxide layer with thickness of about 12 nm thick with a mixture Ti2O3 and NiO in the films deposited at substrate temperature 573K and 723K, respectively. In the film/substrate interface a layer with thickness of about 12 nm contains Ti suboxides (TiO) and metallic Ni in the films deposited at ambient temperature. The composition of the films deposited at RT and 573K are Ti-51.06 at.% Ni, while that of the film deposited at 723K is Ti-52.0 at.% Ni. In the film deposited at ambient temperature the oxygen atoms concentration is greater than that of the film deposited at 573K. The XRD pattern of the NiTi film at 573K substrate temperature after heat treatment at 923K for 0.5h exhibits the specimen is with B2 austenite and B19' martensite structure, the thin film at 723K substrate temperature precipitates Ni3Ti and TiO2.After annealed at 673K for 1 h the film deposited on 673K substrate is with B2 and B19' phase, the As of thickness of 3μm is higher. As the oxidation occurs, titanium atoms diffuse outward while oxygen atoms diffuse inward. The absorption of interstitial oxygen atoms in the lattice during the oxidation and the atoms concentration of TiNi thin film could affect the martensite transformation temperature. The microstructure of TiNi film annealed at 743K for 1 h shows Ni4Ti3 precipitate and B2 phase. The samples grow at room temperature contain fine grain of B2 phase and B19' martensite phase after annealed at 923K for 0.5 h.Ni3Ti phase precipitate and grow during tensile testing. The average dislocation density and the microhardness values increase with the strain. A series of parallel cracks grow in a concerted fashion across the film. The cracks are equally spaced.