The Study Of Microstructure And Phase Transformation Behavior Of TiNi Shape Memory Alloy Thin Film

As a functional material, TiNi 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). Since the phase transformation behavior of TiNi thin film has a great influence on the properties of micro-actuator, both Ti-rich and Ni-rich TiNi thin films' phase transformation behavior were studied in this stud)'. Electrical resistance, DSC, XRD and TEM measurements were used to study the crystallization, phase transformation and microstructure of TiNi thin film. In addition, a new synthesis method was done to prepare PbTiO3/TiNi composite film. The conclusions are summarized as follows:1) The crystalline temperature of amorphous TiNi film prepared by magnetic sputtering system is 450 癈, and the electrical resistance of TiNi film abnormally changes during crystallization. The electrical resistance method is more sensitive than DSC to study the crystallization of the amorphous TiNi thin film.2) The martensitic transformation temperature and the thermal hysteresis of both R phase and martensitic transformation of the Ti₅1Ni₄9 film increase with increasing the annealing temperature, but R phase transformation temperature is constant. In Ni-rich Ti₄9. 4Ni₅0. 6 film R phase transformation temperature and the thermal hysteresis of both R phase and martensitic transformation decrease with increasing the aging temperature, while the martensitic transformation temperature increases. To shorten the responding time of micro-actuator by decreasing the thermal hystersis of TiNi film, Ti₅1Ni₄9 and Ti₄9. 4Ni₅0. 6 film should be heat treated at 550℃ and 500℃, respectively.3) As the number of thermal cycling increases R phase transformation temperature, martensitic transition temperature and thermal hystersis of Ti₅1Ni₄9 film annealed at 550 癈 for 0.5 h increase, while they are constant after the thermal cycling is more than 30 times. In the Ti₄9.4Ni₅0.6 film aged at 500℃ for 0.5 h the martensitic transformation temperature decreases with increasing the number of thermal cycles, while thermalhystersis increases. They are constant after the thermal cycling is more than 10 times. To stabilize the micro-actuator using TiNi film, the number of thermal cycling should be more than 30 times.4) After small deformation, the <011>II type twin bands in the martensitic variant of TiNi thin film disappear in some martensite plates showing evidence of detwinning, while the (001) compound twins increase. The maximum recovery strain of Ti51Ni49thin film is 4.5%, but the plastic strain increases quickly when the total strain is more than 4%.5) As the deformation strain increases the reverse transformation temperature and thermal hystersis of TiNi film increase on the first heating, while the martensitic transformation temperature decreases and R phase transition temperature is constant. Therefore, the deformation has no effect on the properties of the micro-actuator using R phase transformation of TiNi film. However, for the micro-actuator using the martensitic transformation of TiNi film, the deformation strain of TiNi film should be less than 3%.6) After incomplete phase transformation in the TiNi thin film, two reverse transformations occur on the second heating, but it returns back to one reverse transformation again on the third heating. Therefore, the incomplete phase transformation has no effect on the property of micro-actuator using TiNi thin film.7) By using in-situ hydrothermal synthesis and sol-gel complex method a stoichiometry tetragonal perovskite PbTi03 ceramic film was successfully prepared on TiNi substrate. Scratch test showed that the adhesion load was about 65 N, which is 2 times than that of PbTiO3 ceramic film prepared by sol-gel method.