S. Miyazaki
TiNi-based shape memory alloy (SMA) thin films have been developed by the sputter-deposition method since 1990. They can now show the stable shape memory effect and superelasticity which are equivalent to those of bulk materials produced by conventional melting methods. Because of the unique microstructures consisting of non-equilibrium thin plate precipitates in the sputter-deposited thin films, they can sometimes show the stable properties even better than the bulk materials. Utilizing the Ti-Ni thin films deposited on Si substrates, prototypes of microactuators have been fabricated to reveal the possibility of applications in the field of micromachines. Recently, two types of high-speed microactuators utilizing TiNi-based thin films were also developed. One type is a microactuator utilizing a Ti-Ni-Pd thin film which has high transformation temperatures. Cooling rate of a micro-actuator increases with increasing the temperature difference between the transformation and atmosphere temperatures, thus increasing the response of actuation. The other type is a microactuator using the R-phase transformation of a Ti-Ni thin film. In the case of the R-phase transformation, the forward and reverse transformations occur with a narrow temperature hysteresis, thus being effective to increase the response of actuation. Both types of actuators are promising for high response applications. The working frequency of the microactuator utilizing the Ti-Ni-Pd thin film reached 100Hz, and that of the microactuator using the R-phase transformation reached 125Hz. More challenges for developing fabrication methods of thin films and microdevices are requested for many scientists and engineers to extend the possibility of applications utilizing SMA thin films.