Martensitic transformation and shape memory behavior in sputter-deposited TiNi-base thin films

Since 1990, TiNi and TiNiX (X=Cu, Pd, Hf) thin films have been made by sputtering. The motivation for fabricating sputter-deposited TiNi-base shape memory alloy thin films originates from the great demand for the development of powerful microactuators which can drive micromachines, because actuation force and displacement are greatest in shape memory alloys amongst many actuator materials. Stable shape memory effect and superelasticity, which are equivalent to those of bulk alloys, have been achieved in the sputter-deposited TiNi thin films. Narrow transformation temperature hysteresis and high transformation temperatures were also achieved in TiNiCu and TiNi(Pd or Hf) thin films, respectively. In the meantime, unique microstructures consisting of nonequilibrium nanoscale precipitates and nonequilibrium compositions in the matrix have been found in Ti-rich TiNi thin films which were fabricated from amorphous condition by annealing at a considerably low temperature. Several micromachining processes have been proposed to fabricate some prototypes of microactuators utilizing TiNi thin films. The present paper will review the recent development of the above mentioned topics relating to sputter-deposited TiNi-base shape memory alloy thin films.