Material Performance and Phase Transformation of Super Rapid-Solidified Shape Memory TiNiCu Ribbon with Nano-Amorphous Microstructures

The sputter-deposited nano-structured thin films of shape memory TiNiCu alloy show excellent shape memory properties by the addition of Ti composition over 50at%, which are brought by the unique nano-precipitation produced by low temperature crystallization (=Tx) treatment. However, the applicability of thin film actuator is restricted to only very small and tiny micro-machines due to the small mass and, hence, small recovery force as a mechanical actuator. In the present paper, we have investigated the possibility of obtaining larger mass of actuator material as well as improvement of shape memory properties of the same kind of TiNiCu for rapid-solidified (RS) thin ribbons that were about 2-5 times thickness than sputtered film. As a result, 1) an amorphous state was formed in the rapid-solidified Ti-rich Ti50Ni40Cu10 composition (Ti54Ni40Cu6,at%) under a roll-speed over 45m/sec. condition as in the sputtering process, 2)by annealing just below Tx(=754K) for amorphous ribbons, nano-crystalline structures with nano-precipitation of bct (non-equilibrium phase) were observed in the matrix. These nano-microstructured TiNiCu ribbons showed good material properties of tensile strength and almost the shape recovery stress to those of sputtered films.