Two-way shape memory NiTi sputter-deposited film fabrication

This paper describes work on two-way shape memory (TWSM) training of 52at.% Ti--48at.% Ni thin films. The amount of recoverable strain of shape memory alloys (SMA) with TWSM is about 2%. With TWSM, NiTi films will remember different high-temperature and low-temperature shapes. These shapes may be cycled fairly reproducibly by simply changing the temperature. In this work, NiTi films were deposited by RF magnetron sputtering from an NiTi target with atomic composition of 56at.% Ti--44at.% Ni. The atomic composition of the sputtered films was determined to be 52at.% Ti--48.0at.% Ni by electron microprobe. Solution treatment of the as-deposited films was required to crystallize and memorize a high-temperature shape, followed by age treatment to increase the transformation temperatures to above room temperature. The crystal structure of the solution-treated films was determined to be B2. The transformation temperatures of the age-treated films were determined by differential scanning calorimeter to be 311 K (A*) and 307 K (R*). TWSM training was carried out by over deforming the specimen while in the R-phase. Below Rf, a load was applied to the specimen beyond the usual strain limit for completely recoverable shape memory. Then, the load was removed prior to the next heating step, upon which the reverse transformation occurred under zero stress. With similar loads and temperatures, the procedure was then repeated. This paper will present details of the fabrication techniques, measurement results and its application.