Strengthening of Ti-Ni shape-memory films by coherent subnanometric plate precipitates

It has been found that subnanometric thin plate precipitates are formed in sputter-deposited Ti-rich Ti-Ni shape-memory films. This has been achieved by heating the as-sputtered amorphous sample at relatively low temperatures. The precipitates are produced along the {100} planes of the bcc (B2) parent phase and are completely coherent with the matrix. They have a disc-like shape with a thickness of only two to three lattice planes and a diameter of 5-10 nm. The distance between neighbouring precipitates is dependent on the heat treatment temperature, varying in the range 5-30 nm. Owing to the existence of these precipitates in the parent phase, the critical stress for slip in the parent phase is greatly increased and, as a result, excellent shape-memory properties, such as 6% recoverable shape strain and 670 MPa recovery strength, are obtained for Ti-Ni shape-memory thin films. It is concluded that the precipitates have a Ti-rich composition and are produced only in the off-stoichiometric amorphous Ti-Ni alloys with a Ti-rich composition.