Internal structures and shape memory properties of sputter-deposited thin films of a Ti–Ni–Cu alloy Original Research Article

Low-temperature heat treatments of the sputter-deposited amorphous films, which were previously proved to be a new method to produce very good shape memory properties for Ti-rich Ti–Ni alloys, have been applied to a ternary Ti–43.0Ni–6.2Cu alloy (at.%). The basically same nanometric structures as in the binary alloy are formed, i.e. the nanometric structures consist of extremely thin plate precipitates of bct structure, which are formed on {100} planes of the parent B2 structure and have the c-axis normal to the habit planes. High-shape recovery stresses of about 500 MPa with recoverable shape strains of 5% are obtained without accompanying any permanent strains. A shape recovery stress of more than 870 MPa is attained if it is allowed to involve about 1% permanent strain. Although these bct precipitates have large tetragonalities, they are perfectly coherent with the parent bcc lattice. The maximum shape recovery stress is nearly twice that of the Ti-rich Ti–Ni binary alloy having a similar nanometric structure. It is suggested that this remarkable increase in recovery stress may be attributed to the change in Burgers vector of dislocations caused by partial disordering in Ti–Ni–Cu alloys. It is emphasized that the shape recovery stress in this ternary alloy is four times that of the Ti2Ni containing samples and 10 times that of a bulk Ti–45Ni–5Cu alloy.