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

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 bet 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 bet 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 Ti,Ni containing samples and 10 times that of a bulk Ti-45Ni-5Cu alloy. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.