Latent strain in titanium-nickel thin films modified by irradiation of the plastically-deformed martensite phase with 5 MeV Ni2+

Lattice damage brought on by heavy ion irradiation is able to alter the displacive transformation characteristics of near equiatomic titanium-nickel. Irradiation of sputtered TiNi thin films can modify thermomechanical response to a depth of more than a micron, and may thus be used to create a perfectly bonded heterophase that deploys materials of sharply differing latent thermal strain on opposite sides of a thin sheet. If the alloy film is first martensitized, and then deformed in tension prior to partial-depth exposure to ion beam damage at temperatures well below A,, a novel active-passive bilayer results that expresses pronounced bending displacements on subsequent heating. In the present paper, describing experiments on stretched 6-mu m thick sputtered Ti50.2Ni49.7 films irradiated with 5 MeV Ni2+, we show that ion-induced latent bending can be cyclically reversed in temperature-displacement space, and that appreciable mechanical work can be extracted. Marked effects are observed at doses as low as 5 x 10(13) Ni2+ cm(-2). The approach, in which nominally planar processing is used, derives mechanical robustness from a naturally diffuse interface between the beam-damaged stratum and the adjacent unmodified shape-memory layer. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. AII rights reserved.