MICROSTRUCTURAL MODIFICATIONS INDUCED BY SWIFT IONS IN THE NITI INTERMETALLIC COMPOUND

The conditions for track formation by irradiation with swift ions, in the shape memory NiTi metallic compound are studied. We observe that t racks are only induced in the monoclinic structure but not in the cubi c one and only when the linear rate of energy deposition by electronic excitation ([dE/dx](e)) exceeds 46 keV/nm. We show that the tracks ar e amorphous in their centre and that a decrease of the monoclinic towa rds cubic transformation temperature is observed at the periphery of t he track region. For [dE/dx](e) = 32 keV/nm, no individual tracks can be observed at low fluences, only a monoclinic --> cubic structure tra nsformation is observed at high fluences. If[dE/dx] less than or equal to 17keV/nm, swift ions are unable to induce any visible structural m odification by electronic excitations. The track formation in this all oy is discussed within the framework of the classical Coulomb explosio n and thermal spike models. (C) 1998 Elsevier Science B.V, All rights reserved.