Radiation Induced Grain Growth in Transition Metals
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RADIATION INDUCED GRAIN GROWTH IN TRANSITION METALS P. Borgesen 1 , D. A. Lilienfeld 2 , and H. Msaad' 1) Department of Materials Science and Engineering 2) National Nanofabrication Facility Cornell University Ithaca, N.Y. 14853 ABSTRACT Nanocrystalline self supporting thin films of Ni, Co, Cr, V, and Ti were irradiated with 600 keV Xe ions at liquid nitrogen temperature and the resulting grain growth was measured by transmission electron microscopy. Average grain sizes were seen to increase from about 10 nm to 18-35 nm, depending on the metal. Systematic variations did not support current thermal spike models. INTRODUCTION Over the past decades, systematic studies have led to a greatly improved understanding of ion-solid interactions and the resulting modifications of materials composition and structure. In particular, the development of the 'collision cascade', and the effects of binary collisions of ions or recoils with target atoms at rest, is now quite well described in terms of so-called ballistic models (1-3]. However, a large number of phenomena and dependencies remain which cannot be explained within such models. For metal targets, where ionization and charge effects can usually be ignored, two mechanisms are commonly invoked to explain deviations from predictions based on ballistic models [4]. At 'elevated' temperatures, radiation induced defects become mobile and 'radiation enhanced diffusion' models are applied. At low temperatures, thermal spike mechanisms have been proposed to explain effects that appear insensitive to ambient temperature. The thermal spike concept is, at best, an approximation to the extreme opposite limit of that described by the ballistic models. Nevertheless, it has achieved considerable popularity, at least in part because it allows for reasonably simple, semiquantitative modeling [5-7]. Indeed, spike like phenomena probably do occur. Molecular dynamics simulations [8] do suggest that so-called 'space filling' subcascades [9] may develop in medium-Z metals such as Cu and Ni. However, truly quantitative estimates of the effects are not available. In recent years, most investigations of non-linear (sub-eV) effects have focussed on ion beam mixing of bilayer or marker systems [5, 7]. However, the effects of composition variations and chemical driving forces are not easily predicted by ab initio models. In many ways, ion induced grain growth can be argued to represent a 'simpler' type of ion-solid interaction phenomenon. Alexander et aL [10, 11] showed that the basic mechanisms must be the same as for mixing. EXPERIMENT deposited on NaCI 550 A thick films of Ni, Co, Cr, V, and Ti were e-beam 7 substrates in an oil free vacuum.The base pressure was 2x10 Torr before and 5x10-7
Mat. Res. Soc. Symp. Proc. Vol. 201. c 1991 Materials Research Society
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Torr during deposition. The films were floated off onto Cu grids in de-ionized water and analyzed by Transmission Electron Microscopy (TEM) before and after irradiation. The grids were mounted on Al plates with silver paint, and irradi
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