Atomic Transport in Irradiated Solids
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ATOMIC TRANSPORT IN IRRADIATED SOLIDS R.S. AVERBACK, MAI GHALY, Y.S. LEE, and H. ZHU Department of Materials Science and Engineering and the Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801 ABSTRACT Atomic transport in irradiated solids has been investigated in both the prompt and delayed regimes. Prompt effects are revealed on an atomic level through molecular dynamics computer simulations. It is demonstrated that for metals like gold, which have high atomic numbers and low melting points, thermal spikes play a primary role in the cascade dynamics and that concepts like melting and rapid quenching are useful descriptions. Surface effects in these metals are also discussed. For metals with higher melting points and lower atomic numbers, the cascade dynamics are determined almost exclusively by energetic collisions far above thermal energies. This is illustrated by simulations of cascades in NiAl. The effect of the high ordering energy in this intermetallic compound on the radiation-induced defect structure has also been studied. Atomic transport in the delayed regime is illustrated by two examples: an order-disorder alloy, Cu3Au, and an amorphous alloy, NiZr. The first example is used to illustrate various aspects of radiation enhanced diffusion (RED): ion beam mixing, diffusion kinetics, the effects of primary recoil spectrum, and the importance of chemical order. The second example illustrates that the basic theory of RED, which was developed to describe crystalline materials, appears to work adequately for amorphous metal alloys, suggesting that similar mechanisms may be operating. It is shown, however, that the kinetics of RED observed in amorphous alloys are not unique to point defect models. INTRODUCTION Atomic transport in irradiated solids is generally classified according to two time scales. Prompt effects take place within some pico to nano seconds of an energetic particle impinging on a solid, while delayed effects occur afterwards. Atomic transport in the prompt period is localized, and it is driven by the energy supplied by the implanted ion. Because the kinetic energies of the atoms are high, chemical and crystalline disorder are created in the lattice. Atomic motion in the delayed period is thermally activated and spatially uniform; it tends to restore global equilibrium. In pure metals and dilute alloys, a detailed, if not complete, picture
of atomic transport in both time frames has been developed over the past thirty to forty years (1). In more complex systems, for example, ordered or amorphous alloys, semiconductors and insulators, or when surfaces or interfaces are nearby, the level of understanding of radiation induced transport is still rudimentary; but it is these more complex systems and geometries that are of most concern for materials processing and questions of phase stability. In this paper, some examples of such complexities are discussed. Molecular dynamics computer simulations (MD) of cascades in pure Au and in an intermetallic alloy, NiAl,
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