The Roles of Energetic Displacement Cascades in Ion Beam Modifications of Materials
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THE ROLES OF ENERGETIC DISPLACEMENT CASCADES IN ION BEAM MODIFICATIONS OF MATERIALS R.S.
Averback*,
S.-J. Kim
, and T. Diaz de la Rubia***
*Argonne National Laboratory,
Argonne, Il. 60439 (New Address: University of Illinois at Urbana, 61801) **California Institute of Technology, Pasadena, Ca. 91125 SUNY at Albany, Albany,
N.Y.
12222
ABSTRACT The roles of energetic displacement cascades are ubiquitous in the These fields of radiation damage and ion beam modifications of materials. For the first, which lasts roles can be described on two time scales. 10-11 s, small cascade volumes are characterized by large supersaturations of point defects, structural disorder, and energy densities in excess of some tenths of eV's per atom. During this period, the system can be driven far from equilibrium with significant rearrangement of target atoms and the Experimental studies of ion beam mixing in production of Frenkel pairs. conjunction with molecular dynamics computer simulations, have contributed At later largely toward understanding these dynamic cascade processes. times, the microstructure of the material evolves as cascades begin to overlap, or at elevated temperatures, point defects migrate away from their nascent cascades. It will be shown how the primary state of damage in cascades influences this microstructural development. Examples involving radiation-enhanced diffusion and ion-induced amorphization will be discussed. INTRODUCTION The use of ion beams for materials modifications applications has increased dramatically in recent years, and with this growth, has come a Ion beam modification renewed interest in basic radiation effects in solids. processes can be conveniently divided into two parts, one involving The first equilibrium thermodynamics and a second involving kinetics. determines where a system wants to go during irradiation; the second determines how it gets there. This paper is concerned primarily with kinetic processes. Two regimes of kinetic processes can be distinguished; they are In the prompt sometimes referred to as the prompt and delayed regimes. regime, which takes place within - 10-11 s after the ion impinges on the target, two processes occur: (1) Many atoms are displaced from their lattice sites through a series of high energy (> 5eV) collisions, producing Frenkel pairs, vacancies and self interstitial atoms (SIAs), in a collision cascade. (2) As the collision cascade ends, most of the initial kinetic energy becomes partitioned among the atoms in the volume containing the cascade, thereby Additional atomic rearrangements creating a thermal spike-like condition. occur during this thermal spike. At the end of the prompt regime, the cascade In the delayed regime, the region is left in a highly nonequilibrium state. point defects created in the cascade migrate far beyond the original As diffusion in the delayed regime boundaries of the collision cascade. occurs at the ambient temperature of the target, it tends to return the system toward global equilibrium. However, many metastable states
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