A hybrid binary-collision/continuum model for temperature and defect evolution in displacement cascades was used to investigate the effect of a nearby surface upon vacancy formation in cascade events. The shape, vacancy and thermal conditions in cascade zones that melted and intersected the surface during the thermal spike phase were analyzed. The simplified geometries of such zones were simulated by means of molecular dynamics, on the basis of these data, and mechanisms of vacancy generation in the melt-zone were analyzed. It was demonstrated that additional vacancies formed, in the re-solidified zone below the surface, via a combination of thermal sputtering and viscous flow of atoms from the cascade core. Quantitative relationships between the geometry and size of the melt zone, and the number of additional vacancies, were considered and were used in the hybrid model in order to calculate the depth, yield and size distribution of vacancy dislocation loops in samples which were bombarded with 50keV Ni+ ions. The results were in good agreement with experimental data which had been obtained by the transmission electron microscopy of ion-irradiated thin foils.

Mechanism of Surface Influence on Dislocation-Loop Yield in Copper and Cu3Au. V.G.Kapinos, D.J.Bacon: Physical Review B, 1999, 60[6], 3829-38