Mechanism of surface influence on dislocation loop yield in copper andCu3Au

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A recently developed hybrid binary-collision/continuum model [V. G. Kapinos and D. J. Bacon, Phys. Rev. B 52, 4029 (1995)] of temperature and defect evolution in displacement cascades has been used to investigate the influence of a nearby surface on vacancy defect formation in cascade events. The shape, vacancy, and thermal conditions in cascade zones that melt and intersect the surface during the thermal spike phase have been analyzed. Simplified geometries of such zones have been simulated by molecular dynamics on the basis of these data and the mechanisms of vacancy generation in the melt zone analyzed. It has been demonstrated that additional vacancies are formed in the resolidified zone under the surface through 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 number of additional vacancies have been considered, and used in the hybrid model to calculate the depth, yield, and size distribution of vacancy dislocation loops in Cu and ${\mathrm{Cu}}_{3}\mathrm{Au}$ irradiated with 50-keV ${\mathrm{Ni}}^{+}$ ions. The results are in good agreement with experimental data obtained by transmission electron microscopy of ion-irradiated thin foils.