Defect production by near-surface displacement cascades in α-zirconium

Recent computer simulations have shown that a nearby surface can engender a significant increase in the number of vacancy defects produced by displacement cascades is pure metals of cubic crystal structure (e.g, M. Ghaly and R.S. Averback, Phys. Rev. Lett. 72 (1994) 364). In the present work, the influence of a surface on the production of lattice defects by displacement cascades of 2 keV in energy in a-zirconium has been studied. This HCP metal is of interest because self-interstitials migrate preferentially along the basal planes, and so the crystallographic orientation of the surface may influence the final defect state of cascade damage. The primary recoil atom was chosen to be an atom on either a basal- or a prism-plane surface, and recoil directions making either a high or a low angle to the surface normal were considered. The resulting production of vacancies and interstitials below the surface has been compared with equivalent data for simulations of cascades in the bulk. The creation of vacancies is not strongly affected by the crystallography, but the number of interstitials, adatoms and sputtered atoms is. The production efficiency of the latter two forms is much larger for the prism-plane surface, whereas stable interstitial defects are more readily generated below a basal-plane surface, These results can be interpreted in terms of cascade mechanisms and defect properties in zirconium.