Defect production by near-surface displacement cascades in Ni3Al

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Abstract
Recent computer simulations have shown that a nearby surface can engender a significant increase in the number of vacancy defects produced by displacement cascades in pure metals (as found for example by Ghaly and Averback). In the present work, the influence of a surface on the production of lattice defects by displacement cascades of up to 10 keV in energy in the ordered alloy Ni3Al has been studied. This alloy system was chosen because previous simulations have shown that antisite defects, rather than Frenkel pairs, are the dominant defect species arising from cascades in the bulk. By choosing the primary recoil atom to be a surface atom, the effect of the surface on the damage mechanisms was maximized. The production efficiency for vacancies is higher in the near-surface events (at approximately 0.5–0.6 of the Norgett-Robinson-Torres standard value at all energies), whereas the corresponding efficiency for interstitials is much reduced. Most of the extra atoms are created as adatoms on the surface, for the number of sputtered atoms is insensitive to the cascade energy. As in cascades in the bulk, the dominant defects are the antisite atoms, and their production in a disordered zone is enhanced by 25—50% by the surface. The surface results in a strong increase in vacancy clustering, to the extent that vacancy dislocation loops can be produced in the disordered zone by cascade collapse at low cascade energy levels. These results are assessed in the context of current understanding of cascade processes.