The influence of a surface on defect production by 10 keV displacement cascades in Ni3Al

Recent computer simulations, for example by Ghaly and Averback (Phys. Rev. Lett. 72 (1994) 364), have shown that a surface can produce a significant increase in the number of vacancy defects produced by displacement cascades in pure metals. In the present work, the influence of a surface on the production of lattice defects by displacement cascades of 10 keV in the ordered alloy Ni3Al has been studied. This alloy system is of interest because previous simulations, for example by Gao and Bacon (Phil. Mag. A 71 (1995) 43), have shown that antisite defects, rather than Frenkel pairs, are the dominant defect species arising from cascades in the bulk. We find that for the near-surface cascades, the dominant defects are again the antisites atoms and their production in a disordered zone is enhanced by about 41% due to the surface. The production efficiency for vacancies is also much higher in the near-surface events and most of the extra atoms are created as adatoms on 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. The formation mechanism of a vacancy loop is investigated in detail.