The influence of the surface on the production and development of displacement cascades in Ni₃Al and Cu₃Au

Abstract

Stereo transmission electron microscopy has been used to characterize the distribution in depth of disordered zones and associated dislocation loops in the ordered alloys Ni₃Al and Cu₃Au after heavy-ion irradiation, most extensively for Ni₃Al irradiated with 50 keV Ta⁺ ions at a temperature of 573 K. The Cu₃Au specimen was irradiated with 50 keV Ni⁺ ions at a temperature of 373 K with the ion beam incident at 45[ddot] in order to see more clearly the shapes and sizes of the cascades parallel to the beam direction. The defect yield in Ni₃Al, that is the probability for a disordered zone to contain a loop, was found to be strongly dependent on the depth of the zone in the foil, varying from about 0.7 for near-surface zones to about 0.2 in the bulk. The sizes and shapes of disordered zones were independent of the depth of the zones in the foil, except for a small population of zones very near the surface which were strongly elongated parallel to the incident ion beam. In Cu₃Au the surface had a smaller effect, with the yield falling from about 0.7 close to the surface to 0.5 for the deeper zones. These measurements of the depth dependence are interpreted in terms of the annihilation of freely migrating point defects at the surface. The results are compared with recent computer simulations.