Effects of the Surface on Displacement Cascades Produced by Heavy-Ion Irradiation of Ni₃Al and Cu₃Au

Abstract

Stereo transmission electron microscopy has been used to characterise 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 an incident angle of 45° at a temperature of 373 K. In Ni₃Al the defect yield, i.e. 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 only weakly dependent on depth, 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 but still significant effect on the defect yield. The dependence of the tranverse disordered zone diameter d on ion energy E for Ta⁺ irradiation of NiA was found to follow a relationship d =k₁, E^1/α withk, = 2.4 ± 0.4 and α = 3.3 ± 0.4. A similar relationship with the same value of α is valid for a wide variety of incident ion/target combinations found in the literature.