Displacement cascade collapse at low temperatures in Cu 3 Au

Abstract

A systematic study by transmission electron microscopy has been made of factors affecting the collapse of displacement cascades in ordered Cu 3 Au. Thin foils of ordered Cu 3 Au were bombarded with Ar + , Cu + and Kr + ions of energies 50 keV and 100 keV to nominal doses of 10 15 ions m -2 both at room temperature and at 30 K. The damage induced at low temperature was observed in situ and after subsequent warming to room temperature. The imaging techniques employed allowed disordered zones produced at individual cascade regions, together with their associated dislocation loops, to be identified and characterized separately, and so permitted defect-yield values to be measured accurately independently of the ion dose. Collapse of displacement cascade to vacancy dislocation loops was observed to occur with moderately high probability under all irradiation conditions at low temperature, with no subsequent increase in the number of loops on warming to room temperature. Collapse, however, occurred with significantly greater probability in corresponding irradiations performed at room temperature. An increase in collapse probability with ion mass was also found, although no dependence on ion energy was observed. Detailed correlations between collapse and the sizes and shapes of disordered zones at cascade sites are reported. The observations are shown to be consistent with cascade collapse resulting from systematic vacancy migration during the thermal spike phase of the cascade. The disordering of cascade peripheries, probably by point-defect migration, is also considered to occur during this phase.