The influence of a dilute magnesium addition on the growth ant shrinkage of dislocation loops in aluminium

Abstract

The influence of a dilute magnesium addition to pure aluminium on the climb of dislocation loops has been studied by isothermal annealing of thin foils of Al-0·65% Mg. Faulted loops were observed to shrink at temperatures above 130°C, whereas prismatic loops above a critical size grew at temperature? ≳200°C; in pure aluminium they generally shrink. The various mechanisms previously proposed to account for excess vacancies have been critically examined and shown to be inconsistent with the experimental observations, and an alternative mechanism based on the growth of a surface oxide is proposed. Experimental observations consistent with the oxide growth mechanism are described. The variation of shrinkage rate with temperature for both prismatic and faulted loops over a wide temperature range has been determined. The results, which do not conform to emission controlled kinetics, have been analysed in terms of diffusion controlled behaviour to deduce an accurate value for the intrinsic stacking-fault energy of Al-0·65% Mg. Comparison of the annealing rate of faulted loops in the alloy and pure aluminium indicates that the Mg atom-vacancy binding energy is extremely small.