Observations on the structural changes accompanying recovery in super-purity aluminium

Abstract

The structure of super-purity aluminium has been examined by metallographic and X-ray examination directly after cold rolling at room and liquid air temperatures and after annealing at temperatures in the range, room temperature to 375†C. Hardness and micro-hardness tests have also been carried out. A small amount of work has been done on a super purity aluminium-1% magnesium alloy. It is shown that there exists directly after cold working at room temperature a subgrain structure, the subgrains of which become increasingly perfect on annealing. This increase in perfection of the subgrains is accompanied by a decrease in hardness and it is found that the percentage softening brought about by this recovery process is about 50–60% irrespective of the amount of cold work. Furthermore it is shown that recovery can proceed both before and at the same time as recrystallization. The subgrain size decreases with increasing cold reduction, decreasing annealing temperature and with additions of magnesium. It is concluded that during recovery dislocations within the subgrains diffuse, those of opposite sign cancelling each other out and others diffusing into the subgrain boundary regions. It is shown that this residual strain energy in the subgrain boundaries is probably the driving force for the growth of recrystallization nuclei.