Kinetics of growth and coarsening of faceted hexagonal precipitates in an f.c.c. matrix—I. Experimental observations

Observations are reported on the precipitation kinetics of a range of Al-Ag alloys under different conditions of aging temperature and supersaturation. For most of each precipitation reaction a large and constant aspect ratio is seen together with the thickening and lengthening rates expected for diffusion-controlled growth. The aspect ratios increase with mean precipitate spacing. The interpretation of the results is that at the start of the reaction thickening is inhibited due to a low density of growth ledges. The thickening inhibition causes an increase in the aspect ratio, continuing until the precipitates, growing on nonparallel {111} matrix habit planes, intersect each other. After intersection, growth ledges are generated and appear to have become sufficiently numerous to allow diffusion limited thickening. However observations of coarsening at both 406°C and at 225°C suggests that the thickening reaction is again inhibited, despite residual precipitate intersections, under the very low driving forces of the coarsening reaction.