Influence of interfacial properties on the kinetics of precipitation and precipitate coarsening in aluminium-silver alloys

Abstract

The lengths and thicknesses of hexagonal Ag 2Al precipitates in AI-Ag were studied under two conditions; firstly during free precipitate growth in supersaturated solid solutions before soft impingement and secondly during Ostwald Ripening. Measurement of the lengths and thicknesses of freely growing precipitates by optical microscopy and replica electron microscopy showed agreement with the kinetics expected for diffusion controlled growth of plate-like precipitates with the observed very large aspect ratios. However a study of the precipitates at the start of precipitation by transmission electron microscopy (TEM) showed initial inhibition of the thickening reaction which led to the large aspect ratios observed. This latter observation supports Aaronson's theory of precipitate morphology. The study of Ostwald Ripening showed two effects: firstly the retention of the large- non-equilibrium, aspect ratios and secondly a rapid increase of the mean precipitate length. A model, that successfully accounted for these observations was developed. The model was based on two assumptions, inhibited thickening at the low supersaturations of Ostwald Ripening, but diffusion limited lengthening.