Characterization of gamma plate-shaped precipitates in an Al-4.2 at.% Ag alloy - Growth kinetics, solute field, composition and modeling

The lengthening and thickening rates of gamma plate-shaped precipitates in an Al-4.2 at.% Ag alloy were determined by transmission electron microscopy (TEM) for aging times of 120-720 s at 400 degreesC. The compositions of th e gamma plates and the solute fields surrounding them were measured quantit atively using energy-dispersive X-ray spectroscopy (EDS) and revealed quali tatively by energy-filtering TEM (EFTEM). The shape of the solute field aro und the plates was also revealed by employing a double-step aging treatment to produce GP zones in the supersaturated a matrix around the precipitates , but not in the solute-depleted zone surrounding the plates. The combined data obtained from these measurement were then compared with the Ham, Horva y and Cahn (HHC) model for precipitate growth. The results show that: (1) t he lengthening rate of the plates ranged from 77 nm/s at 120 s aging to 15 nm/s at 720 s aging, the thickening rate ranged from 0.2 nm/s at 120 s agin g to 0.09 nm/s at 720 s aging, and the aspect ratio increased rapidly with time; (2) the solute gradient around the plates is approximately two plate thicknesses wide and has a faceted-elliptical shape for all aging times exa mined; (3) the solute concentration in the matrix immediately adjacent to t he faces of the plates is higher than adjacent to the edges, indicating inh ibited growth at the faces, (4) the Ag concentration in the gamma plates is significantly lower than that given by the equilibrium Al-Ag phase diagram ; and (5) the HHC model for the growth of an oblate spheroidal precipitate predicted the lengthening and thickening rates of the plates reasonably wel l when the time dependence of the aspect ratio was included in the calculat ions. (C) 2000 Elsevier Science Ltd. Published by Elsevier Science Ltd. All rights reserved.