Characterization of γ plate-shaped precipitates in an Al–4.2 at.% Ag alloy — growth kinetics, solute field, composition and modeling

Abstract The lengthening and thickening rates of γplate-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°C. The compositions of the γ plates and the solute fields surrounding them were measured quantitatively using energy-dispersive X-ray spectroscopy (EDS) and revealed qualitatively by energy-filtering TEM (EFTEM). The shape of the solute field around the plates was also revealed by employing a double-step aging treatment to produce GP zones in the supersaturated α 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, Horvay and Cahn (HHC) model for precipitate growth. The results show that: (1) the 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 aging 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 examined; (3) the solute concentration in the matrix immediately adjacent to the faces of the plates is higher than adjacent to the edges, indicating inhibited growth at the faces, (4) the Ag concentration in the γ 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 well when the time dependence of the aspect ratio was included in the calculations.