The growth of γ plates in an Al-15% Ag alloy

Abstract

In order to test theories of the formation of plate-shaped precipitate morphologies, the mechanisms and kinetics of both thickening and lengthening have been studied in h.c.p. γ plates grown in an Al-15% Ag alloy. Hot-stage electron microscopy was the major tool employed. It was observed that thickening was accomplished by the passage of partial dislocations (ledges) across the γ broad faces, rather than by the uniform advance of these faces. Ledge velocity, however, was usually controlled by volume diffusion. Edges of the plates were observed to be faceted. Consistent with this morphological observation, lengthening also occurred by a ledge mechanism; the movement of the ledges, from 100 to 500 Å in height, was easily visible in the hot-stage experiments. The measurements of both thickening and lengthening have been successfully interpreted in terms of recent ledge theories of growth.