Strong interaction of an Al2Cu intermetallic precipitate with its boundary

NMR, X-ray diffraction (XRD) and transmission electron microscopy (TEM) experiments have been undertaken to establish the nature of Ω-platelets which form during heat treatment of aluminium alloys containing Cu, Mg (Mg lean) and Ag of the order of 0.1 at. % 1]. The platelets lie on (111) planes of the Al host lattice, separated from the Al on either face by a thin layer, one or two atoms thick, of Mg and Ag atoms. At temperatures between 185°C and 250°C the platelets have been previously shown to coarsen (thicken) slowly with time but more rapidly at 300°C 2,3]. TEM observations are described which confirm that the platelets remain on (111)_α for heat treatments up until at least 5?h at 300°C. The NMR and XRD results indicate that for thick platelets the bulk of the platelet material, sufficiently distant from the two bounding interfaces, is exactly tetragonal Al₂Cu θ-phase, but that platelets of the order of 2–4?nm thick (e.g. 100?h at 185°C) have a structure strongly influenced by interaction with the platelet boundary, which removes the axial symmetry of the Cu atom. Both NMR and XRD observations have shown a gradual transition between these two limits.