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 Omega-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 degrees C and 250 degrees C the platelets have been previously shown to coarsen (thicken) slowly with time but more rapidly at 300 degrees C [2,3]. TEM observations are described which confirm that the platelets remain on (111)(alpha) for heat treatments up until at least 5 h at 300 degrees 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 theta-phase, but that platelets of the order of 2-4 nm thick (e.g. 100 h at 185 degrees 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.