The precipitate Ω phase in Al-Cu-Mg-Ag alloys

Abstract

The combined addition of small concentrations of Ag and Mg to Al-Cu alloys promotes precipitation of a phase, designated Ω, that forms as thin, hexagonal-shaped plates on matrix {111} α planes. The structure, morphology and composition of this phase have been examined in two quaternary alloys using transmission electron microscopy, electron microdiffraction and energy dispersive X-ray spectroscopy. Electron microdiffraction patterns from the precipitate phase may be indexed in accordance with an orthorhombic structure ( a = 0.496 nm, b = 0.859 nm, c = 0.848 nm) and the orientation relationship between precipitate and matrix lattices is such that (001) Ω∥(111) α and [010] Ω∥[101̄] α. The morphology of the precipitate phase is consistent with the intersection point group ( 2 m ) defined by symmetry elements common to the two lattices in the observed orientation relationship. The plate shape parallel to (111) α is determined by a pinacoid parallel to the common 2-fold axis, [010] Ω. The hexagonal form in this plane is defined by four equivalent prism facets with a common direction perpendicular to [010] Ω and the truncation of this prism section by a second pinacoid normal to this 2-fold axis. The six facets thus defined are equivalent geometrically and may be associated with the hexagonal lattice on which the orthorhombic structure of the precipitate phase is based. Energy dispersive X-ray microanalysis indicates that the Ag in quaternary alloys partitions to the Ω phase during ageing and there is evidence that it may segregate at the interface with the matrix. On the other hand, Ag does not partition to the phase θ′ when it coexists with Ω in these alloys or is present alone in the ternary alloy Al-Cu-Ag. There is also evidence to suggest that Mg associates itself with the Ω phase, but the observations are not conclusive.