Solute segregation induced sandwich structure in Al-Cu(-Au) alloys

In this work atomic-resolution techniques of high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDXS)-STEM and first-principles calculations have been combined to study a hitherto unreported sandwich structure formed in aged samples of Al-Cu and Al-Cu-Au alloys. This sandwich structure comprises a stack of regularly spaced plates of metastable precipitate phases of GP zones, θ″ and θ′. Within the sandwich structure, the separation between the broad surface of θ′ and its adjacent GP zone, as well as that between two neighbouring GP zones, is always three {002}_α planes. This sandwich structure is observed for θ′ precipitates of various thicknesses. Based on experimental results and calculations, it is inferred that the formation of the sandwich structure is induced by the θ′/Al interfacial segregation of Cu atoms, rather than the misfit associated with θ′ formation. In the sandwich structure formed in the ternary alloy, Au atoms distribute mainly in the central part of θ′ plate, but not at the θ′/Al interface or in the GP zone. Calculations confirm the experimental observations and further indicate that, energetically, Au atoms prefer to substitute for Cu, rather than Al, atoms within θ′.