Effect of Ag addition on the precipitation evolution and interfacial segregation for Al?Mg?Si alloy

Abstract

The effect of Ag addition on the precipitation evolution and interfacial segregation for Al–Mg–Si alloys was systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), atom probe tomography (APT) and density functional theory (DFT) calculation. At the early aging stage, Ag atoms could enter clusters and refine the distribution of these clusters. Then, Ag atoms preferentially segregate at the GP zone/α-Al and β"/α-Al interfaces at the peak aging stage by the replacement of Al atoms in FCC matrix. With prolonging aging time, Ag atoms generally incorporate into the interior of β" precipitate, facilitating the formation of QP lattice (a hexagonal network of Si atomic columns) and the local symmetry substructures, Ag sub-unit (1) and Ag sub-unit (2). At the over-aged stage, the Ag sub-unit (1) and Ag sub-unit (2) could transform to the β′Ag (i.e. β′Ag1 and β′Ag2.) and Q′Ag unit cells, respectively. All the precipitates at the over-aging stage have a composite and disordered structure due to the coexistence of different unit cells (β′Ag1, β′Ag2, Q′Ag and β′) and the non-periodic arrangement of Ag atoms within the precipitate. In the equilibrium stage, the incorporated Ag atoms in the precipitates release into the α-Al matrix as solute atoms or form Ag particles. In general, Ag atoms undergo a process of “segregate at the precipitate/matrix interface → incorporate into the interior of precipitate → release into the α-Al matrix” during the precipitation for Al–Mg–Si–Ag alloys. Besides, Ag segregation is found at the interfaces of almost all metastable phases (including GP zone, β″, β′/β′Ag phase) in Al–Mg–Si–Ag alloys. The Ag segregation at the β′/α-Al interface could increase the length/diameter ratio of β′ phase and thus promote the additional strengthening potential of these alloys. These findings provide a new route for precipitation hardening by promoting the nucleation and morphology evolution of precipitates.