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Abstract

Evolution of precipitate and precipitate/matrix interface in artificially aged Al-Zn-Mg-Cu (-Ag) alloys has been systematically studied. In the early stage of ageing, Ag, as a fast diffuser, can promote the formation of solute pairs and small clusters. Solute clusters are further demonstrated to be able to act as precursors for η’ precipitates by in-situ STEM heating. With prolonged ageing time, the precipitate/matrix interface evolves from the Zn-dominated interface between early-stage η’ and Al matrix to the Zn and Mg co-segregated η’/Al and η₂/Al interfaces. The η’/Al interfacial layers are shown to precede the formation of η’, while the η₂/Al interfaces are found to be closely related to the thickening process of η₂ and the involved particular atomic movements are specified. Experimental observations and DFT calculations reveal that for η’ and η₂, Ag can dissolve into the precipitate as well as locate at the precipitate/matrix interface without showing preference. For Cu, its dissolution in the precipitate and segregation on the interface mainly occur for η₂ rather than η’. The incorporation of Ag and Cu does not change the defined precipitate structure.