Evolution of precipitate and precipitate/matrix interface in Al-Zn-Mg-Cu (-Ag) alloys

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