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

Abstract

• The phase evolution of Al-Zn-Mg-Cu alloys with and without Ag addition during ageing at 120 °C and over-ageing at 180 °C is revealed. • The mechanism controlling the early-stage solute clustering and the promoting effect of Ag are elucidated systematically. • The role of solute clusters on the formation of η’ precipitates is clarified by in-situ STEM heat treatment. • With longer ageing time, the precipitate/matrix interface shows an evolution from Zn dominating for the interface between early-stage η’ and Al matrix to Zn and Mg co-segregating for η’/Al and η 2 /Al interfaces. • The role of precipitate/matrix interface in the growth of η’ and η 2 is revealed. • The exact positions of Ag and Cu atoms are specified. 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 η 2 /Al interfaces. The η’/Al interfacial layers are shown to precede the formation of η’, while the η 2 /Al interface is found to be closely related to the thickening process of η 2 and the involved particular atomic movements are specified. Experimental observations and DFT calculations reveal that for η’ and η 2 , 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 η 2 rather than η’. The incorporation of Ag and Cu does not change the defined precipitate structure.