The effect of Ag addition on the microstructure evolution in peak aging and retrogression and re-aging processes of an Al-Zn-Mg-Cu-Zr alloy

Abstract

The evolution of precipitates in a novel Al-Zn-Mg-Cu-Zr-Ag alloy during artificial aging was experimentally studied using density functional theory (DFT) calculations and experimental methods. DFT calculations indicate that Ag plays an important role in enhancing the strength of the alloy through the Mg-Ag and Mg-Zn-Ag clusters. Transmission Electron Microscopy (TEM) and Three-Dimensional Atom Probe (3DAP) results reveal that in early aging, Ag quickly combines with Mg and Zn atoms to form solute clusters, promoting nucleation and second-phase precipitation. This leads to a rapid increase in hardness during the early aging stage, reaching peak aging. After retrogression and re-aging, a large amount of needle-shaped GPII precipitates form, which directly grow and transform into η'. Subsequently, the precipitate phase accumulates layer-by-layer in a disc-like manner perpendicular to the long interface. Additionally, Zr and Ag-alloying improves the second phase's stability, limits coarsening during retrogression and re-aging, and reduces grain boundary width. Furthermore, we found that retrogression and re-aging increase the interaction between different orientations of the second phase, allowing the alloy to maintain high mechanical properties and achieve high conductivity. The findings offer guidelines for the development of high-strength and high-conductivity aluminum alloys.