Abstract: The influence of minor Ag on the precipitation evolution of the Al-4.2Zn-2.8Mg-1.0Cu (wt.%) alloy from early stages to over-aged stages at 150 °C was investigated. Surprisingly, co-precipitation of strengthening phases T′ and η′ are found in both Ag-free and Ag-added alloys. With Ag addition, precipitation of both T′ and η′ is refined and increased, such that the age-hardening capabilities and peak-aged tensile strength are improved. In addition, the quantitative proportion of η′ precipitates increases with the increase of Ag content due to the increase in the (Zn+Cu)/Mg ratio of nucleating particles. The narrowed precipitate-free zones (PFZs) are considered responsible for the undiminished fracture elongation in Ag-added alloys. Essentially, these effects of Ag are closely related to the strong Ag-vacancy and Ag-solute interactions. In over-aged stages, the Ag-added alloys still possess higher hardness values compared to the Ag-free alloy, which is related to precipitate coarsening mechanisms. The Ag-free alloy follows classical coarsening behavior by solid solution mediated diffusion, while the Ag-added alloy follows two possible coarsening mechanisms, coalescence of aggregates and diffusion of atoms. The smaller average size and higher residual number density of precipitates benefited from the slow diffusion-controlled coarsening behavior depending on the precipitate composition characteristics of the two-stage differentiation and the precipitate distribution characteristics of high-density dispersion in early-aged stages could explain why the hardness of Ag-added alloy keeps at a higher level than that of Ag-free alloy even after 1000 h ageing. Meanwhile, the transformation of metastable phases to stable phases is inhibited due to the addition of Ag, such that GP zones, T′, η′, η and T phases coexist even after 14 d of ageing. In terms of phase composition, the addition of Ag decreases the ratio of Mg/(Al+Zn) in T-type phase. For the Ag-added alloy, the sum concentration of Zn+Mg in η′ phase is about 10 at.% higher compared to T′ phase, and η phase continues to have a high sum concentration of Zn+Mg, besides, the Zn/Mg ratio and Cu concentration exhibit obvious differences from T phase.

Key words: Al-Zn-Mg-Cu alloy with low Zn/Mg, Trace Ag, Co-precipitation, Evolution of precipitation, Atom probe tomography