Investigation of secondary phases evolution and mechanical properties of ultra-high strength aluminum alloy driven by Cu element

Abstract

The synergistic effect of multi-scale secondary phases is the primary source of the excellent mechanical properties for ultra-high strength aluminum alloys. Investigating the role of Cu, a key element in secondary phase formation, is fundamental to achieving precise microstructure-property control. In this study, Al-10.50Zn-2.35Mg-xCu-0.12Cr-0.1Mn-0.1Zr-0.06Ti alloys (x = 0.0, 1.3, 2.2 wt.%) were selected to examine the influence of Cu content on the morphology of secondary phases, elemental distribution, and mechanical properties in both single-stage aging (T6) and retrogression and re-aging (RRA) conditions. The results indicate that, compared to the 0Cu sample, the 1.3 wt.% Cu alloy exhibits a significant increase in the volume fraction of precipitates in the T6 condition, with a transformation of the dominant precipitates from GPI zones to GPII and η′ phases. Consequently, the ultimate tensile strength (UTS) increases from 739 ± 2 MPa to 779 ± 3 MPa, with minimal change in elongation. However, in the 2.2 wt.% Cu alloy, the T6 treatment leads to the formation of micron- and submicron-sized σ phases (Al, Zn, Mg, Cu), consuming solute elements necessary for nanoscale precipitate formation. This reduces the nucleation sites for early-stage precipitates and promotes precipitate coarsening, ultimately leading to a decrease in both strength and ductility. In the RRA condition, the primary precipitate in the 0Cu alloy is the T′ phase. The addition of Cu facilitates the transformation of T′ to η′ phase while inhibiting the η′ to η phase transition and suppressing precipitate coarsening. With increasing Cu content, the average precipitate size in all three RRA alloys decreases, and the UTS gradually improves. Furthermore, Cu content exhibits a positive correlation with UTS_RRA/UTS_T6, suggesting that increasing Cu content can enhance the strength of the RRA-treated samples beyond that of the T6-treated ones.