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The effects of aging temperature (25–220 °C) on precipitation behavior and the resulting alloy strength were characterized for the Al–6Mg–9Si–10Cu–10Zn–3Ni (wt%) natural composite. At an aging temperature of 25 °C, spherical and ellipsoidal Guinier–Preston (GP) zones (Zn clusters) are formed within the Al matrix. The fastest kinetics of Zn clustering is exhibited at 70 °C, which is caused by the combined effects of accelerated Zn diffusion and decreased phase thermal stability with increasing aging temperature. Thus, the Zn clusters and precipitates are formed prior to those of Cu (GPI and θ'') at 70 °C. As the precipitation kinetics of θ accelerates and Zn decelerates at above 70 °C, the precipitation sequence changes at 170 °C; clusters and precipitates of Cu are formed prior to those of Zn at this temperature. At an aging temperature of 220 °C (above the solvus of hcp Zn), θ'-Al 2 Cu and Q′-Al 4 CuMg 6 Si 6 phases are mainly precipitated instead of Zn clusters and precipitates. The matrix strengthening induced by the fine clusters and precipitates that form below 70 °C results in a significant improvement in the strength of the composite. However, high-temperature aging has only a small strengthening effect due to the lower matrix hardening caused by precipitate coarsening and the softening caused by the decreased volume fraction and spheroidization of the secondary phases. [ABSTRACT FROM AUTHOR]

We investigated the microstructural evolution and precipitation behavior of a multicomponent Al 83 Zn 5 Cu 5 Mg 5 Li 2 alloy using transmission electron microscopy (TEM) and atom-probe tomography (APT) during solution treatment, cooling, and natural aging. The as-cast alloy consisted of high volume fractions of V-Al 5 Cu 6 Mg 2 and η -Mg(Zn,Cu,Al) phases, which were partially dissolved during solution treatment at 450 °C. Slow furnace cooling (FC) after solution treatment resulted in the formation of polygonal V-phase, elongated η -phase, and platelet Y-phase precipitates. The Y-phase precipitates consisted of one Mg-rich central layer and three adjacent Zn/Cu-rich layers, both lying on the {111} Al planes. Instead of Y-phase precipitates, air cooling (AC) induced the formation of S- and Zn-phase precipitates with well-developed orientation relationships. No precipitation occurred during water quenching (WQ). The microhardness increased significantly during the natural aging of the AC and WQ alloys, whereas no hardening was observed in the FC alloy. TEM and APT analyses demonstrated that the natural aging induced the formation of fine solute clusters with a chemical composition of 52.3Al–21.3Li–18.3Zn–4.9Mg–3.1Cu (at%), contributing to the natural age-hardening of AC and FC alloys. The solute clusters were fully coherent with the Al matrix and exhibited an interfacial region rich in Zn and two central regions rich in Li and Zn. • Al 83 Zn 5 Cu 5 Mg 5 Li 2 cast alloy consists of highly interconnected V and η phases. • Cooling rate-dependent precipitation of V-, η-, Y-, S-, and Zn-phases is confirmed. • Y-phase comprises a Mg-rich layer and adjacent Zn/Cu-rich layers lying on {111} Al. • Li- and Zn-rich coherent solute clusters contribute to natural age-hardening. [ABSTRACT FROM AUTHOR]