Formation of solute nanostructures in an Al-Zn-Mg alloy during long-term natural aging

Nature aging (NA) is believed to produce solute nanostructures, including solute clusters and GPI zones in Al-Zn-Mg alloys. Here we report the first observation that GPII (GP(eta)') zones with an average Zn/Mg ratio near 1.3-1.4 formed in the Al-Zn-Mg alloys during long-term natural aging. High-angle-annular-dark-field scanning transmission electron microscopy and atom probe tomography were used to study the evolution of solute nanostructures during natural aging. Early clusters were found to have a wide spread of Zn/Mg ratios and their number density increased sharply with a dramatic hardening effect during the first month ageing. Further ageing up to 3 months made the Zn/Mg ratios of the solute clusters gradually converge to near 1.2 (GPI zones), but produced no change in the morphology, size distribution and volume fraction of the solute nanostructures, as well as hardness. At room temperature, GPII (GP(eta)' type) zones formed very slowly and provided stronger hardening effects than early clusters. Importantly, unlike solute-rich clusters, GPI and GP(eta)' zones are easy to transform into (eta)' during subsequent artificial ageing, avoiding hardness drop at the early stage of subsequent artificial aging at 120 degrees C. (C) 2019 Elsevier B.V. All rights reserved.