Evolution of strengthening precipitates in Al-4.0Zn-1.8Mg-1.2Cu (at%) alloy with high Zn/Mg ratio during artificial aging

Evolution of strengthening precipitates, including their types, sizes, volume fraction, and chemical compositions in Al-4.0Zn-1.8Mg-1.2Cu (at%) alloy during artificial aging, were investigated using anomalous small-angle X-ray scattering (ASAXS), three-dimensional atom probe techniques (APT), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and density functional theory (DFT) calculation. Upon aging, Cu-containing clusters merge with adjacent clusters to form larger clusters, which then act as nucleation sites for GP _{η`</sub> zones. As aging proceeds, GP <sub>η`} zones grow up to η` <sub>I</sub> metastable phase with seven atomic layers on {111} <sub>Al</sub> planes, and this process is controlled by diffusion of Mg atoms. Cu atoms have a low diffusion rate and do not participate into the growth of precipitates within the first 60 mins of aging. As aging time increases, Cu atoms enter the η` _I metastable phase, leading to a significant increase in Cu content and (Zn+Cu)/Mg ratio; η` <sub>I</sub> metastable phase grow, and after exceeding about 7 nm, gradually transform into the η` _II metastable phase with ten atomic layers by adding three new atomic layers at one side. GP _{η`</sub> zones, η` _I metastable phase and η` <sub>II</sub> metastable phase have symmetric interface structures. The precipitation sequence during aging to peak state can be summarized as: atomic clusters → GP <sub>η`} zones → η` <sub>I</sub> metastable phase → η` _II metastable phase.