Effect of Cu on precipitation hardening and clustering behavior of Al-Zn-Mg alloys in the early stage of aging

The effects of Cu (0, 1, 2.6 wt%) on precipitation hardening and clustering behavior of Al-9.0Zn-1.5Mg (wt%) alloys in the early stage of aging at 120 °C were investigated by hardness tests, in-situ small-angle X-ray scattering (SAXS), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), atom probe tomography (APT) experiments, differential scanning calorimeter (DSC) tests, positron annihilation lifetime spectroscopy (PALS) and density functional theory (DFT) calculations. A higher Cu addition leads to a higher hardening response rate. In the alloy without copper, Zn-Mg and Zn-Zn clusters contribute to rapid hardening, and their growth is governed by the diffusion of Mg. The addition of 1 % Cu accelerates the annihilation of vacancies at grain boundaries and inhibits the diffusion of Zn and Mg, thereby reducing the growth rate of clusters; however, it can promote the formation of Cu-containing clusters, which have a better modulus hardening effect than those without Cu, and thus enhance hardness. The addition of 2.6 % Cu leads to a further increase of the number density of Zn-Cu clusters, and decreases significantly the average distance between them. Many of these clusters then contact with neighboring clusters by kicking out Zn through internal vacancies and then combine together to quickly grow to a larger cluster. The higher density and larger size of the clusters in this alloy than other two alloys contribute to higher hardness.