The role of trace Ag in controlling the precipitation and stress corrosion properties of aluminium alloy 7N01

Abstract

The effect of trace Ag on the early precipitation and the resistance to stress corrosion cracking (SCC) of peak-aged 7N01 alloy (Al-4.5Zn-1.55Mg-0.12Cu) were investigated. A characterization method known as three-dimensional atom probe (3DAP) was used in this research to investigate the precipitation behaviors of the alloy. When 0.12 wt % Ag is present in the alloy, the solid solubility of the alloy increases after the solid-solution treatment at 485 °C and quenching in water. In the early stage when the alloy is aged at 120 °C, Ag atoms will enrich around the clusters (GPI zones and GPII zones) giving rise to more nucleation sites for them. As the ageing time goes on, these clusters will transform into high-density η′ phase and then the hardness and mechanical strength of the alloy will increase. Three kinds of precipitates with different sizes in the alloy after peaking ageing were found by high resolution transmission electron microscopy (HRTEM). The results obtained from the high angle annular dark field of scanning transmission electron microscope (HAADF-STEM) show that Ag atoms are distributed around η′ phase in the matrix and η phase in the grain boundaries, which significantly increases the corrosion resistance of the alloy in NaCl solution after peak ageing. The results from the slow strain rate test (SSRT) and the observation of fracture morphologies show that for the alloy with trace Ag addition and after peak ageing, the resistance to SCC is improved, as well as the size and the distribution distance of the precipitate (MgZn2) in the grain boundaries increase.