Effect of pre-ageing on nucleating of GP zones and precipitation, strength and stress corrosion properties of 7N01 alloy

Three ageing treatments, known as T6, T74, and P-T74 (a pre-ageing treatment followed by a T74 treatment) were carried out to investigate their effects on aluminium alloy 7N01. The aspects investigated included the alloy's mechanical properties, microstructure, electrochemical behaviours, and resistance to stress corrosion cracking (SCC). The experimental results showed that in comparison with T74, P-T74 can effectively increase the density of GPI zones in the early stage giving rise to more nucleation sites for the subsequent eta ' phase. T74 guarantees more GPII zones than does T6, but only a fraction can transform into the eta' phase, which leads to the lowest hardness and mechanical strength of all treated alloys studied here. Compared with the T6 and T74 ageing treatments, P-T74 showed the highest self-corrosion potential, lowest self-corrosion current density, and largest spacing between the eta phase along the grain boundaries, which was an indicator of its increased resistance to SCC.