Unravelling age hardening behavior of multicomponent Al₅₀Mg₃₀Si₁₀Cu₅Zn₅ alloy using TEM, APT, and SANS

We investigate the microstructural evolution and precipitation behavior of multicomponent Al₅₀Mg₃₀Si₁₀Cu₅Zn₅ alloy using transmission electron microscopy (TEM), atom-probe tomography (APT), and small-angle neutron scattering (SANS). The as-cast alloy contains primary Mg₂Si, S–Al₂CuMg, and Al/η-Mg(Zn,Cu,Al)₂ eutectic phases, with a total fraction of ∼65%. Solution treatment at 450 °C causes partial dissolution and spheroidization of secondary phases. Natural aging induces the precipitation of Guinier-Preston (GP)I and GPII zones with radii of ∼1 nm, resulting in increased microhardness and decreased electrical conductivity. The maximum hardness is achieved after 1 h of artificial aging at 120 °C, due to the formation of GPI and GPII zones. Artificial aging for 8 h causes the formation of GPII zones and η′ precipitates, which contribute to maintaining the high hardness. The chemical composition of GPII zones and η′ precipitates is measured to be 45.53Al–23.89 Mg–28.91Zn–1.33Cu (at.%) using APT. The average radii of the precipitates measured by TEM, APT, and SANS correspond to 2.0, 1.1, and 1.7 nm, respectively. APT and SANS analyses show volume fractions of 1.8% and 2.6% for the GPII zones and η′ precipitates. The overaging for 1000 h causes the formation of coarse η nanoprecipitates (∼3.7 nm by SANS), which reduces microhardness and increases electrical conductivity. The strengthening mechanism of precipitation-strengthened Al₅₀Mg₃₀Si₁₀Cu₅Zn₅ alloy is discussed based on the quantitative analysis of nanoprecipitates.