Quantifying the strength-ductility trade-off induced by precipitation evolution in an over-aged Al–Mg–Si alloy

The strength and ductility are generally considered to exhibit an inverse relationship with aging time in Al-Mg-Si alloys. However, differences in the contributions of strengthening effects and ductility within multi-precipitates coexisting systems may lead to variations in this trend. In this study, the influence of multi-precipitates on the evolution of strength and ductility with aging time has been quantitatively analyzed in Al-Mg-Si rolled sheets. Quantitative characterization and computational simulations reveal that differing strength effects of β" and β' phases lead to a rapid decrease in strength after peak aging. Meanwhile, the increase in precipitate volume fraction during the early over-aging stage also reduces ductility. Once phase transformations are completed and the alloy enters the β' coarsening stage, the ductility gradually increases, indicating that the volume fraction of β' also significantly affects the ductility of the Al-Mg-Si alloy. Furthermore, the surface Mg oxide layer primarily forms during solid solution treatment and exhibits no significant changes during low-temperature aging heat treatment. This indicates that no substantial element diffusion occurs during heat treatment, thus having no significant impact on the ductility of the alloy. These findings could provide theoretical support for selecting suitable heat treatment processes for these alloys.