Precipitation strengthening mechanisms during natural ageing and subsequent artificial aging in an Al-Mg-Si-Cu alloy

An Al-Mg-Si-Cu alloy was water quenched after solute heat treatment at 540 ℃ for 1 h, followed by natural aging and subsequent artificial aging. The hardness measurement and tensile tests show that the hardness and yield strength increased slightly after 7 days natural aging. However, applying natural aging could decrease the peak hardness and yield strength during subsequent artificial aging. For peak aged condition, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) observation revealed that the β" precipitates in the natural aged sample are much smaller in size and volume fraction than that of sample without natural aging, resulting in a better elongation and a decline of peak-aging hardness and yield strength. It seems that the formation of Mg-Si clusters induced by natural aging is beneficial for the hardness and yield strength, and the clusters still occupied a mount of Mg, Si atoms and quenched-in vacancies during artificial aging, which inhibited the precipitation and growth of β" phase. Moreover, the precipitation strengthening effect of the β" phase was quantitatively evaluated by Orowan mechanism.