Solute nanostructures and their strengthening effects in Al–7Si–0.6Mg alloy F357

Abstract The solute nanostructures formed in the primary α-Al grains of a semi-solid metal cast Al–7Si–0.6Mg alloy (F357) during ageing at 180 °C, and the age-hardening response of the alloy, have been systematically investigated by transmission electron microscopy, atom probe tomography and hardness testing. A 120 h natural pre-ageing led to the formation of solute clusters and Guinier–Preston (GP) zones. The natural pre-ageing slowed down the precipitation kinetics 6-fold during 1 h ageing at 180 °C, but this effect diminished after 4 h when the sample reached the same hardness as that without the pre-ageing treatment. It reduced the number density of β″ needles to approximately half of that formed in samples without the treatment, and postponed the peak hardness occurrence to 4 h, four times that of the as-quenched sample. A hardness plateau developed in the as-quenched sample between 1 and 4 h ageing corresponds to the growth of the β″ precipitates and a significant concurrent decrease of solute clusters and GP zones. The average Mg:Si ratio of early solute clusters is