Abstract
Needle-shaped precipitates and atomic clusters were evaluated in two model Al-Mg-Si alloys with different Mg/Si ratios by transmission electron microscopy (TEM) and atom probe tomography (APT). Following a solution heat treatment with rapid quenching, specimens were naturally aged for about 3 month and then artificially aged for various times at 170 °C. In the peak-aged condition (aged for 86.4 ks) the Si-rich alloy (6M9S) produced a higher fraction of perfect β" phase with a narrower size distribution than the Mg-rich alloy (8M7S). This resulted in a Mg-depleted matrix in the 6M9S alloy. Clusters with a mean radius of about 1 nm were observed in-between the needle precipitates in both alloys. Their number density was clearly higher in the 8M7S alloy than in the 6M9S alloy in the peak-aged condition. The number fraction of Mg-rich clusters is increasing in the Mg-rich alloy with prolonged aging time, indicating a higher stability of these clusters. Finally, the mechanical properties of the two alloys were predicted using precipitate and cluster statistics measured by APT and TEM. These simulations indicate that clusters present in-between the needle precipitates contribute significantly to strength.