PRE-PRECIPITATE CLUSTERS AND PRECIPITATION PROCESSES IN Al-Mg-Si ALLOYS

––The solute clusters and the metastable precipitates in aged Al-Mg-Si alloys have been characterized by a three-dimensional atom probe (3DAP) and transmission electron microscopy (TEM). After long-term natural aging, Mg-Si co-clusters have been detected in addition to separate Si and Mg atom clusters. The particle density of β′′ after 10 h artificial aging at 175°C varies depending on pre-aging conditions, i.e., pre-aging at 70°C increases the number density of the β′′ precipitates, whereas natural aging reduces it. This suggests that the spherical GP zones formed at 70°C serves as nucleation sites for the β′′ in the subsequent artificial aging, whereas co-clusters formed at room temperature do not. Atom probe analysis results have revealed that the Mg:Si ratios of the GP zones and the β′′ precipitates in the alloy with excess amount of Si are 1:1, whereas those in the Al-Mg2Si quasi-binary alloy are 2:1. Based on these results, the characteristic two-step age-hardening behavior in Al-Mg-Si alloys is discussed.