The solute clusters and the metastable precipitates in aged Al-Mg-Si alloys have been character- ized 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 l75℃ varies depending on pre-aging conditions, i.e. pre-aging at 70℃ increases the number density of the β"precipitates, whereas natural aging reduces it. This suggests that the spherical GP zones formed at 70℃ serve as nucleation sites for the β" in the sub- sequent 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-Mg_2Si quasi-binary alloy are 2:1. Based on these results, the characteristic two-step age-hardening behavior in Al-Mg-Si alloys is discussed.
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