Solute clustering in Al-Mg-Si-Cu-(Zn) alloys during aging

Abstract We report on a study of the role of solute clustering and partitioning in an Al-Mg-Si-Cu alloy containing 1.0 wt % Zn. Our results demonstrate that the addition of 1.0 wt % Zn enhances the hardening response as evidenced by an increase of 65 and 135 MPa following natural aging (T4) and pre-aging (T4P) treatments, respectively. Using results obtained from atom probe tomography we report, for the first time, that co-clusters comprised of Mg, Si, Cu and Zn atoms were formed in the Zn-containing alloy. Moreover, the composition, number density, distribution and clustering kinetics of these co-clusters are all different from those associated with Al-Mg-Si-Cu. During natural aging, the aggregation of Mg and Si atoms in the Al-Mg-Si-Cu alloy can be reduced by the addition of Zn, but increasing the Mg/Si ratios in the co-clusters facilitates the nucleation of precipitates. During pre-aging, a higher Si concentration appears in the co-clusters in the two alloys, especially for the Zn-containing alloy, while Zn concentration in the solute-rich features decreases slightly with increasing their sizes. Our findings demonstrate that Zn additions facilitate the formation of fine co-clusters containing Mg, Si, Cu and Zn atoms, which are believed to be responsible for the enhanced hardening response documented during the T4 and T4P aging treatments. The partitioning of solute atoms and precipitation kinetics of the alloys are discussed in detail.