Clustering evolution during low temperature aging and thermal stability during two-step aging in Al-Mg-Si alloys

The age-hardening response during two-step aging in Al-Mg-Si alloys depends on the nanoclusters formed at the early stage of phase decomposition. However, understanding of the evolution and thermal stability of the nanoclusters is still lacking. This paper discusses clustering behavior during natural aging (NA) and pre-aging (PA) at 50 ℃ through various experiments [hardness, differential scanning calorimetry (DSC), electrical resistivity and three-dimensional atom probe (3DAP)]. A rapid increase in hardness is confirmed due to the acceleration of cluster formation during PA, and a larger area of an endothermic peak is confirmed during DSC thermal analysis compared to NA specimens. Hardness and electrical resistivity were clearly decreased in the PA specimens at the initial stage of two-step aging since the formation of thermally unstable clusters, the Mg-enriched clusters identified by 3DAP, was accelerated during PA. A long aging time at 50 ℃ produces Mg-enriched clusters rather than facilitating the transition from Cluster (1) to Cluster (2). To investigate the atomic arrangement of a cluster, we conducted further analysis for chemical composition of clusters through the normalization for the different sizes of cluster. We found that the Mg-rich clusters with the core-shell structure where Mg atoms are segregated around the cluster-matrix interface, cause thermally unstable at the initial stage of two-step aging. The direct experimental evidence primarily proves that atomic arrangement inside a cluster is a more critical factor than its size for the thermal stability of a cluster in Al-Mg-Si alloys.