Three-Dimensional Atom Probe Characterization of Nanoclusters Responsible for Multistep Aging Behavior of an Al-Mg-Si Alloy

The characterization of nanoscale clusters (nanoclusters) was performed using differential scanning calorimetry (DSC) and a three-dimensional atom probe (3DAP), to clarify the complicated aging behavior of an Al-Mg-Si alloy. The DSC results conducted over the temperature range 223 to 473 K revealed that two types of nanoclusters, i.e., Cluster(1) and Cluster(2), were formed near room temperature (RT) and 373 K, respectively. In the present work, the quantitative estimation of atom maps of the 3DAP analysis revealed the difference in the growth mechanism and the composition distribution of the two types of nanoclusters. The distribution of both the size and Mg/Si ratio of Cluster(1) does not change during prolonged natural aging. On the other hand, Cluster(2) grows gradually with preaging time. The Mg/Si ratio of the larger-sized Cluster(2) approaches a constant value that is equal to that of the β″ phase. The difference in the two-step aging behavior can be explained by the different growth mechanisms and chemical compositions of the two nanoclusters. This means that only Cluster(2) can easily transform continuously into the β″ phase during the BH treatment, due to its size and compositional similarity.