Microstructural evolution at the initial stage of two-step aging in an Al-Mg-Si alloy characterized by a three dimensional atom probe

Microstructural evolution at the initial stage of two-step aging in an Al-0.9 Mg-1.0 Si (mass%) alloy was studied using 3DAP. The chemical composition, cluster size, number density, and volume fraction change were analyzed through the maximum separation method. This study confirmed that the clusters with Si-rich and Balanced compositions accounted for 97% of the total, and the larger the size, the more similar to the composition of the alloy. At two-step aging at 170 °C for 0.6 ks, the smaller and lower number density clusters were formed compared with the clusters formed after natural aging for 2419.2 ks. Thus, we found that the dissolution of the nanocluster predominantly occurred at the initial stage of two-step aging at 170 °C after natural aging for 2419.2 ks. Then, the cluster size was increased and the formation of pre-β" was dominant during two-step aging for 1.2 ks. The pre-β″, nuclei of β″, formed is considered to be similar to the Mg:Si ratio of β″. In order to investigate the internal structure change of clusters at the initial stage of two-step aging, the atomic number density of clusters with an average size of 2 nm or more was analyzed. The atomic number density for the two-step aging at 170 °C for 1.2 ks was higher than 30 nm-3, while the atomic density of natural aging for 2419.2 ks was lower than 30 nm-3. In Al-Mg-Si alloys, we propose for the first time that a nanocluster can act as the nucleus of precipitate, pre-β ", with a size and an atomic density of more than 2 nm and 30 nm-3. In addition, the relationship was analyzed between the hardness change, the volume fraction, and the number density in the initial stage of two-step aging.