Impact of Sn addition and Mg:Si ratio on the natural aging and subsequent artificial aging characteristics of Al–Mg–Si alloys

The impact of Sn on the natural aging (NA) and artificial aging (AA) behavior of the Al–Mg–Si alloys with high (1.7), balanced (1.0), and low (0.6) Mg:Si ratios was explored via hardness testing, transmission electron microscopy, and atom probe tomography (APT). Additionally, density functional theory (DFT) studies were also undertaken, which revealed a reduction in the binding energy of the Sn-vacancy within the Sn-containing clusters as a result of increasing the Mg atoms within the clusters, indicating that more trapped vacancies can be released to accelerate the development of NA clusters. The APT results revealed that the Mg:Si ratio of NA clusters increased with an increase in the Mg:Si ratio of the alloys. This, together with the DFT results, is supposed to be the reason for the decreasing Sn-induced NA suppression effect with an enhancement in the Mg:Si ratio of the investigated alloys. The APT results revealed that the Sn atoms increased the Mg:Si atomic ratio of the NA clusters. Therefore, addition of Sn led to a greater deviation in this ratio for the NA clusters in alloys with a high and balanced Mg:Si ratio, compared to the β″ phase, resulting in suppressed precipitation of β″ during subsequent AA. However, the introduction of Sn resulted in a Mg:Si atom ratio for the NA clusters that was closer in value to that of the β″ phase in alloys with a low Mg:Si ratio and promoted the precipitation of β″ during subsequent AA.