Natural aging behavior in pre-aged Al–Mg–Si–Cu alloys with and without Zn addition

This study investigates the clustering behavior and its correlation with the change in hardness during natural aging (NA) after pre-aging (PA) at 80 °C for 15 min in Al–Mg–Si–Cu alloys with and without Zn addition. The results show that NA after PA causes the growth of clusters in the Zn-free and Zn-added alloys, which is related to the partitioning of solute atoms from the Al matrix into clusters. Si exhibits considerably stronger partitioning than Mg during NA after PA, which leads to an increase in the number of Si-rich clusters. Cu can also be partitioned into clusters of two alloys when the NA after PA is conducted. Zn is present in the clusters of the Zn-added alloy during NA after PA. The addition of Zn increases the growth rate of clusters. In addition, the increased volume fraction of clusters explains the increased hardness of the Zn-free and Zn-added alloys during NA after PA. Compared with the Zn-free alloy, the Zn-added alloy exhibits a more significant change in volume fraction of clusters, thus achieving an enhanced age hardening response.