Nanocluster formation and two-step aging behavior in Al–Mg–Si(-xCu: X = 0–4 mass%) alloys

Abstract The effects of Cu addition on the clustering and two-step aging behavior in Al–Mg–Si(-Cu) alloys were analyzed by differential scanning calorimetry (DSC), the Vickers hardness and tensile tests, the three dimension atom probe (3DAP) and the transmission electron microscope (TEM). A high amount of Cu up to 4 mass % was added to the Al–Mg–Si alloys. Several types of clusters, namely, Cu–Mg, Cu–Si, Cu–Cu and Mg–Si–Cu, after natural aging for 2419.2 ks were formed in the Al–Mg–Si–Cu alloys. The size distribution and chemical composition of the clusters were characterized using 3DAP based on the maximum separation method. The cluster formation was promoted by 1Cu addition, but it was suppressed by 2Cu and 4Cu additions based on the analyses of cluster formation temperature and activation energy. Thermally unstable clusters were confirmed at the early stage of two-step aging at 170 °C after natural aging for 2419.2 ks. The amount of thermally unstable clusters was increased based on the hardness results. The peak hardness during single aging at 170 °C increased with an increasing Cu content. The negative and positive effect of two-step aging at 170 °C after natural aging for 2419.2 ks was clarified in Al–Mg–Si and Al–Mg–Si–Cu alloys, respectively. The formation temperature, activation energy, cluster size, and hardness changes with the cluster formation during natural aging and dissolution of cluster and precipitate formation during two-step aging at 170 °C are quantitatively described and those behaviors are discussed based on the age-hardening phenomena.