Effect of added Mg on the clustering and two-step aging behavior of Al–Cu alloys.

The effects of added Mg on cluster formation and two-step aging behavior in Al–Cu(-Mg) alloys were investigated using differential scanning calorimetry (DSC), Vickers hardness test, transmission electron microscope (TEM) and three dimensional atom probe (3DAP). When Mg was added to Al–Cu alloys, cluster formation was accelerated because Mg lowered formation temperature. Clusters with Mg/(Mg + Cu) values of 0.1, 0.4 and 0.5 predominantly formed in 40C5M, 40C10 M and 40C15 M alloys, respectively. The dominant cluster size was determined to be 0.5 nm regardless of the Mg content. The growth behavior of clusters in the Al–Cu alloys during natural aging was rarely affected, even though the chemical composition inside the clusters was changed by the amount of Mg. It was found that clusters formed during natural aging partially dissolved during two-step aging at 170 °C. The relationship between the thermal stability of the clusters and the decrease in hardness at the initial stage of two-step aging was clarified. Importantly, it was determined that the thermal stability of clusters is affected by the interatomic structure of the cluster, rather than its size, based on 3DAP analyses. The peak hardness of Al–Cu alloys after two-step aging significantly decreased when the amount of added Mg exceeded a certain threshold. Thermally stable clusters formed during natural aging do not directly transform into precipitates during two-step aging in Al–Cu–Mg alloys. [ABSTRACT FROM AUTHOR]