Abstract
The evolution of the Mn-bearing dispersoids in two Al-Mg-Mn 5xxx alloys and their impact on the mechanical properties and recrystallization resistance have been investigated. A large number of Mn-dispersoids were precipitated in the aluminum matrix by applying a multistep heat treatment. The traditional homogenization treatment was also applied for the comparison. Results showed that the type, size and number density of these dispersoids after heat treatment and rolling were strongly related to the Mg levels: fine Al6Mn and α-Al(Mn,Fe,Cr)Si dispersoids were observed in Al-3Mg-Mn alloy, while the dispersoids changed to relatively large Al4Mn and Al6Mn dispersoids in Al-5Mg-Mn alloy. The mechanical properties after cold rolling as a function of the annealing temperature were studied. The mechanical strengths of two alloys with the multistep heat treatment were significantly improved compared to the samples subjected to traditional homogenization treatment. With increasing the annealing temperature, the strengths decreased, and the elongation increased. In addition, the deformed grain structures after cold rolling and annealing were characterized, and the recrystallization fractions of two alloys were quantified. Results demonstrated that fine and dense Mn-dispersoids generated during multistep heat treatment could effectively hinder the dislocation and sub-grain boundary migration and improve the recrystallization resistance of cold-rolled sheets.