Tailoring structures through two-step annealing process in nanostructured aluminum produced by accumulative roll-bonding

Due to structural and textural heterogeneities and a high content of stored energy, annealing of nanostructured metals is difficult to control in order to avoid non-uniform coarsening and recrystallization. The present research demonstrates a method to homogenize the structure by annealing at low temperature before annealing at high temperature. By this two-step process, the structure is homogenized and the stored energy is reduced significantly during the first annealing step. As an example, high-purity aluminum has been deformed to a total reduction of 98.4% (equivalent strain of 4.8) by accumulative roll-bonding at room temperature. Isochronal annealing for 0.5 h of the deformed samples shows the occurrence of recrystallization at 200 °C and above. However, when introducing an annealing step for 6 h at 175 °C, no significant recrystallization is observed and relatively homogeneous structures are obtained when the samples afterwards are annealed at higher temperatures up to 300 °C. To underpin these observations, the structural evolution has been characterized by transmission electron microscopy, showing that significant annihilation of high-angle boundaries, low-angle dislocation boundaries, and dislocations characterizes the low-temperature annealing step. In a discussion, the observed annealing behavior is related to these structural changes.