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, lowangle dislocation boundaries, and dislocations characterizes the low-temperature annealing step. In a discussion, the observed annealing behavior is related to these structural changes.