Linking recovery and recrystallization through triple junction motion in aluminum cold rolled to a large strain

Recovery mechanisms and kinetics have been studied in commercial purity aluminum (AA1050) cold rolled to a true strain of 5.5 (99.6% thickness reduction) and annealed at low temperatures from 140 to 220°C. Transmission electron microscopy, electron backscatter diffraction (EBSD) and electron channeling contrast (ECC) are used to characterize the microstructural evolution during annealing. The microstructural characterization shows that a deformed lamellar structure coarsens uniformly during annealing by triple junction motion while maintaining the lamellar morphology, leading to a gradual transition into a more equiaxed structure, where recrystallization nuclei start to evolve. The apparent activation energy for the microstructural coarsening is estimated separately for different stages characterized by an increase in the lamellar boundary spacing measured by EBSD and ECC. The apparent activation energy increases during annealing, from 110kJmol−1 at the beginning to 230–240kJmol−1 at the end of uniform coarsening, linking the recovery stages to recrystallization. The increase in activation energy underpins operation of different diffusion mechanisms for migration of boundaries and their junctions during coarsening, and solute drag may become increasingly important as the structure coarsens. These findings form the basis for a discussion of the thermal behavior of a fine lamellar structure produced by cold rolling to a large strain of both scientific and applied interest.