Annealing Behavior of Nanostructured Aluminum Produced by Cold Rolling to Ultrahigh Strains

The isochronal annealing behavior of nanostructured commercial purity aluminum (AA1100 and AA1200) produced by either cold rolling (CR) or accumulative roll bonding (ARB) up to ultrahigh strains of about 99.5 pct reduction in thickness has been studied in the temperature range from 200 °C to 420 °C. Microstructural and texture measurements were made using data from electron backscatter diffraction (EBSD) investigations, and the change in mechanical strength was followed using hardness measurements. A large effect of the rolling strain is observed on recovery at temperatures below 200 °C to 220 °C. This effect is exemplified by samples deformed to the largest strain, where a rapid decrease in the stored energy from approximately 2 MJ/m³ in the deformed state to less than 0.5 MJ/m³ is seen, accompanied by a large decrease in the hardness. A new method for analyzing the uniformity of the structural coarsening, based on analysis of the crystallite size distribution with respect to the mode, is described. The analysis demonstrates that annealing leads to locally nonuniform changes in the microstructure, and to a description of the annealing process at temperatures greater than 220 °C as one of conventional recrystallization.