Quantification of annealed microstructures in ARB processed aluminum

High-purity aluminum (99.99% purity) deformed by accumulative roll bonding to an equivalent strain of 4.8 has been characterized by transmission electron microscopy and electron backscatter diffraction techniques in the deformed state and after various annealing treatments. These two techniques are compared and discussed. The majority of the structure in the as-deformed state is composed of equiaxed crystallites surrounded mostly by high-angle boundaries with misorientation angle above 15°. However, regions that contain a high concentration of low-angle boundaries with misorientation angle below 15° in a lamellar structure elongated along the rolling direction have also been observed. The microstructure is correlated with the local texture as the equiaxed structure contains a mixture of texture components whereas the lamellar structure has orientations corresponding to rolling or shear texture components. Annealing of deformed specimens causes structural coarsening without removing the regions with a high concentration of low-angle boundaries. Heat treatment for a short period of time (0.5 h) leads to the start of recrystallization at 200 °C, whereas a heat treatment at 175 °C for an extended period (6 h) allows heat treatments to be carried out at temperatures above 200 °C without initiation of recrystallization. The two-step process is a promising route for producing almost uniform equiaxed structures by annealing of samples that have been deformed to large strains.