Mechanisms of formation of submicron grain structures by severe deformation

The grain refinement mechanisms operating during severe deformation processes, like ECAE, are discussed using data obtained from model Al-alloys. Refinement occurs predominantly by orientation splitting, micro and macroshear banding, and the geometric requirement for high angle boundary area to increase with strain. The deformation structure evolution is affected strongly by the processing route, die geometry, and material parameters. Both cell bands and microshear bands show strong alignment with the dies shear plane. Shear bands are promoted by changes in the shear plane orientation each extrusion cycle. At high strains a steady state is approached, where the grain size converges with the subgrain size, controlled by dynamic boundary migration.