Microstructural evolution in a 5024 aluminum alloy processed by ECAP with and without back pressure

The microstructural evolution of an Al–4.57Mg–0.2Sc–0.09Zr–0.64Mn (in wt%) alloy processed by equal channel angular pressing (ECAP) without and with back pressure (BP) was investigated at 300°C. ECAP up to a strain of ε∼12 without and with BP provided the formation of fine recrystallized grains with a size of ∼870nm; their volume fraction was 77% and 86%, respectively. The application of a BP ensures the homogeneity of the billet processed and leads to an increased rate of continuous dynamic recrystallization (CDRX). The positive effect of BP on grain refinement through ECAP is attributed to the fact that BP facilitates the formation of deformation bands during the first to second passes. These deformation bands evolve uniformly on the macroscale of whole billets and mesoscale of initial grains and then rapidly transform into chains of recrystallized grains under further straining. The uniformity of deformation banding ensures an extensive decrease in the unrecrystallized fraction with strain. The mechanisms of new grain evolution, as well as factors promoting grain refinement, are discussed.