Effect of strain path on microstructure evolution during severe plastic deformation of Al

Microstructure evolution during severe plastic deformation of unalloyed aluminum was investigated to establish the effect of processing route and purity level on grain refinement and subgrain formation. Two lots of unalloyed aluminum with different purity levels (99.99%Al and 99%Al) were subjected to large plastic strains at room temperature. Four different deformation processes were used: equal channel angular extrusion, sheet rolling, conventional conical-die extrusion, and uniaxial compression. Orientation-imaging microscopy was employed to characterize the final microstructures. The various deformation routes yielded an ultrafine microstructure with a ∼1.5-μm grain size in commercial-purity aluminum. For high-purity aluminum, however, the minimum grain size produced after the various deformation routes was ∼20 μm. The high fraction of high-angle grain boundaries and the absence of subgrains/deformation bands in the final microstructure suggested the occurrence of discontinuous recrystallization in high-purity aluminum deformed to large plastic strains.