Bulk ultrafine-grained AA3003 alloy processed by equal- Channel angular pressing

Abstract

Hot rolled billets ofAA3003 alloy were deformed by equal channel angular pressing in a die having equal channels of 10 mm diameter intersecting at an inner angle of 120° and outer arc of 60°. Billets were repeatedly pressed up to eleven passes to introduce an equivalent strain of 6.6 adopting route Be. Samples were characterized by optical microscopy, electron back scattered diffraction, transmission electron microscopy and tensile testing. Mechanism of grain refinement and strengthening are explored Grain refinement is strongly dependent on amount of strain. The refinement process involves several steps viz., elongation of grains by shear deformation, subdivision of grains to bands, subgrain formation in bands and fragmentation of bands into grains. Finally conversion of subgrains to grains takes place by progressive lattice rotation. Grain size was reduced from 20 fxm to 0.55fim after eight passes. The kinetics of refmementwas enhanced by the presence of second phase particles. Final microstructure has a lower dislocation density in the interior of grains/subgrains in comparison to the microstructure obtained in the initial passes. The yield strength and ultimate tensile strength were increased by a factor of 2.4 and 1.6 respectively. The yield and tensile strengths increase rapidly at first few passes and then reach to a saturation level at six passes or an equivalent strain of 3.6. The observed improvement in mechanical strength at initial passes of ECAP is due to work hardening and refinement in grain size and formation of subgrain or dislocation cell formation. Strengthening at large number of passes (eight passes or an equivalent strain of 4.8) is due to mainly by grain refinement.