Plastic deformation and fracture of ultrafine-grained Al-Mg alloys with a bimodal grain size distribution

Four different ultrafine-grained (ufg) Al-7.5 wt.% Mg alloys were synthesized by consolidation of a mixture of as-received and cryomilled Al-Mg powders with a ratio of 1:9, yielding a bimodal microstructure consisting of coarse grains (grain sizes, dcg, typically of several micrometers) evenly distributed in the ufg matrices (average grain sizes d = 120, 142, 197, and 338 nm). The deformation behavior under uniaxial compression and tension of the as-extruded alloys was investigated. The Ramberg-Osgood equation was used to fit the compressive stress-strain curves of the bimodal ufg alloys. The compressive yield stresses of the ufg matrices with different average grain sizes indicated a reduced slope in the Hall-Petch relation. The plastic deformation of the ufg Al-Mg alloys with a bimodal microstructure was highly localized. The fracture of the alloys was attributed to shear localization under the compressive tests, and to a combination of shear localization, cavitation, and necking under the tensile tests.