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

Abstract 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, d cg , 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.