Mechanism-based modelling of plastic deformation in magnesium alloys

The plastic deformation of two different rolled magnesium sheets (AZ31 and ZE10) under quasi-static tensile and compressive loading conditions at room temperature is studied. Beside glide by dislocation motion, deformation twinning leads to evolving flow stress asymmetry and evolving plastic anisotropy in these alloys. These mechanisms cause a significant change in the shape of the yield surface with accumulated plastic deformation which cannot be described by traditional hardening concepts. A phenomenological plasticity model in which the primary deformation mechanisms, slip and (extension) twinning, are treated separately is developed here and incorporated in the finite element framework. Deformations caused by these mechanisms are modelled by a symmetric and an asymmetric plastic potential, respectively. The hardening functions are coupled to account for the latent hardening. The necessary input for model parameter calibration is provided by mechanical tests along different orientations of the rolled sheets. Tensile tests of notched samples and shear tests are furthermore incorporated in the parameter optimisation strategy, which is based on minimising the difference between experimental behaviour and FE prediction over the entire deformation range up to failure. The model accounts for the characteristic tension-compression asymmetry and the evolution of strain anisotropy. Both, the convex-up and the concave-down shaped stress strain response are predicted. The computational model is exemplified by studying the evolution of the plastic multipliers during a shear test. It is shown that despite its phenomenological character, the model predicts the dominant deformation mechanism present at monotonous loading of the magnesium sheets investigated. (C) 2015 Elsevier Masson SAS. All rights reserved.