Effect of dislocation transmutation on modeling hardening mechanisms by twinning in magnesium

Textured hexagonal close packed double-lattice structures show stronger anisotropy than textured cubic structures. The reason lies behind the necessity to activate deformation twinning and hard slip dislocation modes. Although the mechanisms behind activation of dislocations with non-basal Burgers vectors are still not fundamentally understood, the effect of twinning on hardening presents the most substantial challenge to polycrystal plasticity modelers. The origin of the increasing strain hardening rate regime (Regime II) upon profuse twinning is still not fundamentally clear. Previous successful attempts to fit the stress-strain behaviors based on a Hall-Petch effect by twin segmentation had systemically led to discrepancies in predicting intermediate textures and/or twin volume fraction evolutions. A recent dislocation-based hardening rule incorporated into the Visco-Plastic Self-Consistent (VPSC) model allows slip and twinning to be physically coupled in the simulations. In this paper, we investigate hardening mechanisms in pure magnesium and apply a dislocation based formalism to model anisotropy. In contrast to magnesium alloys, we show that pure magnesium under large strains develops substantial multivariant twinning and multifold twinning. These twinning phenomena are accompanied by a marked grain refinement and blunting of former twin boundaries. This blunting suggests severe accommodation effects in the soft matrix that caused the twin boundary to lose coherency. Thus, multivariant and multifold twinning take place to accommodate further deformation, but the subsequent twin-twin interactions arise to contribute in material hardening. The strain path anisotropy related to the saturation stresses revealed major missing links for comprehending hardening by twinning and substantiated dislocation transmutation effect by twinning shear. (C) 2011 Elsevier Ltd. All rights reserved.