Modeling the effect of twinning and detwinning during strain-path changes of magnesium alloy AZ31

Hexagonal materials deform plastically by activating diverse slip and twinning modes. The activation of such modes depends on their relative critical stresses, and the orientation of the crystals with respect to the loading direction. To be reliable, a constitutive description of these materials has to account for texture evolution associated with reorientations due to both dislocation slip and twinning, and for the effect of the twin boundaries as barriers to dislocation propagation. We extend a previously introduced twin model, which accounts explicitly for the composite character of the grain formed by a matrix with embedded twin lamellae, to describe the influence of twinning on the mechanical behavior of the material. The role of the twins as barriers to dislocations is explicitly incorporated into the hardening description of slip deformation via a directional Hall-Petch mechanism. We introduce here an improved hardening law for twinning, which discriminates for specific twin/dislocation interactions, and a detwinning mechanism. We apply this model to the interpretation of compression and tension experiments done in rolled magnesium alloy AZ31B at room temperature. Particularly challenging cases involve strain-path changes that force strong interactions between twinning, detwinning, and slip mechanisms. (C) 2008 Elsevier Ltd. All rights reserved.