Limitation of current hardening models in predicting anisotropy by twinning in hcp metals: Application to a rod-textured AM30 magnesium alloy

When a strongly textured hexagonal close packed (HCP) metal is loaded under an orientation causing profuse twinning or detwinning, the stress-strain curve is sigmoidal in shape and inflects at some threshold. Authors have largely attributed the dramatic stress increase in the lower-bound vicinity of the inflection point to a combined effect of a Hall-Petch mechanism correlated to grain refinement by twinning, and twinning-induced reorientation requiring activation of hard slip modes. We experimentally and numerically demonstrate that these two mechanisms alone are unable to reproduce the stress-strain behaviors obtained under intermediate loading orientations correlated to in-between profuse twinning and nominal twinning. We argue based on adopting various mechanistic approaches in hardening model correlations from the literature. We used both a physics dislocation based model and a phenomenological Voce hardening model. The HCP material is exemplified by an extruded AM30 magnesium alloy with a <math display='block'> <mrow> <mrow><mo>&#x2329;</mo> <mrow> <mn>10</mn><mover accent='true'> <mn>1</mn> <mo>&#x00AF;</mo> </mover> <mn>0</mn> </mrow> <mo>&#x0232A;</mo></mrow> </mrow> </math>$$\left\langle {10\bar 10} \right\rangle$$-fiber parallel to the extrusion direction.