Experiments and numerical simulations were used to study the anisotropy of flow and microstructure evolution in extruded Mg AZ31 sheet during deformation under conditions representing a typical hot blow forming process. Tensile specimens were cut so that the tensile axis was orientated 0°, 45° and 90° from the extruded direction, and were subjected to strain rates between 1.0x10-4/s and 0.1/s within a temperature range of 350°C to 450°C. The strain rate sensitivity m=d(log(stress))/d(log(strain rate)) was measured and the evolution of grain size was recorded. The strain rate sensitivity was observed to decrease from values of order 0.3 to a value of order 0.2 as strain rate was increased from 1.0x10-4/s and 0.1/s, indicating a transition in deformation mechanism from grain boundary sliding to dislocation creep. Grain size measurements provide further evidence of a mechanism transition. Both the strain rate sensitivity and microstructure evolution were found to be strongly sensitive to the orientation of the tensile axis with respect to the extruded direction. Specimens loaded with the tensile axis parallel to the extruded direction show strain sensitivity of order 0.3 at 1.0x10-4/s strain rate, where with the tensile axis perpendicular to the extruded direction were found to have strain rate sensitivity of order 0.5. The difference in strain rate sensitivity decreases with strain rate, and is negligible at 0.01/s. The change in strain rate sensitivity leads to a substantial improvement in ductility for the specimens loaded perpendicular to the extrusion direction, and is accompanied by a change in the evolution of grain size with straining. Finite element simulations were used to investigate possible origins of this anisotropy. The simulations idealized the microstructure as an array of single crystal HCP grains, separated by grain boundaries, which allow sliding between neighboring grains. The simulations were used to predict the strain rate sensitivity for the solid and to compute the relative contributions to the total strain rate from grain boundary sliding and slip in the crystal. The simulations suggest that two mechanisms contribute to the observed anisotropy. Texture in the extruded sheets leads to anisotropy in the slip within the grains. In addition, the grains in the extruded sheet are elongated in the extrusion direction, which leads to anisotropy in the resistance of the grain boundary network to sliding.
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