Effect of precipitate shape on slip and twinning in magnesium alloys

The predicted strengthening effect of precipitates of different shape and habit on the basal, prismatic and { 1 0 1 ¯ 2 } twinning deformation systems in magnesium has been calculated. In parent material, rod precipitates parallel to the c-axis are predicted to be more effective than plates parallel to the basal plane in hardening the basal and prismatic slip systems. However, in twinned material, non-sheared basal plates are highly effective in inhibiting the basal slip necessary to relieve incompatibility stresses. The predictions suggest basal plates will reduce asymmetry in strongly textured extrusions by preferentially hardening against twin growth compared to prismatic slip, whereas c-axis rods can have the opposite effect. The predictions have been compared with the measured asymmetry for two magnesium alloys that form either c-axis rods (Z5) or basal plates (AZ91). In agreement with the model, it is shown that precipitation in Z5 leads to an increase in asymmetry, whereas in AZ91 precipitation reduces asymmetry. These results suggest that designed precipitation may provide a useful tool for reducing asymmetry in wrought magnesium alloys.