Reducing yield asymmetry and anisotropy in wrought magnesium alloys – a comparative study

Yield asymmetry in wrought magnesium alloys has been shown previously to be reduced by texture weakening and {-1012} tensile-twin solid-solution strengthening due to rare-earth element (RE) additions. Yield asymmetry can also be reduced by grain-size reduction and precipitation strengthening. In this work these mechanisms were studied in several Mg-Sn-Zn-Al-Na(-Ca) alloys and a Mg-7wt.% Y (W7) alloy to rank their effectiveness. It was shown that texture weakening and tensile-twin solid-solution strengthening were the most effective asymmetry reducing mechanisms but were only achieved in the yttrium containing alloy. For yttrium-free alloys, grain-size reduction and precipitation strengthening must be utilised to overcome asymmetry and anisotropy produced by strong textures. Carefully designed precipitate populations were demonstrated to reduce both asymmetry (in the extrusion direction) and anisotropy. The power of crystal plasticity models, such as the visco-plastic self-consistent model, in aiding this design process has been demonstrated. However, achieving the desired precipitate populations in a practical alloy under realistic processing conditions is difficult. Grain-size reduction is more easily achieved and is effective in reducing asymmetry, but can only combat anisotropy successfully if an alloy is not hindered by an unfavourable wrought texture.