The effect of multiple precipitate types and texture on yield asymmetry in Mg-Sn-Zn(-Al-Na-Ca) alloys

Strong textures produced during thermomechanical processing cause high levels of mechanical asymmetry in wrought Mg alloys. Through careful choice of precipitate type, asymmetry can be reduced by age hardening. In this work various Mg-Sn-Zn(-Al-Na-Ca) alloys were extruded then aged in the direct-aged and solution-treated conditions at 150°C and 200°C to investigate the capability of the combination of basal lath and c-axis rod precipitates to reduce asymmetry. Precipitate populations were found to be sensitive to ageing temperature as ageing at 150°C produced basal ellipsoid precipitates instead of higher aspect ratio basal laths as was expected. In addition, basal ellipsoids and c-axis rods were found to nucleate epitaxially reducing their strengthening effect. A Mg-8.8Sn-4.0Zn-0.9Al-0.3Na (TZA941+0.3Na) (wt.%) extruded alloy was shown to exhibit extrusion-direction tensile and compressive yield stresses of 360MPa and 233MPa in the solution treated condition with a compression to tension ratio of 0.65 after ageing at 150°C. It was also demonstrated that increasing the Sn content in the alloys reduced the time to peak hardness when ageing at 200°C where TZA941+0.3Na reached peak hardness within 12 hours, as opposed to 168 hours when ageing at 150°C. In addition, increasing Na additions served to strengthen extrusion textures, and Ca additions were shown to be ineffective at weakening extrusion textures in these alloys.