The role of Ca, Al and Zn on room temperature ductility and grain boundary cohesion of magnesium

It is know from literature that small additions (<1 wt%) of Ca, Al and Zn significantly improve the intrinsic ductility of Mg. The exact role of each element, both qualitatively and quantitatively, and their combined effects, however, are poorly understood. Here we achieved a much clearer view on the quantitative role of each element with respect to ductility improvement and on the collaborative effect, particularly of Ca and Zn in Mg. Some of our findings and conclusions are in disagreement with data and interpretation found in literature. Four different alloys, namely, Mg-0.1 Ca, Mg-0.1 Ca-1 Al, Mg-0.05 Ca-1 Al, Mg-0.1 Ca-2 Al-1 Zn (all are in wt%) were selected for this investigation. All alloys were treated such that approx. similar grain sizes and textures were obtained. This largely excludes the effect of extrinsic factors on ductility. EBSD-guided slip trace analyses reveal that the addition of Ca eases activation of prismatic and pyramidal II slip systems. Using in-situ deformation experiments in SEM and atom probe tomography observations of grain boundaries direct evidence is given for the individual and synergetic effects of Ca and Zn on grain boundary cohesion as an important contribution to improve the ductility of these alloys. We conclude that Ca reduces the slip anisotropy and ameliorates ductility, however, the weak grain boundary cohesion in the Mg-0.1 wt% Ca alloy limits the material's tensile ductility. The addition of Zn alters the Ca segregation at the grain boundaries and helps to retain their cohesive strength, an effect which thus enables higher ductility and strength. The further addition of Al primarily improves the strength. The results show that the balanced influence of reduced slip anisotropy on the one hand and increased grain boundary cohesion on the other hand allow to design a high strength high ductility rare-earth free Mg alloy.