The role of Ca on the microstructure and tensile properties of Mg-Al-Zn-Ca alloys

Synergy between Ca and other alloying elements such as Al and Zn can modify the microstructure and mechanical properties of Mg alloys. In this work, we compared four extrusion alloys: Mg-6Al-2 Zn, Mg-6Al-2Zn-0.5Ca, Mg-6Al-2Zn-1.0Ca, and Mg-6Al-2Zn-2.0Ca (all in wt.%). Tensile properties were remarkably not improved with Ca addition. The underlying reasons were explored using in situ synchrotron X-ray diffraction, electron microscopy, and cyclic loading tests. It is found that solute Ca imposes a softening effect on Mg's slip systems. The slight increase of the yield strength by the formation of Al2Ca particles is fully offset by the reduced Al solute strengthening. Texture weakening and resultant ductility enhancement reported in Mg-Zn-Ca ternary alloys is largely compromised by the high amount of Al. Nevertheless, Al2Ca particles can enhance the materials' strain hardening by inducing back stress. An elastic-viscoplastic self-consistent (EVPSC) model was employed to simulate the deformation of the four alloys. Discrepancies between the simulation results and the experimental measurements mostly occur in the microscopic level (e.g. lattice strains and dislocation density evolution), and the reasons will be discussed.