Comparisons of microstructure, thixoformability and mechanical properties of high performance wrought magnesium alloys reheated from the as-cast and extruded states

The solid grain size, degree of spheroidization, contiguity and practical liquid fraction of Mg-Zn-Y-Zr alloys reheated from the as-cast and extruded states are characterized and compared. Thixoforming tests are performed to examine thixoformability of the as-cast and extruded alloys. Tensile mechanical properties for Mg-Zn-Y-Zr alloys thixoformed from the as-cast and extruded alloys are also determined. Reheating from the extruded alloy has been shown to produce ideal, fine semi-solid structures, in which completely spheroidal primary solid grains are essentially free of entrapped liquid. However, the as-cast alloy exhibits non-uniform microstructures with a large amount of entrapped liquid, which are less spheroidal compared with the extruded alloy in the semi-solid state. The grain coarsening rate is relatively low in the extruded alloy than that in the as-cast alloy during reheating. During thixoforming, despite the low liquid fraction, the fine and spheroidal solid grains are weakly connected and slide over each other without any plastic deformation in the extruded alloy. Therefore, the as-cast alloy which has heterogeneous and coarse solid grains is not as thixoformable as the extruded alloy. The tensile mechanical properties for Mg-Zn-Y-Zr alloys thixoformed from the extruded alloy are better than those of Mg-Zn-Y-Zr alloys thixoformed from the as-cast alloy.