Mechanical Behavior of As-Cast and Extruded Mg-Si-Ni-Ca Magnesium Alloys

The mechanical performance of as-cast and hot extruded Mg-4Si-4Ni-xCa alloys was studied. In the as-cast microstructures, the dendritic primary Mg2Si particles, eutectic alpha-Mg + Mg2Si constituent, blocky chi-phase (Mg8Ni7Si5 intermetallic), and alpha-Mg phase were observed. Moreover, the needle-shaped CaMgSi intermetallic at high Ca additions was detected. The modification effect of Ca was discussed based on the size refinement and morphological enhancement of Mg2Si phase. Accordingly, the tensile properties of the as-cast alloys were enhanced by Ca addition no more than 1 wt.%. However, large additions (3 wt.%) resulted in the deterioration of strength and ductility due to the appearance of the needle-shaped CaMgSi intermetallic. Compared to the cast ingots, the wrought alloys (obtained by hot extrusion) showed superior mechanical performance with much larger work-hardening exponents and tensile toughness values. For instance, the tensile toughness of the extruded alloy with 1 wt.% Ca was determined as 78.65 MJ/m(3), which is much higher than the value of 9.32 MJ/m(3) for the as-cast 0 wt.% Ca alloy. As the best combination of tensile properties, the ultimate tensile strength and the total elongation of the extruded alloy with 1 wt.% Ca were 348.4 MPa and 16.6%, respectively.