Effects of Ca/Al ratio and extrusion process on Mg-Al-Ca alloys to produce a high toughness in-situ composite

The microstructures and tensile properties of Mg-Al2Ca-Mg2Ca in situ composites (Mg-17Al-8Ca, Mg-14Al-11Ca and Mg-12.5Al-12.5Ca) with different Ca/Al ratios have been studied in both as-cast and extruded conditions. The results indicated that by increasing Ca/Al ratio, new Mg2Ca intermetallic introduces to the Al2Ca phase in eutectic structure. Computer-aided cooling curve analysis confirmed the formation of these phases during solidification. Extrusion process not only altered the size of large bulk Al2Ca intermetallic, but also changed the size and morphology of intermetallics in eutectic structure considerably. The results showed that with increasing Ca/Al ratio, tensile properties of cast composites changes slightly, but significant enhancement is observed after extrusion process. The strength and elongation values of Mg-12.5Al-12.5Ca (Ca/Al = 1) alloy improved from 166 MPa and 2% in as-cast condition to 465 MPa and 12% in hot-extruded condition. The reason for the improved toughness may be attributed to the formation of finer and well-dispersed distribution of hard (Al2Ca) and ductile (Mg2Ca) phases. It was found that hot extrusion easily deforms ductile Mg2Ca phase in comparison with Al2Ca phase. In as-extruded condition, there are more very fine dimples than as-casted condition because extrusion process leads to formation of fragmented tiny particles and more uniformity distribution of Al2Ca particles.