Die-cast alloys with high strength and acceptable elongation under as-cast condition have been designed and developed in Al-Si-Cu-Mg alloys by optimizing the synergistic strengthening effect of Q and θ phases. It is found that the Q phase fraction was controlled only by Mg level, but θ phase fraction was affected by both Cu and Mg levels. Experimental results demonstrated that a series of θ and Q combinations could provide a same high yield strength but at different cost of elongation, and the reason can be attributed to the different morphological forms and distribution of θ and Q phases. Microstructural observation confirmed that numerous and well-distributed nanoscale Q precipitations (mostly under 200nm) were obtained inside α-Al matrix of the developed die-cast alloy. Be different with the normal methods, these nanoscale Q precipitations were formed via solid reactions during solidification process, rather than high temperature heat treatment. Meanwhile, a few large lamellar Q phase that formed directly from the melt also appeared and aggregated at the α-Al grain boundaries. θ intermetallics tend to locate at the grain boundaries and have blocky or lamellar morphologies. It is also found that over-limit of Mg or Cu levels would induce too much strengthening phases located at α-Al grain boundaries, decreasing the elongation obviously. The optimised strength-ductility trade-off could be achieved with 3.0Cu and 0.9Mg in Al-Si-Cu alloys, which can offer a yield strength of 213MPa, an ultimate tensile strength of 357MPa and a satisfactory elongation of 4.3% under as-cast condition
