Microstructure and mechanical properties of new die-cast quaternary Al-Cu-Si-Mg alloys

Three new ultrafine hypoeutectic die-cast alloys based on quaternary Al-Cu-Si-Mg system were developed. The CALPHAD thermodynamic modelling of Al-Cu-Si-Mg system based on Scheil description was conducted to predict the volume fractions of the eutectic mixtures. The designed volume fraction of eutectic mixtures of Al-xCu-2.2Si-1.1Mg (x = 5, 6.6 and 10.6 wt%) alloys were determined to be 0.2, 0.25 and 0.3, respectively. With the increase in eutectic fraction, the yield strength and ultimate tensile strength increased from 219 MPa to 267 MPa, and 344.7 MPa–395 MPa respectively, while the elongation to fracture decreases from 7.72% to 3.4%. The microstructure of the alloys mainly consists of Si, Al2Cu, α-Al and Al4Cu2Mg8Si7 (Q) phases. The Al5Cu2.2Si1.1Mg and Al6.6Cu2.2Si1.1Mg alloys had similar microstructures that consisted of coarse binary/ternary and fine quaternary eutectic regions, surrounding α-Al matrix. The Al10.6Cu2.2Si1.1Mg alloy contained α-Al grains and almost only one type of eutectic structure. The orientation relationships among Al2Cu, α-Al and Q phases were found in the eutectic region. Al2Cu and α-Al has a coherent interface, while the interfaces between Q and Al2Cu as well as Q and α-Al, were found to be semi-coherent. Our results broaden a new approach to the design of die-cast alloys with multi-phase and multi-component microstructure for high-performance applications. © 2020