Microstructure, Intermetallic Phases, and Fractography of the Cast Al-5.8Zn-2.2Mg-2.5Cu Alloy by Controlled Diffusion Solidification

The cast specimens for this study were prepared with a final composition of Al-5.8Zn-2.2Mg-2.5Cu alloy by using a relatively new casting process that is also known as controlled diffusion solidification (CDS). In an earlier publication, evaluation of the uniaxial tensile properties of these castings was reported and discussed. In the present study, the effect of the CDS process was examined on the formation of secondary and intermetallic phases (IMPs) evolving during solidification. A quantitative image analysis was carried out using the light optical microscopy (LOM) and scanning electron microscopy (SEM) techniques. The quantitative metallography showed that the total phase fraction of the eutectic phases is greater than that predicted by the thermodynamic calculations, assuming the Scheil–Gulliver (S–G) (nonequilibrium) paradigm, which strongly suggests a significant shift from conventional wisdom regarding solute redistribution during solidification. Furthermore, the fractography results confirmed an intergranular ductile fracture mode in the castings under tension load. The microcracks/voids were initiated from shrinkage cavities or fractured IMPs such as Mg2Si and Cu2FeAl7; these phases solidified at the grain boundary (GB) areas, alongside the eutectic phases, such as sigma-Mg(Zn,Cu,Al)2 and S-(CuMgAl2), during the final stage of solidification. It was shown that the GB eutectic phases would further provide an easy pathway for the growth and propagation of initial microcracks/voids.