Combined Two- and Three-Dimensional (2D and 3D) Characterization of Directionally Solidified Al-Cu-Ag-Mg Quaternary Eutectic

Quaternary eutectic presents far more complexity than binary and ternary eutectics, and its understanding can offer fundamental knowledge of the microstructure formation in a multi-component system. Accurate characterization of the quaternary eutectic structure can provide experimental evidence to validate simulation models for the multi-phase solidification. In this study, a quaternary eutectic alloy with a composition of Al-16.74 Cu-40.54 Ag-6.46 Mg in wt% was directly solidified with a velocity of 0.4 μm/s in a vertical type Bridgeman furnace. The composition of the constituent phases in the quaternary eutectic was determined by electron probe micro-analyzer, and their crystal structures were examined by x-ray diffraction. The quaternary eutectic is composed of α-Al, θ-Al2Cu, AgAlMg, and a new intermetallic phase considered as Ag5Mg3Al2. The morphology of the eutectic phases, and their spatial distribution were studied in 3D by a serial sectioning technique combined with the focused ion beam. The quaternary eutectic exhibits regular rod-like or fibrous morphology despite of the intermetallic nature of its three eutectic phases. Both the rod splitting and merging were observed common for all the eutectic phases in the 3D analysis, while only the rod splitting can be determined in the 2D analysis. Interconnected complex structures of split and merged rods of the AgAlMg and Ag5Mg3Al2 phases were observed in the 3D structure. Their formation is attributed to the higher tendency of the AgAlMg and Ag5Mg3Al2 phases to split and branch to compete for the Ag and Mg elements. The phase fractions of the eutectic phases were determined in both 2D and 3D, and it is concluded that the 3D measurement gives relatively better result.