Influence of Eu on the Oxide Structure of A356 Melts

Abstract Europium (Eu) has recently attracted attention as an alternative modifier to conventional elements such as Sr, Na, and Sb for eutectic Si refinement in Al–Si cast alloys. Although the modification effect of Eu has been widely investigated, its influence on oxidation behavior and oxide evolution during melt processing remains unclear. In this study, the effects of Eu addition on the microstructure and oxidation behavior of the A356 alloy were investigated using optical microscopy, SEM–EDS, X-ray diffraction (XRD), differential scanning calorimetry (DSC), oxidation experiments, and FactSage thermodynamic modeling. Eu additions refined eutectic Si and promoted its transition toward a fine fibrous morphology. Simultaneously, Al 2 Si 2 Eu intermetallic compounds formed in both fine and coarse morphologies, with increased Eu content promoting the growth and quantity of coarse intermetallics. While fine Eu-rich particles are proposed to contribute to eutectic modification through TPRE and IIT mechanisms, excessive Eu additions promoted coarse Al 2 Si 2 Eu formation and altered intermetallic evolution. Oxidation experiments showed that 0.1 wt.% Eu had negligible influence on oxidation kinetics, whereas 0.4 wt.% Eu significantly increased oxidation rates. SEM–EDS and XRD analyses revealed no direct incorporation of Eu into the oxide layer; instead, Al 2 Si 2 Eu intermetallics preferentially formed near oxide–metal interfaces and are proposed to disrupt oxide continuity, accelerating oxidation. Thermodynamic calculations showed reasonable agreement with experimentally observed phase evolution. The results demonstrate a trade-off between microstructural refinement and oxidation resistance, indicating that excessive Eu additions may compromise melt stability despite improving eutectic modification.