Effect of Sc Microalloying on Interfacial Microstructural Evolution in Al/Mg Bimetallic Composites Fabricated by Lost Foam Compound Casting

Sc microalloying was introduced into A356 Al inserts to regulate interfacial microstructural evolution in A356/AZ91D bimetallic composites fabricated by lost foam compound casting. Nominal Sc additions of 0, 0.6, 0.9, and 1.2 wt.% were investigated using scanning electron microscopy, energy-dispersive spectroscopy, electron backscatter diffraction, elemental mapping, microstructural size statistics, and Vickers hardness testing. The results showed that Sc addition markedly reduced the interfacial reaction region. The total interface thickness decreased from 1750 μm in the Sc-free specimen to 1069, 1088, and 922 μm at 0.6, 0.9, and 1.2 wt.% Sc, respectively. Among the Sc-containing specimens, 0.6 wt.% Sc produced the most pronounced microstructural refinement, reducing the mean equivalent circular diameter of indexed interfacial microstructural units from 23 to 19 μm and that of Mg2Si from 17 to 14 μm while decreasing the amount of coarse γ-Al12Mg17 dendrites. EBSD revealed no marked change in the major reliably indexed phases or overall preferred orientation after Sc addition, whereas elemental mapping showed discrete Sc-enriched regions rather than a continuous Sc-rich interfacial layer. These results indicate that Al-side Sc microalloying regulates the interface mainly by restricting the expansion of the interfacial reaction region and modifying local solidification and second-phase growth, thereby promoting interface thinning and microstructural refinement.