Study of the evolution mechanisms of Fe-rich phases in Al-Si-Fe alloys with Mn modification using synchrotron X-ray imaging

The plate-like Fe-rich intermetallics phases in the recycled Al-Si alloys significantly deteriorate the mechanical properties. In this study, the effect of Mn addition on Fe-rich phases formation and the properties of the Al-7Si-1.2Fe alloys were systematically studied by optical microscopy, scanning electron microscopy, differential scanning calorimetry, thermodynamic calculations, synchrotron X-ray radiography and tomography. The results show that with the addition of Mn, the two-dimensional (2D) morphology of the Fe-rich phase change from plate-like to Chinese-script and star-like. The synchrotron X-ray tomography results indicate that the three-dimensional (3D) morphology of Fe-rich phases change from interconnected plate-like to separated hollow polyhedron with increasing Mn content. The in-situ synchrotron X-ray radiography results reveal that the formation temperature, size, and area fraction of the primary Fe-rich phase in the Al-7Si-1.2Fe-1.09Mn alloy gradually decreased, but the number density increased as the cooling rate increased. The thermodynamic calculation results indicate that β-Al ₅FeSi could be eliminated when the Mn content exceeded 1.06%, which was consistent with the experimental results. However, excessive Mn addition caused the formation of large primary Fe-rich phases, resulting in a significant reduction in the elongation of the alloys. Reducing the equilibrium partition coefficient difference of Mn and Fe and reducing the formation of primary Fe-rich phases with high Mn/Fe ratios are good strategies to neutralize the negative effect of Fe-rich phases.