Intermetallic phases in high purity Al-10Si-0.3Fe cast alloys with and without Sr modification studied by FIB tomography and TEM

Abstract Structure and chemical composition of intermetallic phases in unmodified and Sr-modified Al-10Si-0.3Fe casting alloys based on pure elements are identified by transmission electron microscopy and energy-dispersive X-ray spectroscopy. A three-dimensional characterization of intermetallic phases is performed by focused ion beam tomography. Both alloys were found to contain Fe-rich α-Al 14 Fe 3 Si 2 with a bcc structure and the τ 2 phase Al 4.5 FeSi, also called γ phase AlFeSi and both exhibit the “Chinese script” morphology in two dimensions, appearing as complex shaped objects in three dimension visualizations. The crystal structure of the τ 2 phase is shown to be trigonal R -centred rather than monoclinic as claimed by all published studies. It is related to the structure of R phases such as Mn 85.5 Si 14.5 . In the Sr-modified alloy, the tetragonal δ-Al 4 FeSi 2 phase is observed, which appears as thin platelets in 3D. No monoclinic β-Al 5 FeSi phase is found in any of the alloys unlike in industrial alloys that contain further impurities (Mn, Cr, Cu, P). Thus, the common β-phase is not a feature of the pure Al-Si-Fe system. In summary, addition of the modifier strontium to Al-10Si-0.3Fe alloys containing no further alloying elements markedly promotes the formation of δ-Al 4 FeSi 2 , but suppresses the formation of α-Al 14 Fe 3 Si 2 , whereas τ 2 phase Al 4.5 FeSi is found in similar fractions in both alloys.