A crystallographic identification of intermetallic phases in Al–Si alloys

Iron-rich intermetallic phases are well known to be strongly influential on mechanical properties in Al–Si alloys. Two important phases have been previously designated as the relatively benign α-phase (with a ‘Chinese Script’ morphology) and the detrimental β-phase (plate or needle-shaped). While the common morphologies and effects of these phases are generally agreed upon, there is some confusion in the literature regarding the chemistry and/or crystallography of these phases. Using a combination of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), electron backscatter diffraction pattern analysis (EBSD), transmission electron microscopy (TEM) and convergent beam electron diffraction (CBED), the present work showed that intermetallic phases in cast eutectic Al–Si alloy intermetallics are often misidentified in the current metallurgical literature. The α-phase particles were consistent with cubic Al19Fe4MnSi2 rather than hexagonal. The phase often identified a β-phase is actually consistent with the tetragonal Al3FeSi2 (more correctly designated as δ-phase) rather than monoclinic or orthorhombic.