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 alpha-phase (with a 'Chinese Script' morphology) and the detrimental beta-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 AI-Si alloy intermetallics are often misidentified in the current metallurgical literature. The alpha-phase particles were consistent with cubic Al19Fe4MnSi2 rather than hexagonal. The phase often identified a beta-phase is actually consistent with the tetragonal Al3FeSi2 (more correctly designated as delta-phase) rather than monoclinic or orthorhombic. (c) 2005 Elsevier B.V. All rights reserved.