Distribution of Fe-rich phases in eutectic grains of Sr-modified Al–10 wt.% Si–0.1 wt.% Fe casting alloy

Abstract

The addition of Sr to Al–Si-based alloys is known to modify the morphology of the eutectic Si phase and to influence the nucleation of eutectic grains. Understanding the distribution and the morphology of small constituent phases in eutectic grains such as Fe-rich intermetallic phases can further yield an insight into the cellular substructure of eutectic grains formed during solidification. The addition of 200ppm Sr to an Al–10Si–0.1Fe alloy and its influence on the formation of Fe-rich phases was comprehensively studied by several microscopy techniques. Optical microscopy combined with scanning electron microscopy revealed the existence of two types of Fe-rich phases at cell boundaries of eutectic grains. The Fe-rich phases were identified as α-Al14Fe3Si2 and δ-Al4FeSi2 by transmission electron microscopy. Both Fe-rich phases are located at distinct regions in the eutectic grain, namely in the transition region (region 2: α-phase) and the outer region (region 3: δ-phase) of the eutectic grain. The three-dimensional morphology of the eutectic Si phase and the Fe-rich phases at the eutectic cell boundaries was investigated by focused ion beam tomography. The Fe-rich α-phase was found to form concentrated networks with the 3D morphology of “sheets”, whereas the Fe-rich δ-phases exist as thin platelets. The distribution of Fe-rich phases in the cellular substructure of eutectic grains are described on the basis of the evolution of the eutectic solidification front by a qualitative solidification model.