Eutectic growth morphologies in magnesium-aluminium alloys

The Mg-Mg17Al12 eutectic exhibits a wide range of morphologies depending on the alloy composition and cooling conditions. An alloy of eutectic composition, Mg-33 wt% Al, exhibits a lamellar morphology at low growth rates and a fibrous morphology at higher growth rates. However, in low aluminium content magnesium;aluminium alloys such as the most common commercial magnesium alloys AZ91 and AM60, the eutectic has a different morphology, described as either completely or partially divorced. This paper reports the results of an investigation into the effect of aluminium content and cooling rate on the morphology of the eutectic in permanent mould cast magnesium-aluminium alloys. Magnesium alloys with aluminium contents ranging from 9 to 33 wt% were cast into steel moulds. The effect of aluminium content was determined by comparing equivalent regions in the different castings while the effect of cooling rate was determined by comparing the microstructures obtained at the edge and the centre of each casting. As the aluminium content of the alloys increased, the dominant eutectic morphology changed progressively from fully divorced to partially divorced, to granular, to fibrous and, finally, to lamellar. Increasing the cooling rate produced a small change in the opposite direction with these transitions occurring at higher aluminium contents. The influence of both aluminium content and cooling rate on eutectic morphology results from the effect of these variables on dendrite morphology and the consequential size of the eutectic regions