Al₈Mn₅ Particle Settling and Interactions with Oxide Films in Liquid AZ₉₁ Magnesium Alloys

Abstract

Al₈Mn₅ particles form as primary solidification phases in Mg-Al-based alloys and are important for ensuring adequate corrosion resistance. However, excessive Al₈Mn₅ formation above the α-Mg liquidus temperature can lead to sludge formation, and the clustering of Al₈Mn₅ particles on oxide films can generate deleterious casting defects. Here, we use analytical SEM and real-time synchrotron x-ray radiography to study Al₈Mn₅ particle settling, the formation of a sludge layer, and the mechanism of Al₈Mn₅ clustering on oxides in AZ₉₁ containing two Fe levels. It is shown that settling Al₈Mn₅ can become trapped in entrained oxide and that new Al₈Mn₅ particles also seem to nucleate on oxide films. On cooling, these Al₈Mn₅ particles continue to grow, creating large Al₈Mn₅ clusters.