Al 8 Mn 5 particles form as primary solidification phases in Mg-Al-based alloys and are important for ensuring adequate corrosion resistance. However, excessive Al 8 Mn 5 formation above the a-Mg liquidus temperature can lead to sludge formation, and the clustering of Al 8 Mn 5 particles on oxide films can generate deleterious casting defects. Here, we use analytical SEM and realtime synchrotron x-ray radiography to study Al 8 Mn 5 particle settling, the formation of a sludge layer, and the mechanism of Al 8 Mn 5 clustering on oxides in AZ91 containing two Fe levels. It is shown that settling Al 8 Mn 5 can become trapped in entrained oxide and that new Al 8 Mn 5 particles also seem to nucleate on oxide films. On cooling, these Al 8 Mn 5 particles continue to grow, creating large Al 8 Mn 5 clusters.

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Magnesium alloys are commonly melted and held in steel or cast iron crucibles and small but important amounts of Fe and C can dissolve into Mg-rich melts. Here, carbide formation is studied during interface reactions between solid Fe-xC alloys (x= 0 -3.6 wt.%) and liquid Mg-9Al-0.7Zn-0.2Mn (wt.%, AZ91) at temperatures from 700 -800 C. Two ternary carbides, AlFe3C and Al2MgC2, formed in the reaction layers between the Fe-C and AZ91, and T2-Al2MgC2 additionally formed within the AZ91 alloy due to carbon pickup. T2-Al2MgC2 grew in liquid AZ91 as hexagonal plates that were commonly twinned. A reproducible orientation relationship was measured between T2-Al2MgC2 and -Mg, and the grain refinement of magnesium by heterogeneous nucleation on T2-Al2MgC2 is explored.

Manganese-bearing intermetallic compounds (IMCs) are important for ensuring adequate corrosion performance of magnesium-aluminium alloys and can be deleterious to mechanical performance if they are large and/or form clusters. Here, we explore the formation of Al-Mn IMCs in Mg-9Al-0.7Zn-0.2Mn produced by two industrial casting processes, high-pressure die casting (HPDC) and direct chill (DC) casting. As Al8Mn5 starts forming above the α-Mg liquidus temperature in this alloy, we consider its formation during melt handling as well as during casting and heat treatment. In HPDC, we focus on sludge formation in the holding pot, partial solidification of IMCs in the shot chamber, and Al-Mn IMC solidification in the die cavity. In DC casting, we focus on interactions between Al-Mn IMCs and oxide films in the launder system, Al-Mn IMC solidification in the billet, and the partial transformation of Al8Mn5 into Al11Mn4 during solution heat treatment. The results show that minimising pre-solidification in the shot sleeve of HPDC and controlling pouring and filtration in DC casting are important for ensuring small Al-Mn intermetallic particles in these casting processes.