Effects of manganese on heterogeneous nucleation and grain refinement of Mg-3Al alloys with carbon inoculation were investigated by using ICP

SEM and EDS

and so on. As-cast microstructure of the Mg-Al alloy after and before Mn addition was observed. The results show that the addition of Mn element can refine effectively the grain size in Mg-3Al alloys. The average grain size of primary Mg in the Mg-Al alloy can be refined to about 66 μm with 0.32% Mn addition. However

the grain size is coarsened with further increasing in manganese content. It is found that small rod-like or particle AlMn phases

which can act as possible nucleating substrate of primary Mg grain

is formed with the Mn content lower than 0.32%. With increasing in Mn content

supersaturated Mn and Al elements can react with Al4C3 and Al-C-O particles in the melt and subsequently large cross petal-like Al-Mn-C-O phases are formed

leading to the coarseness of primary Mg grain as a result of the decrease of effective heterogeneous nuclei.