Crystallography of grain refinement in Mg-Al based alloys

An edge-to-edge matching model has been used to analyze the crystallographic features between magnesium matrix and three commonly accepted heterogeneous nucleants for magnesium grains, Al4C3, Al2CO and Al8(Mn,Fe)5. Although certain orientation relationships between magnesium and each of these compounds have been predicted, the potency of each of these three compounds as nucleating substrates for magnesium grains is different. Based on calculated interatomic spacing misfits along matching directions and d-value mismatches between matching planes, the model has predicted that Al2CO is the most effective nucleant of the three compounds assessed for Mg grains and is probably responsible for the grain refinement in both the superheating and carbon inoculation processes. Al4C3 can be a nucleant for Mg grains, but it is not as effective as Al2CO. This prediction is consistent with most experimental results reported recently. The Al8(Mn,Fe)5 intermetallic compound is predicted to have the lowest efficiency as a nucleant for magnesium grains. The model may be used to provide theoretical guidance for selection of new and more powerful grain refiners for Mg-Al based alloys.