On grain coarsening and refining of the Mg–3Al alloy by Sm

•The Mg–3Al alloy was significantly coarsened by the addition of 0.7 wt. % Sm, but dramatically refined by 2.1 wt. % Sm.•Sm coarsened the Mg–3Al alloy by inhibiting the Al–Fe–C–O particles nucleant potency.•2.1% Sm refined the Mg–3Al alloy because of the solidification of Al2Sm particles.•“Interdependence Theory” was applied to describe the nucleant selection process in the Mg–3Al–2.1Sm alloy.

The effect of Sm on the microstructure of a Mg–3Al alloy has been investigated where it was found that the size of the primary α-Mg grains was significantly coarsened by the addition of 0.7 wt. % Sm, but dramatically refined by 2.1 wt. % Sm. It is proposed that the coarsening effect of Sm is due to a decrease in the nucleation potency of the native Al–Fe–C–O particles when they transform to the lower potency Al–Fe–Sm–C–O particles. The grain refinement observed when 2.1 wt. % Sm is added, is caused by the formation of potent Al2Sm nucleant particles prior to the solidification of α-Mg. The “Interdependence Theory” was then applied to describe the nucleant selection process in the Mg–3Al–2.1Sm alloy.