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.