Segregation of Ca at the Mg/MgO interface and its effect on grain refinement of Mg alloys

Abstract

It has been reported that native MgO particles in Mg alloy melts can act as heterogeneous nucleation substrates such that grain refinement of Mg alloys is achieved. A recent study showed the addition of Ca, combined with the native MgO particles, significantly improves grain refinement of Mg and its alloys. However, the mechanism underlying the grain refining phenomenon is not well understood due to the lack of direct experimental evidence. In this work, we investigated the segregation of Ca atoms at the Mg/MgO interface and its effect on grain refinement in Mg-0.5Ca alloys by utilizing advanced analytical electron microscopy. The experimental results focus on the chemical and structural information at the interface between MgO and the Ca solute. Adsorption layers rich in Al, N and Ca have been detected on {1 1 1} facets of MgO particles, with the lattice structure resembling the structure of MgO. It is suggested that the significant grain refinement improvement can be attributed not only to the growth restriction due to the presence of Ca addition but also to the specific chemistry and structure of the adsorption layers.