Grain refinement of Mg-Ca alloys by native MgO particles

In Mg-Ca alloys the grain refining mechanism, in particular regarding the role of nucleant substrates, remains the object of debates. Although native MgO is being recognised as a nucleating substrate accounting for grain refinement of Mg alloys, the possible interactions of MgO with alloying elements that may alter the nucleation potency have not been elucidated yet. Herein, we design casting experiments of Mg-xCa alloys varied qualitatively in number density of native MgO, which are then comprehensively studied by advanced electron microscopy. The results show that grain refinement is enhanced as the particle number density of MgO increases. The native MgO particles are modified by interfacial layers due to the co-segregation of Ca and N solute atoms at the MgO/Mg interface. Using aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy, we reveal the nature of these Ca/N interfacial layers at the atomic scale. Irrespective of the crystallographic termination of MgO, Ca and N co-segregate at the MgO/Mg interface and occupy Mg and O sites, respectively, forming an interfacial structure of a few atomic layers. The interfacial structure is slightly expanded, less ordered and defective compared to the MgO matrix due to compositional deviations, whereby the MgO substrate is altered as a poorer template to nucleate Mg solid. Upon solidification in a TP-1 mould, the impotent MgO particles account for the grain refining mechanism, where they are suggested to participate into nucleation and grain initiation processes in an explosive manner. This work not only reveals the atomic engineering of a substrate through interfacial segregation but also demonstrates the effectiveness of a strategy whereby native MgO particles can be harnessed for grain refinement in Mg-Ca alloys.