Mechanisms of enhanced heterogeneous nucleation during solidification in binary Al-Mg alloys

The mechanisms involved in the grain refinement of Al-Mg alloys through varying the Mg content and applying intensive melt shearing were investigated. It was found that the oxide formed in Al-Mg alloys under normal melting conditions is MgAl₂O₄, which displays an equiaxed and faceted morphology with {1 1 1} planes exposed as its natural surfaces. Depending on the Mg content, MgAl₂O₄ particles exist either as oxide films in dilute Al-Mg alloys (Mg < 1 wt.%) or as naturally dispersed discrete particles in more concentrated Al-Mg alloys (Mg > 1 wt.%). Such MgAl₂O₄ particles can act as potent sites for nucleation of α-Al grains, which is evidenced by the well-defined cube-on-cube orientation relationship between MgAl₂O₄ and α-Al. Enhanced heterogeneous nucleation in Al-Mg alloys can be attributed to the high potency of MgAl₂O₄ particles with a lattice misfit of 1.4% and the increased number density of MgAl₂O₄ particles due to either natural dispersion by the increased Mg content or forced dispersion through intensive melt shearing. It was also found that intensive melt shearing leads to significant grain refinement of dilute Al-Mg alloys by effective dispersion of the MgAl₂O₄ particles entrapped in oxide films, but it has marginal effect on the grain refinement of concentrated Al-Mg alloys, where MgAl₂O₄ particles have been naturally dispersed into individual particles by the increased Mg content.