Grain Refinement of Mg and Its Alloy by Inoculation of In Situ MgO Particles

Significant grain refinement of commercial purity Mg and AZ91D Mg alloy was achieved by intensive melt shearing imposed to the melts prior to solidification without addition of grain refiner. Heterogeneous nucleation mechanism was investigated using analytical electron microscopy. It was demonstrated that the grain refinement was resulted from the promoted heterogeneous nucleation by inoculation of in situ MgO particles, which had been effectively dispersed by melt shearing. It was shown that MgO formed in pure Mg and AZ91D alloy melts were {1 0 0} and {1 1 1} faceted, respectively. For pure Mg sample, high resolution TEM revealed two orientation relationships OR I: (1 0 0) [0-1 1] MgO // (0-1 1 2) [0 1-1 1] Mg, and OR II (1 0 0) [0-1 1] MgO //(1-1 0 2) [-2 4-2 3] Mg. For the alloy sample, however, alpha-Mg grain was found to nucleate on the faceted {1 1 1} planes of MgO particles according to the OR III: (1 1 1) [0-1 1] MgO //(0 0 0 1) [1 1-2 0] Mg. The large number of MgO particles dispersed by intensive melt shearing acted as the substrates to promote heterogeneous nucleation process, leading to the significant grain refinement.