Mechanisms of nucleation and defect growth in undercooled melt containing oxide clusters

Mechanisms governing the nucleation and defect growth during solidification of undercooled aluminum (Al) in the presence of oxide clusters are studied by molecular dynamics (MD) simulations. Without serving as heterogeneous nucleation sites or changing the overall average grain size, Al oxides (Al2O3) induce localized variations in nanostructures away from the oxide surfaces. Thermodynamic modeling of solidification based on the terrace-ledge model suggests that the non-wetting nature of the liquid Al- solid Al2O3 interface prevents formation of perfect and twinned structures on oxide surfaces. Delayed crystal nucleation and steady-state solidification followed by a sharp growth of smaller grains are identified to trigger variations in crystal orientation selection and increase the localized fraction of twinned structures in regions between oxide surfaces. Finally, our investigation of formation of growth twin boundaries provides the first computational evidence that both layer-by-layer additions and formation-decomposition of grain boundaries mechanisms govern the formation of five-fold twin boundaries during solidification of Al.