Heterogeneous nucleation of solid Al from the melt by Al3Ti: Molecular dynamics simulations
JS Wang and A Horsfield and PD Lee and P Brommer, PHYSICAL REVIEW B, 82, 144203 (2010).
It has been known experimentally for some time that Al3Ti is a powerful nucleant for the solidification of aluminum from the melt; however, a full microscopic understanding is still lacking. To develop this understanding, we have performed molecular dynamics simulations of the nucleation and early stages of growth using published embedded atom method potentials for Al-Ti, but modified by us to stabilize the D-022 structure. We discover that Al3Ti can indeed be very effective in promoting the growth of solid Al but the manner in which growth takes place depends sensitively on the surface on which the Al nucleates. In particular, complete growth of solid Al from the liquid on the (001) and (110) surfaces of Al3Ti occurs at a lower temperature than on the (112) surface. This anisotropy agrees with observations in previous experiments Greer et al., Acta Mater. 48, 2823 (2000). We explain this observation in terms of interfacial energies. On the preferential (111) surface of Al the solid-liquid interfacial energy is highest while the solid-vacuum energy is lowest. Our simulations also show that the extent of ordering taking place in liquid Al close to the Al3Ti substrate above the melting point correlates well with the effectiveness of the substrate as a nucleant below the melting temperature: this could provide a computationally efficient scheme to identify good nucleants.
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