Using molecular-dynamics simulation coupled with an analysis of equilibrium capillary fluctuations in interfacial position, we compute the magnitude and anisotropy of the interfacial free energy $\gamma$ for a binary hard-sphere system with a diameter ratio $\alpha =0.9$. This system, in which the fluid mixture coexists with a randomly substituted face-centered-cubic solid solution, is a useful reference model for alloys. Our results show that $\gamma$ increases with increasing mole fraction of the smaller sized particle when temperature is held constant. However, after rescaling the results to fixed pressure and varying temperature, we find that $\gamma$ decreases with increased alloying by the smaller particle (corresponding to lower temperatures). Thus, $\gamma$ is seen to decrease with increasing concentration of the lower melting point solute, consistent with earlier simulations on Ni/Cu and Lennard-Jones mixtures. The anisotropy in $\gamma$ is such that the inequality ${\gamma }_{100}>{\gamma }_{110}>{\gamma }_{111}$ holds for all concentrations studied. Using the classification scheme of Haxhimali et al., [Nat. Mater. 5, 660 (2006)] we find that the anisotropy in $\gamma$ is consistent with a predicted $⟨100⟩$ primary dendrite growth direction.

• Received 8 September 2008

DOI:https://doi.org/10.1103/PhysRevB.78.144112