Atomistic simulations of crystal-melt interfaces in a model binary alloy: Interfacial free energies, adsorption coefficients, and excess entropy

Monte Carlo and molecular-dynamics simulations are employed in a study of the equilibrium structural and thermodynamic properties of crystal-melt interfaces in a model binary alloy system described by Lennard-Jones interatomic interactions with zero size mismatch, a ratio of interaction strengths equal to 0.75, and interspecies interactions given by Lorentz-Berthelot mixing rules. This alloy system features a simple lens-type solid-liquid phase diagram at zero pressure, with nearly ideal solution thermodynamics in the solid and liquid solution phases. Equilibrium density profiles are computed for (100)-oriented crystal-melt interfaces and are used to derive the magnitudes of the relative adsorption coefficients (Γ_i^(j)) at six temperatures along the solidus/liquidus boundary. The values for Γ₁⁽²⁾, the relative adsorption of the lower melting-point species (1) with respect to the higher melting point species (2), are found to vary monotonically with temperature, with values that are positive and in the range of a few atomic percent per interface site. By contrast, values of Γ₂⁽¹⁾ display a much more complex temperature dependence with a large peak in the magnitude of the relative adsorption more than ten times larger than those found for Γ₁⁽²⁾. The capillary fluctuation method is used to compute the temperature dependence of the magnitudes and anisotropies of the crystal-melt interfacial free energy (γ). At all temperatures we obtain the ordering γ₁₀₀>γ₁₁₀>γ₁₁₁ for the high-symmetry (100), (110), and (111) interface orientations. The values of γ monotonically decrease with decreasing temperature (i.e., increasing concentration of the lower melting-point species). Using the calculated temperature-dependent values of γ and Γ₁⁽²⁾ in the Gibbs adsorption theorem, we estimate that roughly 25% of the temperature dependence of γ for the alloys can be attributed to interface adsorption, while the remaining contribution arises from the relative excess entropy S_xs⁽²⁾.