Atomistic Simulations of Crystal-Melt Interfaces in a Model Binary Alloy: Interfacial Free Energies, Adsorption Coefficients and Excess Entropy

Published

February 24, 2009

Author(s)

Chandler A. Becker

Abstract

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 interatomic strengths equal to 0.75,and inter-species interactions given by standard 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 (?(j)i ) at six temperatures along the solidus/ liquidus boundary. The values for ?(2)1 , 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 ?(1) 2 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 ?(2) 1 . The capillary fluctuation method was used to compute the temperature dependence of the magnitudes and anisotropies of the crystal-melt interfacial free energy (? ). At all temperatures we obtain the ordering ?100 > ?110 > ?111 for the high-symmetry (100), (110) and (111) interface orientations. The values of ? monotonically decrease with decreasing temperature (i.e., increasing concentration of the lower melting-point species). Using the calculated temperature-dependent values of ? and ?(2) 1 in the Gibbs adsorption theorem, we estimate that roughly 25 % of the temperature dependence of ? for the alloys can be attributed to interface adsorption, while the remaining contribution arises from the relative excess entropy S (2)xs . The analysis yields an estimate for the excess entropy for a pure element of -0.45 ?2 22 for the (100) interface; this value, when multiplied by the melting temperature, accounts for roughly 80 % of the interfacial free energy of the pure element.

Citation

Journal of Physical Chemistry B

Pub Type

Journals

Keywords

Monte-Carlo, Molecular Dynamics, Interfacial Thermodynamics, Crystal-Melt Interfaces

Citation

Becker, C. (2009), Atomistic Simulations of Crystal-Melt Interfaces in a Model Binary Alloy: Interfacial Free Energies, Adsorption Coefficients and Excess Entropy, Journal of Physical Chemistry B (Accessed April 18, 2024)