Atomistic simulations of crystal-melt interfaces in a model binary alloy: Interfacial free energies, adsorption coefficients, and excess entropy
Atomistic simulations of crystal-melt interfaces in a model binary alloy: Interfacial free energies, adsorption coefficients, and excess entropy
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DOI：
10.1103/physrevb.79.054109
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发表时间：
2009-02-01
影响因子：
3.7
通讯作者：
Foiles, S. M.
中科院分区：
文献类型：
--
作者：
Becker, C. A.;Olmsted, D. L.;Foiles, S. M.
采用Monte Carlo和分子动力学模拟研究晶体-熔体界面的平衡结构和热力学性质的模型二元合金系统所描述的Lennard-Jones原子间相互作用与零尺寸失配，相互作用强度的比率等于0.75，和种间相互作用给出的Lorentz-Berthelot混合规则。该合金系统在零压力下具有简单的透镜型固-液相图，在固溶体和液相中具有接近理想的溶解热力学。平衡密度分布计算（100）取向的晶体-熔体界面，并用于推导相对吸附系数（γ（（j））（i））的大小在六个温度沿着固相线/液相线边界。发现较低熔点物质（1）相对于较高熔点物质（2）的相对吸附量Gamma（（2））（1）的值随温度单调变化，其值为正值，并且在每个界面位置的几个原子百分比的范围内。相比之下，Gamma（（1））（2）的值显示出更复杂的温度依赖性，相对吸附量的大峰值比Gamma（（2））（1）的值大十倍以上。毛细管波动方法被用来计算晶体-熔体界面自由能（γ）的大小和各向异性的温度依赖性。在所有温度下，对于高对称性的（100），（110）和（111）界面取向，我们得到了γ（100）> γ（110）> γ（111）的顺序。γ的值随着温度的降低而单调降低（即，增加较低熔点物质的浓度）。使用吉布斯吸附定理中计算的γ和γ（（2））（1）的温度依赖性值，我们估计合金的γ的温度依赖性的大约25%可以归因于界面吸附，而剩余的贡献来自相对过剩熵S-xs（（2））。
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 (Gamma((j))(i)) at six temperatures along the solidus/liquidus boundary. The values for Gamma((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 Gamma((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 Gamma((2))(1). The capillary fluctuation method is used to compute the temperature dependence of the magnitudes and anisotropies of the crystal-melt interfacial free energy (gamma). At all temperatures we obtain the ordering gamma(100)>gamma(110)>gamma(111) for the high-symmetry (100), (110), and (111) interface orientations. The values of gamma monotonically decrease with decreasing temperature (i.e., increasing concentration of the lower melting-point species). Using the calculated temperature-dependent values of gamma and Gamma((2))(1) in the Gibbs adsorption theorem, we estimate that roughly 25% of the temperature dependence of gamma for the alloys can be attributed to interface adsorption, while the remaining contribution arises from the relative excess entropy S-xs((2)).