Monte Carlo simulations of segregation at [001] twist boundaries in a Pt(Au) alloy-II. Discussion

A three dimensional plot of the density of Au atoms versus the position coordinates for the (002) planes, immediately adjacent to the interface, for a θ = 5° [001] twist boundary, suggests the presence of a hill-and-valley structure in the Au segregation behavior at 850 K. The valleys correspond to the regions of good atomic fit between the regions of misfit; the hills correspond to the cores of the pairs of orthogonal primary grain boundary dislocations. A model for the Monte Carlo simulations is developed that leads to a linear expression for S_aver containing a core segregation factor (S_core). For this model S_core has a single value for all twist boundaries at a given T. All the Monte Carlo results can be plotted on a single Arrhenius plot and the Gibbs free binding energy of a Au atom to the cores of the primary grain boundary dislocations extracted; the binding enthalpy and entropy of segregation are 0.095 ± 0.01 eV and κ_B (0.49 ± 0.10) respectively.