MONTE-CARLO SIMULATIONS OF SEGREGATION AT [001] TWIST BOUNDARIES IN A PT(AU) ALLOY .2. 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 theta = 5-degrees [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 k(B) (0.49 +/- 0.10) respectively.