Segregation to Surfaces and Interfaces

The thermodynamics of segregation to surfaces and interfaces establish a framework of solute enrichment behaviour at equilibrium, depending on temperature and bulk concentration. It is represented by the Langmuir-McLean adsorption theory and its modifications. Solute-solute interactions in binary and multi-component systems allow a description of the occurrence of two-dimensional phases, of site competition of segregants and of cosegregation. The kinetics of segregation are mainly governed by the bulk diffusivity of the solute. Calculations of the segregation free energy based on surface energy differences of the components and on strain relief energy serve as a guideline for predictions of enrichment in binary systems and for the influence of surface orientation. Results of computer modelling of the atomistics of grain boundary segregation show the occurrence of specific interface sites corresponding to a spectrum of segregation energies, which help to understand the experimental evidence of segregation anisotropy. Similar conclusions are obtained from molecular orbital cluster calculations which disclose the chemical bond strength and its dependence on the segregant and the structural configuration. The importance of segregation to materials properties is explained for grain boundary cohesion and diffusion