Thermodynamics of Grain Boundary Segregation and Applications to Anisotropy, Compensation Effect and Prediction

Interfacial segregation has long been studied because of its impact on various properties of materials. Usually, theoretical descriptions are based on thermodynamic terms such as segregation enthalpy and entropy. Comparisons of published papers of different authors reveal fundamental differences in our understanding and application of these thermodynamic characteristics. To clarify this situation, the report reviews several types of segregation isotherms and their derivations. Individual sets of thermodynamic state functions appearing in segregation isotherms—Gibbs energy, enthalpy and entropy—are interpreted in detail. We identify the sources of controversial interpretations and the physical meaning of particular sets of thermodynamic state functions. It is shown that for a correct interpretation an unambiguous and systematic use of the term excess in thermodynamic terminology is necessary, as well as a correct evaluation scheme for data obtained with Auger Electron Sepctroscopy (AES). Successful applications of segregation isotherms based on standard thermodynamic state functions of segregation are demonstrated for three examples: anisotropy of grain boundary segregation and its consequences for the classification of grain boundaries, compensation effect applied to grain boundary segregation, and quantitative prediction of grain boundary segregation (including anisotropy).