Interfacial two-dimensional phase transitions and impurity segregation

Impurity segregation to surfaces and grain boundaries are investigated within a phenomenological theory. Next neighbour interactions between segregating species are taken into account in a regular solution model. Resulting phase transitions because of interfacial miscibility gaps are discussed for equilibrium segregation as well as for the kinetics of segregation in ternary systems with repulsive interaction between both segregants. The rate determining step for segregation kinetics remains volume diffusion in these cases. In contrast structural interfacial transitions may delay the kinetics of segregation significantly because nucleation of the second phase requires at least locally an oversaturation up to a critical value compared with the pure thermodynamically determined coverage of the first structure. Computer simulations based on a model which characterizes both structures by different saturation values and segregation energies were carried out. A comparison with surface segregation measurements of Pb in Ag and Sn in Fe shows a good agreement.