Variations in the free energy of a grain boundary/crystal system, due to the transition of vacancies from the bulk into grain boundaries (and their subsequent delocalization in the disordered atomic structure of the boundary) were analyzed. An expression was obtained for the equilibrium excess volume of a grain boundary. Its magnitude was related to the formation energy of vacancies in the crystal lattice. As an elementary act of grain-boundary self-diffusion the formation of a metastable localized grain-boundary vacancy (as a result of a thermal fluctuation of the material density), and a jump of a neighboring atom into this vacancy, was considered. Expressions were obtained for the activation energy and diffusion coefficient in equilibrium and non-equilibrium grain boundaries.

A Unified Approach to the Description of Diffusion in Equilibrium and Non-Equilibrium Grain Boundaries. V.N.Perevezentsev: Physics of Metals and Metallography, 2002, 93[3], 207-10