UNITED APPROACH TO THE DESCRIPTION OF DIFFUSION IN EQUILIBRIUM AND
NONEQUILIBRIUM GRAIN BOUNDARIES AND ITS APPLICATION TO NANOMATERIALS

V.N. Perevezentsev, Blagonravov Nizhny Novgorod Branch of Mechanical
Engineering Research Institute, Russian Academy of Sciences,
Ul. Belinskogo 85, Nizhny Novgorod, 603024 Russia

A change in the free energy of a grain boundary is analyzed in the
case when lattice vacancies come to the boundary and are then
delocalized in its disordered atomic structure. It is shown that the
free energy of the boundary is minimized at some excess atomic volume,
whose value depends on the energy of vacancy formation in the crystal
lattice and the boundary energy. The formation of a metastable
localized grain-boundary vacancy as a result of thermal fluctuations
of the density in a group of atoms, followed by the jump of an
adjacent atom into this vacancy, is taken as an elementary event of
grain-boundary diffusion. Expressions for the activation energy of
diffusion and the diffusion coefficient are derived for equilibrium
and nonequilibrium grain boundaries. Value of excess atomic volume
depends on kinetics of accumulation and annihilation of nonequilibrium
defects in grain boundary. Processes of recovery of nonequilibrium
grain boundaries diffusion properties at the annealing and strain
induced diffusion in nanostructural materials are analyzed in
frameworks of suggested model.

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