Effect of light elements impurity atoms on grain boundary diffusion in FCC metals: a molecular dynamics simulation

Abstract

Effect of carbon and oxygen impurity atoms on diffusion along the tilt grain boundaries with <100> and <111> misorientation axis in metals with FCC lattice was studied by mean of molecular dynamics method. Ni, Ag, and Al were considered as metals. Interactions of metal atoms with each other were described by many-particle Clery-Rosato potentials constructed within the framework of tight binding model. To describe interactions of atoms of light elements impurities with metal atoms and atoms of impurities with each other, Morse pair potentials were used. According to obtained results, impurities in most cases lead to an increase in self-diffusion coefficient along the grain boundaries, which is caused by deformation of crystal lattice near the impurity atoms. Therefore, additional distortions and free volume are formed along the boundaries. It is more expressed for carbon impurities. Moreover, with an increase in concentration of carbon in the metal, an increase in coefficient of grain-boundary self-diffusion was observed first, and then a decrease followed. This behavior is explained by formation of aggregates of carbon atoms at grain boundary, which leads to partial blocking of the boundary. Oxygen atoms had smaller effect on diffusion along the grain boundaries, which is apparently explained by absence of a tendency to form aggregates and lesser deformation of crystal lattice around impurity. The greatest effect of impurities on self-diffusion along the grain boundaries among the examined metals was observed for nickel. Nickel has the smallest lattice parameter, impurity atoms deform its lattice around itself more than aluminum and silver, and therefore they create relatively more lattice distortions in it and additional free volume along the grain boundaries, which lead to an increase in diffusion permeability. Diffusion coefficients along the high-angle boundaries with misorientation angle of 30° turned out to be approximately two times higher than along low-angle boundaries with a misorientation angle of 7°. Diffusion along the <100> grain boundaries flowed more intensively than along the <111> boundaries.