Migration of an incommensurate intercrystalline boundary and boundary self-diffusion

Abstract

Most grain boundaries found in polycrystalline metals are not special. The angle of their mutual disorientation is an arbitrary value, and the axis of rotation is arbitrarily oriented to the plane of the boundary. Periodic atomic structures such as lattices of coincident nodes, alternating polyhedra, and others do not arise within such boundaries. They are called boundaries of a general type, non-special, arbitrary, or incommensurate. The general theory of relaxation processes at such boundaries has not yet been sufficiently developed. The aim of the study was the development of the model of migration of an incommensurate intercrystalline boundary at the atomic level and the description of the process of self-diffusion along it. A circle called the main region is described around each boundary lattice node of one of the crystallites. If there is an atom in the node, then an atom of another crystallite is excluded from entering it. In the case of a vacant node in the main region, such an atom can be located. An atom in a planar picture means an atomic series in the three-dimensional case. The distribution of vacant nodes of the growing crystallite is uniform over the flat reduced main region. The migration mechanism involves the implementation of the following main processes: local rearrangement of atomic configurations and selfdiffusion of atoms in the transverse direction of the slope axis. The characteristic times of these processes and the expression for the migration rate were found. The migrating boundary contains a large number of delocalised vacancies. This leads to the high diffusion mobility of atoms. Most vacancies in the boundary are not of thermal origin, but are determined only by the geometric atomic structure of the boundary. In this case, the expression for the boundary self-diffusion coefficient does not contain a multiplier depending on the activationenergy of vacancy formation. This leads to the fact that the coefficient of self-diffusion along the migrating boundary is significantly higher than in the stationary boundary. The model of an incommensurate boundary allows us to describe its migration and calculate the self-diffusion coefficient.

Keywords

• incommensurate intercrystalline boundaries
• delocalised vacancies
• boundary self-diffusion
• migration mobility