Grain Boundary Structure in Body-Centered Cubic Materials

摘要

A method has been developed to study the atomic structure of grain boundaries in bcc materials. Rigid body displacements between two grains are calculated. These displacements result in groups of atoms that retain first or second-nearest neighbor distances across the boundary. After further individual atom relaxations, the atoms at the boundary form distorted polyhedra whose edges have the length of the first or second-nearest neighbor distances. The boundary structure can be described by these structural units. Certain favored boundaries are composed of only one type of polyhedron while intervening boundaries are composed of mixtures of the polyhedra of the favored boundaries. Several structural unit descriptions are possible for each boundary. The most likely structure is chosen on the basis of the criteria of highest symmetry, maximum coordination and minimum excess volume at the boundary. A study of the symmetry of the grain boundaries in terms of color symmetry groups is given. Some structures are related by the addition of one layer of atoms parallel to the boundary, or, since the excess volume is equal for those boundaries, by an in-plane displacement. Possible transformations between these structures have important implications in the phenomena of grain boundary segregation. The crystallographic concepts to study rigid-body translations and their equivalent in-plane displacements are described. (ERA citation 11:022957)