An atomistic study of grain boundary segregation and cracking

A Monte Carlo model is applied to study a Σ = 5 (310) fcc tilt boundary structure and impurity segregation to this boundary. The Johnson-type potential functions are used to express atomic interactions for two binary alloys of NiCu and CuSb systems. Through atomic relaxation near absolute zero, the basic structural unit of the Σ = 5 CSL boundary is found to dissociate into two subunits. For the NiCu system, copper segregation to the boundary follows the Gibbsian equilibrium segregation behavior in which the segregation is localized on a few atomic layers, its amount increases with increase in the bulk concentration but decreases with increase in temperature. For the CuSb system, an assumption of the CuCu bond weakening due to the strong, adjacent CuSb bonds is also incorporated to realize the embrittling effect of Sb atoms. The preferential site for the large Sb atoms is the vertex of a pentagonal bipyramid in the Σ = 5 grain boundary. Without bond weakening, the Sb-segregated grain boundary maintains a structure combined with a distorted pentagonal bipyramid and a capped trigonal prism. However, with bond weakening some bonds of the trigonal prisms are disrupted in order to form new pentagonal bipyramids. When a uniaxial strain is applied in the direction perpendicular to the grain boundary plane, the weakened copper bonds become the source of microcracks thus enhancing brittle fracture along the Sb-segregated grain boundary.