Accommodation of the stress field at a grain boundary under heterogeneous shear by initiation of microcracks

Abstract

Polycrystalline deformation leads to the creation of deformation ledges at grain boundaries undergoing heterogeneous shear. The large strain fields of the deformation ledges can be relieved either by nucleation of further slip or by initiation of microcracks to accommodate the interface strains set up by the deformation of adjacent grains. The formation of accommodation microcracks from the deformation ledges is considered in detail. It is shown that the type of the accommodation microcrack—tensile or shear, transgranular or intergranular—depends on the type of the deformation ledge, and can be spontaneously nucleated. Stability, equilibrium, and the shape of the microcrack are determined from a discrete dislocation approach. Computer techniques are used in this study and the results are compared with those of the continuum mechanics approach. The effect of the application of an external stress is considered in detail and it is shown that a given stable microcrack has a critical stress at which it becomes unstable with respect to unlimited extension. Growth of a microcrack at a stress smaller than the critical stress is studied as a heterogeneous growth.