A Grain Boundary Model and the Mechanism of Viscous Intercrystalline Slip

Abstract

A study of the activation energy associated with the viscous intercrystalline slip shows that the conventional theories of grain boundary, e.g., the intercrystalline amorphous cement theory and the abrupt transitional theory, are both untenable. A grain boundary model is described in which the transition region at the boundary is considered as consisting of numerous disordered groups of atoms or diffused holes. The intercrystalline slip occurs through the atomic rearrangement by thermal agitation within each ``disordered group'' by a shear process involving as units of flow only a few atoms. This grain boundary model and slip mechanism are consistent with experimental facts and furnish, furthermore, a unified viewpoint as to the mechanism of the viscous intercrystalline slip, the volume diffusion in metals, and the constant rate creep of metal crystals under small stress. Further experiments are described concerning the influence of previous deformation and impurities on grain boundary viscosity. It has been found that the grain boundary viscosity is lower in a specimen subjected to a heavier deformation prior to its recrystallization. A very small amount of impurities was found to be able to block partially or completely the grain boundary slip in aluminum, iron, and copper. These observations are readily understood on the basis of the proposed grain boundary model and slip mechanism.