Cooperative Grain Boundary Processes in Superplastic Flow

摘要

Cooperative grain boundary sliding (CGBS) and cooperative grain boundary migration (CGBM) are important processes for superplastic deformation. Experimental studies performed on a number of superplastic materials and review of literature indicates sliding of grain groups suggesting a process of cooperative movement of grains. Special techniques were used to study CGBS, including targeted observations of superplastic flow in-situ SEM and use of macroscopic marker lines. Observations performed on the macroscopic scale showed sliding of grain groups at the surfaces of maximum shear stress. The pattern of the CGBS surfaces is consistent with that of slip lines predicted by the slip band field theory. Observations performed at the mesoscopic scale showed that spacing of CGBS surfaces determining size of sliding grain blocks depends on deformation conditions. The sequential manner of CGBS can be modeled in terms of cellular dislocations, topological defects in an ideal grain array. At the microscopic scale, sliding of individual grain boundaries forming a CGBS surface has been studied. Movement of grain boundary dislocations associated with a step at the core causes coupling of sliding and grain boundary migration, and cooperative manner of grain sliding leads to a long range correlation in grain boundary migration. It has been demonstrated that processes of microstrucrural evolution occurring in superplastic materials, such as cavitation, grain growth, formation of second phase Stingers, and break up of elongated second phase particles is influenced by the CGBS phenomenon.