Continuous Recrystallization Phenomenon in High Purity Copper during Equal Channel Angular Pressing up to High Strain at Room Temperature

Abstract

Equal channel angular pressing (ECAP) was conducted at room temperature to a high strain level of ~24 in high purity copper. Tensile testing, Transition Electron Microscopy (TEM) and Electron backscatter diffraction (EBSD) were used to characterize the microstructure and property evolution with the increase of ECAP strain. It was found that tensile yield strength and the stored energy increases upon ECAP processing until a peak reached at 8~12 passes of ECAP, and their saturation was observed at higher ECAP passes. Continuous recrystallization phenomenon in microstructure was observed, where dislocation free crystallites with large misorientation to their surrounding matrix and smaller than the nuclei for discontinuous recrystallization were observed embodied in the matrix of deformed structure with high dislocation density. A two-step process was observed for the formation of these small crystallites, first the condensing of dislocation tangles into a narrow boundary, mostly low angle boundary; And second local migration (in sub-micrometer range) of short grain boundaries, in strong contrast to the dramatic migration of long large angle grain boundaries during discontinuous recrystallization to swallow the deformed matrix, was observed leading to vanish of small subgrains.