Evolution in Microstructures and Mechanical Properties of Pure Copper Subjected to Severe Plastic Deformation

Considering the great advantage of improving the pressing speed in reducing the number of repeated deformation passes and grain size, high-speed equal channel angular pressing (ECAP) technology has been applied to achieve the perfect distribution of grain size in a sample volume of pure copper. The microstructures and mechanical properties of pure copper suffering ECAP are investigated. The results show that the well refined grains are obtained through three stages, during which the grain reorientation and recrystallization occurs. After ECAP, the volume fraction of high-angle grain boundaries is reduced, but shows an obvious increase in the population for high-pass extruded sample. Besides, ECAP can significantly increase the proportion of the special boundaries probably through the transformation from annealing twins to deformation twins. The tensile strength of ECAP-ed sample greatly increased to 1.87 times larger than that of the annealed bar, and then improved slowly regardless of increasing strain. The relationship between strength and grain size still obeyed Hall-Petch formula. The cloud maps of microhardness distribution illustrated that the hardness is severely improved after the first three passes, but leveled off after that. Besides, the cloud maps also present the moderate inhomogeneity of microhardness with higher values in the edge and lower values towards the center.