Microstructure Evolution and Microhardness Distribution of Copper Processed Using Multiple Passes of Elliptical Cross-Sectional Spiral Equal-Channel Extrusion

The aim of this work is to study the effect of six-pass elliptical cross-sectional spiral equal-channel extrusion (ECSEE) on the microstructure and performance of ultrafine-grained (UFG) copper. Equiaxed grains of average grain size of less than 1 μm are formed into shear bands in the low strain region of ECSEE deformed specimen. More homogeneous and equiaxed microstructure with high misorientation angles is obtained in the high strain. Moreover, the microstructure evolution of ECSEE-induced copper is a dynamic equilibrium process of shear deformation accompanying the interactions of high dislocation density, cellular structure and high-angle grain boundaries. The grain ECSEE refinement mechanism is described as the formation process of dislocations, cells, local grain sub-boundaries rotation and large angle grain restructure. The significantly non-uniform hardness distribution is consistent with the deformation behavior and microstructure refinement in the ECSEE-induced specimen. The homogeneity of microstructure and hardness improves as the ECSEE pass increases.