Experimental study on pure copper subjected to different severe plastic deformation modes

Equal channel angular pressing (ECAP), elliptical cross-section spiral equal-channel extrusion (ECSEE) and torsion deformation (TD) have been proven as efficient SPD methods for grain refinement. In order to compare the characteristics of grain refinement by these technologies, experimental researches on microstructure evolution and mechanical properties have been conducted by optical microscopy (OM), transmission electronic microscopy (TEM) and microhardness tests. OM observation shows a significant decrease and non-uniformed distribution in grain size on the cross-section of the processed samples, which agrees well with the result of strain distribution. TEM observation shows a similar refinement process undergoing the forming of shear bands, dislocation forest, large-angle grain boundaries and sub-grains. The different morphological structures by TEM have been discussed in terms of the effect of deformation modes including bending-torsion, extrusion torsion and pure torsion on microstructure evolution. Microhardness distribution of pure copper after 6-passes deformation agrees well with the microstructure observed by OM. However, ECAP is different with ECSEE and TD in microhardness distribution along the radial and circumferential directions.