Microstructural Features of Ultrafine Grained Copper under Severe Deformation

Abstract

In this work the microstructural features of pure copper were studied using two methods of severe plastic deformation: equal-channel angular pressing (ECAP) and hard cyclic viscoplastic (HCV) deformation.
During the first step the metal was severely deformed up to 10 Bc routes of ECAP. The ultrafine grained microstructure was received. The elongated laminar substructure has low angle and diffuse grain boundaries, but high dislocation density. Metal shows high hardness and strength but low ductility at tension straining. During the second step – HCV deformation – the strain amplitude of tension-compression cycles was stepwise increased from 0.2 % up to 2.5 % for 30 cycles and for five series. The results show, that under HCV deformation the ultrafine grained microstructure with high-angle grain boundaries was formed. The mechanism of microstructure evolution contains the elongated (ECAP processed) subgrains fracture under shear stresses by atomic layers of crystals and new microstructure with high-angle grain boundaries forming. By this the density of dislocation ribbons was decreased (from ~4.3 × 1014 m–2 to ~2.1 × 1014 m–2) mainly inside of grains and the dislocation density was increased on/near new high-angle grain boundaries. The samples with such microstructure show relative stabile viscoplastic behavior, high level of tension-compression stresses and large uniform elongation under tension straining.