Influence of grain boundaries on steady-state deformation resistance of ultrafine-grained Cu

The deformation behavior in uniaxial compression of ultrafine-grained (UFG) pure Cu produced by severe plastic deformation through equal channel angular pressing (12 passes route C) with initial spacing d(0) of large angle grain boundaries of about 350 nm is compared to that of initially coarse-grained (CG) Cu with d(0) approximate to 50 mum. UFG Cu attains its steady state of deformation within a small strain interval of about 0.05. CG Cu needs much larger strains to approach the steady-state flow stress sigma(v) (Voce stress) terminating stage III hardening and the final steady-state flow stress sigma(infinity). The dependences of the steady-state flow stresses on strain rate (epsilon)over dot and temperature T differ significantly for UFG and CG Cu. In consequence, UFG Cu becomes softer in the steady state of deformation compared to CG Cu with regard to both sigma(v) as well as sigma(infinity). This is explained in terms of annihilation of lattice dislocations at large-angle grain boundaries lowering the steady-state deformation resistance. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.