Transition from strengthening to softening by grain boundaries in ultrafine-grained Cu

Abstract The deformation behaviors of ultrafine-grained (UFG) pure Cu with ≈350 nm grain size produced by severe plastic deformation and of coarse-grained (CG) Cu were investigated as function of temperature T and strain rate ϵ ˙ . Below 450 K (0.33 of melting temperature) the grain structure is fairly stable. Here the grain boundaries in UFG Cu have a strengthening effect with regard to plastic yielding. With increasing strain ϵ, increasing T and decreasing ϵ ˙ , however, the strengthening turns into softening by grain boundaries. This transition is related with distinctly higher strain rate sensitivity of UFG Cu compared to CG Cu and a relatively low activation energy of flow in the order of 100 kJ/mol. At steady state, the range of softening by grain boundaries extends from the stability limit of the grain structure of UFG Cu down to a temperature close to room temperature where UFG Cu was produced. The transition from strengthening to softening is explained in terms of generation and grain boundary diffusion controlled annihilation of lattice dislocations at grain boundaries.