Reversible–irreversible plasticity transition in twinned copper nanopillars

Through computer simulations, we show that plasticity in twinned copper nanopillars can be either reversible or irreversible depending on the applied stress state. Copper nanopillars, containing twinned crystals, are subjected to both compression and tension, and the ratio of the resolved shear ðrRÞ to the normal stress ðrN Þ, R, is adjusted through variation of the orientation of the twin boundary plane with respect to the loading axis. It is found that the yield locus on the rR–rN plane for twinned nanopillars is asymmetric with respect to the sign of R. For a 9 nm diameter copper nanopillar under compression, plastic deformation can be totally reversed when rR is in the range 0:5 6 rR 6 1 GPa, with a corresponding increase in the compressive normal stress, up to 2.5 GPa. It is shown that these conditions are achieved for axial strains <3.3%, and that the transition to plastic irreversibility takes place at larger strains or normal stresses. The mechanism responsible for the plastic reversible–irreversible transition is shown to be a competition between the nucleation of Shockley partial dislocations at the nanopillar surface for irreversible plasticity vs. twinning dislocations for reversible plasticity. Furthermore, the speed of Shockley partials at twin boundaries is subsonic when there is either tension or compression acting on the twin boundary, and slightly supersonic when only shear is applied. 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.