Using computer simulations, it was showd that plasticity in twinned copper nanopillars could be either reversible or irreversible depending on the applied stress state. Copper nanopillars, containing twinned crystals, were subjected to both compression and tension, and the ratio of the resolved shear (σR) to the normal stress (σN), R, was adjusted through variation of the orientation of the twin boundary plane with respect to the loading axis. It was found that the yield locus on the σR–σN plane for twinned nanopillars was asymmetric with respect to the sign of R. For a 9 nm diameter copper nanopillar under compression, plastic deformation could be totally reversed when σR was in the range of 0.5 to 1GPa, with a corresponding increase in the compressive normal stress, up to ≈2.5 GPa. It was shown that these conditions were 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 was 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 was subsonic when there was either tension or compression acting on the twin boundary, and slightly supersonic when only shear was applied.

Reversible–Irreversible Plasticity Transition in Twinned Copper Nanopillars. J.A.Brown, N.M.Ghoniem: Acta Materialia, 2010, 58[3], 886-94