| A theoretical model is suggested which describes transition from plastic shear into rotation deformation mode - deformation accompanied by crystal lattice rotations - in nanocrystalline metals and ceramics. Within the model, the shear deformation occurs through either grain boundary sliding or lattice dislocation slip and results in formation of pile-ups of either grain boundary or lattice dislocations, respectively. The dislocation pile-ups create stress fields initiating the rotation deformation (occurring through formation of immobile disclinations whose strengths gradually increase during the formation process) in neighboring nanograins. These processes provide transition from plastic shear into rotation deformation mode in nanocrystalline metals and ceramics, including single-phase nanocrystalline materials with narrow and bimodal grain size distributions as well as nanocomposites consisting of microscale grains and nanoparticles. The conditions are calculated at which the transition is energetically favorable in nanocrystalline Ni and α-Al2O3 (corundum). |
full paper (pdf, 154 Kb)