Grain-size effect on plastic flow in nanocrystalline cobalt by atomistic simulation

Molecular dynamics simulation is employed to investigate the plastic flows in nanocrystalline (nc) hexagonal close-packed cobalt under uniaxial tensile deformation. In nc-Co samples modeled by a semi-empirical tight-binding potential, different deformation behaviors such as nucleation and growth of disordered atom segments (DAS) inside grains, deformation-induced hexagonal close-packed to faced-centered cubic transformation, partial dislocation activities are identified at different grain sizes (4-12 nm). At high stresses (1.2-3.2 GPa) and low temperatures (77-470 K), growth of DAS and their interaction with stacking faults are found to dominate the deformation process, even when the grain size is as small as 4 nm. A model for plastic flow generated by DAS inside grains is proposed. The strain rates and the inverse Hall-Petch-like behaviors in nc-Co with sub-10 nm grain sizes can be well described by the DAS plastic-flow model.