Transition of deformation mechanisms from twinning to dislocation slip in nanograined pure cobalt

• Gradient nanograined structures were prepared by plastic deformation in a pure cobalt sample. • Atomic-scale observation for microstructure characteristics was conducted in nanograined cobalt. • Comparing deformation mechanisms between nanograins and submicron-grains indicates a transition of governing deformation mechanism as reducing the gain size to nanoscale. Deformation mechanisms of nanograined and submicron-grained pure cobalt processed by means of high strain rate shear deformation at cryogenic temperatures were studied. Microstructural analysis revealed a transition of governing deformation mechanism from deformation twinning and < a > dislocation slip in submicron-grains to < c + a > and < a > dislocations slip in nanograins. Microhardness tests illustrated that the Hall-Petch relation slope changes consequently with the transition of deformation mechanism.