▎ 摘　　要

The Hall-Petch relation predicted strengthening may fail at extremely fine scale, mainly due to the stress-induced dynamic grain growth. In this letter, we report a new strategy of stabilizing grain boundaries by introducing uniformly dispersed graphene into electrodeposited nanocrystalline (NC) Ni. After deformation, the average grain size of graphene-doped NC Ni increases slightly (from 33 nm to 40 nm), while the undoped NC Ni shows a notable increase in grain size (from 35 nm to 51 nm). This discrepancy was attributed to the grain boundary pinning effect from graphene, which in turn activates deformation twinning as a dominate mode for accommodating plastic strains at higher tensile stress level (i.e., 1457 MPa), leading to a 27.6% improvement in strength and a 24.7% enhancement in toughness. The observation of graphene-induced twinning in nano-scaled grains may open a new window for the design of graphene reinforced metallic composites with high strength and toughness.