The transition from an inverse pseudo Hall-Petch to a pseudo Hall-Petch behavior in nanocrystalline graphene

We perform a molecular dynamics simulation to investigate the effect of grain size on the material properties of nanocrystalline graphene, which inherently behaves like brittle material. From a series of simulations, our results reveal a transition from an inverse pseudo Hall-Petch to a pseudo Hall-Petch behavior in nanocrystalline graphene at a critical grain size. We show that since the grain boundary junctions serve as the stress concentration sites where cracks preferentially nucleate, the strength decreases with an increasing density of grain boundary junctions. This inverse pseudo Hall-Petch behavior is observed by decreasing the grain size in nanocrystalline graphene, where the grain size is larger than the critical value. In contrast, when the grain size is reduced below a critical value, the nanocrystalline graphene sustains tensile loads, even after cracks initiate due to a gradual evolution of the cracks. This indicates that the stress concentration of grain boundary junctions decreases with a decreasing grain size by stress cancellation between 5 and 7 defects. We believe that our findings provide a comprehensive and in-depth understanding of the mechanical properties and deformation process of nanocrystalline graphene.