The study of the evolution of grain boundary (GB) structures and the mechanisms of dislocation motion in graphene is of significance in uncovering the physical essence of plastic deformation behavior of graphene. Currently, the dynamic behavior of graphene GBs under non-mechanical loads has been extensively investigated. However, due to the inherent limitations of existing experimental conditions and simulation methods in terms of temporal and spatial scales, the dynamic evolution process of dislocations in graphene under mechanical tensile loads and their intrinsic correlation with plastic deformation are still poorly understood. In this work, a phase-field crystal (PFC) model based on classical density functional theory (DFT) is adopted. Combining periodic density field variables, the model achieves cross-scale coupling between microscopic crystal structures and macroscopic diffusion time scales, enabling efficient simulation of long-term evolution processes. It is particularly suitable for characterizing microscopic mechanisms involving complex defect evolution in graphene, such as dislocation glide and climb, and GB migration.

In this work, the complete deformation process of a graphene bicrystal system containing a GB loop under uniaxial tensile loading is simulated on an atomic scale, including elastic response, elastic-plastic transition, plastic deformation, and fracture. The transformation characteristics of 5|7 dislocation core structures and the topological evolution of the GB loop within the system are systematically investigated. The simulation results reveal that when the applied strain is below a critical value, the system exhibits the elastic response, characterized by a linear relationship between the average response strain and the applied strain. As the strain reaches the critical value, the 5|7 dislocations at the GB loop undergo transformation into 5|7|7|5 dislocations through C—C bond rotation. This transition is accompanied by a significant increase in the strain amplitude at the dislocation cores, marking the onset of plastic deformation. Beyond the critical strain, the system thus enters the plastic deformation stage, during which the GB loop exhibits three different types of evolution behaviors: 1) alternating transformations between 5|7 and 5|7|7|5 dislocation structures driven by repeated C—C bond rotation; 2) a cyclic evolution of dislocations involving “pinning \rightleftharpoons mixed climb/glide motion”, accompanied by energy fluctuations described as “energy storage-dissipation-restorage”; 3) dislocations remaining in a “pinned” state until stress concentration in their core regions initiates transgranular cracking, ultimately leading to ductile fracture of the system.

This study provides important theoretical insights into the physical mechanisms underlying the plastic deformation behavior of graphene.