The evolution law and mechanism of grain boundary structure in the deformation process of graphene are of great significance for understanding the deformation behavior of graphene and optimizing its mechanical properties. In this work, single-layer graphene is taken as the research object and a double crystalline graphene model is established by using the three-mode phase-field crystal method, thereby in depth ascertaining the evolution mechanism of dislocations at small-angle symmetrical tilt grain boundaries in graphene under strain. In view of the relaxation and deformation process, the relationship between the number of multiple dislocations and the grain boundary angle of graphene is studied on an atomic scale, and the deformation and failure mechanism of double crystalline graphene under tensile load are revealed, and also discussed from the perspective of the free energy.

It is found that, after relaxation, with the increase of grain boundary angle, the density of dislocations at the grain boundary decreases, and the number of specific types of dislocations (5|8|7 and 5|7 dislocations) increases. Under stress loading parallel to the grain boundary, the changes of free energy of the systems containing grain boundaries with different angles show the same trend: at first, they fall to the inflection point and then rise abnormally, and the dislocation behavior cannot effectively alleviate the stress concentration caused by continuous loading in the system, leading to failure finally.

Under tensile load, the free energy changes of the systems are divided into four stages, they being stage (I), in which the dislocations at grain boundaries are slightly deformed but do not change their structure, stage (II), in which dislocations at the grain boundaries are transformed into 5|7 or 5|9 dislocation due to C—C bond fracture or rotation, and the dislocations that are “incompatible” have higher energy, making them more conducive to improving the tensile properties of graphene, stage (III), in which the 5|7 and 5|9 dislocations begin to fail, and the free energy shows a tendency to decrease significantly, and stage (IV), in which the double crystalline graphene systems are completely in failure. The system with a grain boundary angle of 10° exhibits the most substantial deduction in free energy in stages (I), (II), and (III), and possesses the highest overall tensile strength.

This work contributes to understanding the micromechanical behavior of graphene on an atomic scale.