Strain induced dislocation evolution at graphene grain boundary by three-mode phase-field crystal method

Abstract

The evolution and role mechanisms of grain boundary structures during the deformation process of graphene are of great significance for understanding the deformation behavior of graphene and optimizing its mechanical properties. This article takes single-layer graphene as the research object and establishes a double crystal graphene model using the three-mode phase-field crystal method, deeply exploring the evolution mechanisms 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 was studied at atomic scale, and the deformation and failure mechanism of double crystal graphene under tensile load was revealed, and it is also discussed from the aspect of the free energy.<br>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 descend to the inflection point and then rise abnormally, and the dislocation behavior can not effectively alleviate the stress concentration caused by continuous loading in the system, leading to failure.<br>Under tensile load, the free energy’s change of the systems is divided into four stages, of which the stage (I): the dislocations at grain boundaries are slightly deformed but does not change its structure. The stage (Ⅱ): dislocations at the grain boundaries are transformed into 5|7 or 5|9 dislocation due to C-C bond fracture or rotation. Dislocations that are "incompatible" have higher energy, making them more conducive to improving the tensile properties of graphene. The stage (Ⅲ): the 5|7 and 5|9 dislocations, begin to fail, and the free energy shows a tendency to decrease significantly. The stage (Ⅳ): the double crystal graphene systems have completely failed. The system with a grain boundary angle of 10° exhibits the most substantial free energy decrease in Stages (Ⅰ), (Ⅱ), and (Ⅲ), and possesses the highest overall tensile strength.<br>This work contributes to understanding the micromechanical behavior of graphene at the atomic scale.