Temperature and strain-rate effects on the deformation behaviors of nano-crystalline graphene sheets

The deformation behavior of nanocrystalline graphene sheets is investigated by molecular dynamics (MD) simulation by coupling the effects of the temperature and strain rate. Mechanical deformation of graphene sheets, which is dominated by the competition between bond breaking and rotation, is essentially an atomic behavior. Similar to single-crystal graphene sheets, nanocrystalline graphene sheets usually exhibit bond breaking induced brittle fracture along grain boundaries after large elastic deformation. The elastic modulus decreases slightly with temperature as a result of softening but does not depend on the strain rate. A brittle-plastic transition by bond rotation and rearrangement under stress appears to occur at high temperature above 1000 K, but the ductility is unexpectedly reduced due to accelerated bond breaking. At small strain rates, it is easier for bonds to rearrange, vacancies to coalesce, and cracks to propagate in grain boundaries and plastic deformation with a larger activation volume occurs. However, at large strain rates, the relaxation time is too short for atomic bonds to rotate and rearrange under stress. Therefore, bond elongation and brittle fracture with a smaller activation volume takes place. The results demonstrate that the atomic behavior in grain boundaries is crucial to mechanical deformation in nanocrystalline graphene sheets, which is temperature and strain rate sensitive.