Phase-Field Crystal Modeling Of Grain Boundary Structure And Deformation Behaviors Of Graphene

Graphene materials are one of the hotspots and frontier issues in the field of materials science and condensed matter physics.The mechanical properties of graphene materials are significantly influenced by grain boundary structure and its deformation behavior.Elucidation of the evolution of grain boundary and its mechanism during deformation would has great significance in understanding the deformation behavior of graphene.To date,most of computational studies of mechanical properties of graphene focused on low-or roomtemperature behaviors which yield brittle fracture and seriously restricts its engineering application as a high-strength material.On the other hand,with the enhancing motion ability of dislocation at ultrahigh temperature conditions,the graphene material may have the ability of dislocation-controlled plasticity.However,due to the slow,diffusive timescales of dislocation motion,researches on the microscopic deformation mechanism of two-dimensional crystal materials such as graphene have brought great challenges.In this dissertation,a PFC model and its corresponding algorithm which can accurately describe the deformation dynamics of tilted and non-tilted grain boundaries of graphene wereproposed.The grain boundary structure,fracture modes,the plastic deformation and brittle-plastic transition mechanism are investigated by the proposed PFC model.This study could provide a theoretical foundation for the structure reliability of graphene materials.The main conclusions are as follows:1.Based on the structural inversion,a phase field crystal model which can describe the2 D graphene structure was established,and then a parameterization can be made via analogy.By systematically analyzed the model,we found two types of the mechanical response of the M-PFC model by simulation the shear deformation of graphene nanoripples.0???1 would corresponds to elastic behavior and reveal elastic behavior while ??1 corresponds to overdamped dynamics and reveal viscoelastic behavior.In addition,three types of shear deformation behaviors of graphene nanoripples have been found,namely,bending,simple shear and the mixed-mode based on the competition of bending and shearing.2.We established an effective computational scheme(C-PFC)which can describe the mechanical relaxation and fracture of graphene structure.The scheme via imposing an additional constraint on the S-PFC model based on the property of linear spatial dependence of atomic displacements,the system would instantaneously reach the mechanical equilibration state from global to local relaxation process.By simulating the effects of large enough characteristic size and loading rate on the mechanical properties of a graphene nanoribbon subjected to uniaxial tension,only the C-PFC scheme yielded the expected linear elasticity behavior and had a much more efficient process of dynamic relaxation and mechanical equilibration,in comparison to the S-PFC and M-PFC methods.3.For tilted grain boundaries(GBs)in graphene,the GBs are composed of a periodic array of 5|7 isolated dislocations with identical Burgers for sym-AC GB and of alternating array of 5|7 isolated dislocations whose Burgers vectors vary by 60° in orientation for sym-ZZ GB,both the disloaction with same proportion.For non-tilted GBs,both the GBs(asym-AC and asym-ZZ GB)are composed of alternating array of 5|7 isolated dislocations with different proportion.In addition,as the misorientation angles increase,the magnitude of strain variation along the GB direction becomes smaller,and the location of strain peaks have a certain periodicity.It means that the array of 5|7 isolated dislocations for non-tilted GBs also has a certain periodicity.4.We clarified the fracture modes of tilted and non-tilted GBs in graphene.When pulled parallel to the GBs,the failure began at the 5|7 membered rings and then the nano-cracks propagated into grains,which exhibited brittle trans-granular cleavage fracture behavior.When pulled vertical to the GBs,the failure began at the 5|7 membered rings and the nano-cracks were interconnected,which exhibited brittle inter-granular cleavage fracture behavior.In addition,we also identified another type of failure dynamics with low-angle zigzag GBs,showing as the disintegration and splitting of GB dislocation array through the migration of nanovoid-bound dislocations.5.We revealed the plastic deformation mechanism at high temperatures for tilted and non-tilted GBs in graphene.Both the tilted and non-tilted GBs gave rise to plasticity instability at high temperature deformation,particularly the phenomenon of jerky plasticity,as seen in both the average local strain curves and stress-strain relation.The jerky plasticity was mediated by the intermittent motion of dislocations(stick and climb-glide motion)for tilted GBs,while was mediated by the continuously moving of dislocations located in special position at non-tilted GBs.The mechanism of plastic deformation is intrinsic to two-dimensional systems,and governed by the motion(climbing,gliding and climb-glide motion)of dislocation at GBs,without the traditional pathways of dislocation generation needed in three-dimensional materials.6.We determined the temperature of brittle-to-ductile transition and clarified its internal mechanism for tilted and non-tilted GBs in graphene.When the stretching temperature is lower than 3384 K,the system is governed by a brittle fracture dynamics.When the stretching temperature is higher than 4047 K,the system is dominated by the dislocation-controlled plasticity.The brittle-to-ductile transition temperature depended on the type of GBs and the loading direction,but all concentrated in the range from 3384 K to 4047 K.The essence of brittle-to-ductile transition is related to enhancing dislocation mobility and hence suppressing the nucleation and expansion of the grain boundary nano-cracks.