Study On The Response Of Grain Boundary Dislocation Of Under External Force By Phase Field Crystal Method

The key of material properties is the internal microstructure.Dislocations,grain boundaries and their coupling are the main factors controlling the evolution of the microstructure.The phase field crystal method is one of the main methods to study the microscopic defects of materials.The introduced free energy function naturally includes the physical properties of the crystal periodic structure such as grain orientation and elastic-plastic deformation.It has been widely used in dislocation migration,grain growth,magnetic films and other fields.In this paper,the pure material is taken as the research object,and the deformation mechanism of the bi-crystal grain boundary is studied by using the two-mode phase field crystal under equal area deformation.The mapping mechanism of asymmetric tilted grain boundaries is focused on the analysis of the type of dislocation reaction,the dislocation arrangement and the critical strain value of dislocation decomposition(dislocation proliferation).The results show that the low-angle asymmetric tilted grain boundaries structure are described by single edge dislocations and dislocation pairs,and the low-angle symmetric tilted grain boundaries are composed of single edge dislocations.Under the action of applied strain,the motion mechanism of asymmetric tilted grain boundaries:edge dislocations slip,the dislocation reacts to form a new edge dislocation.The dislocation pair decomposes and annihilates the edge dislocations with opposite Berger vectors.The edge dislocations perpendicular to each other are close to each other to form new dislocation pairs(dislocations reorganization).The dislocation pairs slip with the increase of strain.A"type I semi-annihilation"reaction occurs by dislocation pair and a single edge dislocation.The edge dislocations slip attract and annihilate each other.Mechanism of symmetric tilted grain boundary motion:the edge dislocations climbing along grain boundary and decomposition.The dislocation pairs formation(recombination)with increase of strain.Some dislocation pairs and single edge dislocations occur"type I semi-annihilation".Some dislocation pairs undergo"type II semi-annihilation"or"full annihilation"with others dislocation pairs.During the whole microstructure evolution process,the numbers of dislocation at the initial equilibrium grain boundary increases with the increase of misorientation angle,and the dislocations distance decreases.The applied strain required for dislocation decomposition is large when there are many grain boundary dislocations,so grain boundary dislocations cannot be decomposed at the same time,the reverse is also true.Different tensile strain directions and grain boundary misorientation angles have a significant impact on the symmetry of grain boundary dislocation arrangement and the law of dislocations movement.When tensile strain in the x-direction is applied,the grain boundary dislocations decompose when the strain reaches a certain critical value.A part of dislocations is emitted into the grains with the strain increases and the other part of the dislocations stills exist at the original grain boundaries.We found that the dislocations with misorientation angles of 6^°and 7^°are roughly arranged in a"double bowknot"symmetry.In particular,sub-grain boundaries are generated when a strain of 0.06 is applied to the system with a grain boundary misorientation angle of 7^°.There are only dislocation pairs in the system and they are neatly arranged to show a"double bowknot".The misorientation angle of 6^°does not have sub-grain boundaries while under the condition of equal strain.There are no dislocation pairs existing and the arrangement of edge dislocations is disordered.The critical strain for grain boundary dislocation decomposition decreases when the y-direction tensile is applied to the twin crystal systems with grain boundary misorientation angles of 6^°and 7^°.In addition,we found that the grain boundary dislocations arrangement and the dislocations response law are sensitive to the grain boundary misorientation angle.The critical strain for dislocations decomposition as a function of strain rate is focused.Properly increasing the strain rate or decreasing the grain misorientation angle can shorten the critical time steps for dislocations decomposition.