Phase Field Crystal Method For Deformation Behavior Of Metal Microstructures

The internal microstructure of the control material is used to improve its processing performance,which is a topic that researchers have been paying attention to.Under the conditions of limited time scale and spatial scale,engineering practice requires a lot of time and cost,and it is difficult to capture the details of the microstructure.Therefore,it is of great significance to study the deformation behavior of metal microstructures by phase field crystal method.In this paper,the advantages of phase field crystal method in microstructure evolution are fully utilized,and the two-dimensional triangular phase in metal crystals is simulated at atomic scale and diffusion time scale.The simulation results of single-mode phase field crystal model show that there are two low-angle symmetric tilting grain boundaries in the nano-double crystal.The structure can be described by the edge-dislocation model with equidistant vertical alignment.The equal area strain is continuously applied in the system,and the metal microstructure is deformed at the atomic level.During the deformation process,the grain boundaries migrate in a fixed direction a nd the grain boundary structure does not change.The final result of the deformation behavior of the metal microstructure is that the dislocation interaction results in complete annihilation of the two grain boundaries,and the nano-double crystal transforms into an undistorted single crystal as the metal defect disappears.According to the quantitative analysis of the atomic density evolution map combined with the energy of the system,the grain boundary annihilation process can be roughly divided into three stages: the first stage grain boundary emits four dislocations with different configurations(I-type,II-type,III-type and IV-Type)and separates at the grain boundary to form a new subcrystal.The second stage of dislocations traverses the interior o f the grain and makes regular slip motion along the face-centered cubic(111)plane.The I-type and IV-type dislocations move to both sides,and the II-type and III-type dislocations approach the middle.In these two stages,the free energy curve of the system increases monotonically due to the thermal coupling of the sample.In the third stage,the grain boundary encounters the dislocations of opposite signs offset each other,and the remaining dislocations continue to slip until the grain boundary annihilation process is completed.The free energy curve in this stage has a local minimum point,which corresponds to a large amount of lattice distortion energy when the grain boundary is annihilated.This paper reveals that the essence of grain boundary annihilation is the process in which the subcrystals engulf the original grains and the energy of the system is released.The growth rate of free energy after grain boundary annihilation is always greater than the growth of free energy in the process of grain boundary migration.Analysis of the influence of different parameters on the grain boundary annihilation process found that the grain boundary is sensitive to temperature and grain orientation angle.High temperatures favor the diffusion of atoms to reduce lattice distortion in the grains.Although the process of dislocation motion,grain boundary migration and annihilation can occur in advance under low temperature conditions.However,from the offset results of the dislocation pair interaction,it can be seen that a proper increase of temperature is helpful to shorten the time of conversion of the double crystal system into a complete single crystal structure under the constant area strain.Especially at higher temperatures,the dislocations can be offset by several pairs to accelerate the annihilation of the grain boundaries.In the range of the small angle tendency angle,the increase of the orientation angle suppresses the occurrence of annihilation and reduces the migration rate of the grain boundary.Under the applied stress,the orientation angle has a certain response relationship with the yield strength of the material.The results of metal microstructure deformation behavior under high temperature conditions show that the independent dislocations surrounded by the liquid phase on the grain boundary are often accompanied by pre-melting.The temperature gradually approaches the melting point,and a liquid phase film is preliminarily precipitated at the grain boundary.The morphology of the liquid phase film is closely related to the orientation difference angle of the crystal grains.During the high temperature pre-melting and annihilation of the grain boundary,the larger the difference angle of the grain orientation,the larger the number of dislocations forming the grain boundary,and the smaller the dislocation spacing.Adjacent two independent dislocations combine the pre-melting zone to form a "liquid phase pool",and the size of the liquid phase film increases as the orientation angle increases.The applied stress does not change the mode of motion of the grain boundaries and the interaction between the dislocations.With the annihilation of the grain boundary,the dislocation area that preferentially pre-melts will also disappear.