Numerical Simulation Of Metal Grain Boundary And Dislocation Configuration Evolution

Grain boundary structures in nanometer-and sub-micro-sized polycrystalline materials during plastic deformation processes have attracted tremendous attention for many years because of their wide application in technologies.Many researches for grain boundary sliding have been done in conventional polycrystalline materials.Migration of grain boundary is a fundamental mechanism in recrystallization and grain growth.At present,the study of dislocation motion of nanocrystals grain boundaries is a hotspot in the evolution of microstructures,especially in the synergistic movement of grain boundaries dislocations under applied strain.For the evolution of nanoscale grain boundaries and dislocations under applied strain,the latest phase field crystal(PFC)model proposed by Elder was used to study the evolution of nanoscale grain boundaries and dislocations.The merit of PFC method is that it could simulate the microstructures of nano-polycrystalline materials in space scale,and the strain rate in diffusion time scale is consistent with the reality.Therefore,it has great advantages to simulate grain boundary migration and grain growth evolution at nanoscale.In this paper,the movement of grain boundary dislocations at two-dimensional small-angle symmetrical inclined grain boundaries and large-angle grain boundaries are studied by using single-mode and double-mode phase field crystal(PFC)methods,respectively.The variation of internal free energy is analyzed,and the micro-mechanism of metal materials under biaxial equal-area strain is revealed.The main results are as follows:1.The dislocation evolution process of small angle symmetrical inclined twin crystal boundaries(STGB)under strain can be divided into three stages:The first stage is the increasing of system energy;The second stage is the decreasing of system free energy,corresponding to the slipping of dislocations and the migration of sub-grain boundaries generated by the coordinated movement of dislocations.Dislocation annihilation occurs when dislocations with opposite Bergs vectors attract each other;The third stage is to repeat the first two stages.With the annihilation of dislocations,both grain boundaries and sub-grain boundaries disappear and finally form a grain.2.For small-angle symmetrically inclined grain boundaries,under biaxial strain,the deformed grain splits between two sub-grain boundaries(SGB)and nucleates through the primary grain boundaries,forming a gap between the two sub-grain boundaries.The migration of sub-grain boundaries is caused by the coordinated movement of dislocations at grain boundaries.During the two processes from dislocation emission to dislocation annihilation,new deformed grains are produced,and the deformed grains have new orientation.3.For large angle grain boundaries under biaxial strain,dislocations at grain boundaries decompose but do not proceed synchronously,and there is no synergistic movement of dislocations.Because dislocation decomposition does not proceed synchronously,at the same time,some dislocations may climb,and the other dislocations slip,resulting in energy offset and plateau period,eventually forming a single crystal.