Study On Strain-induced Polycrystalline Boundary And Dislocation Interaction By Phase Field Crystal Method

The polycrystalline plastic deformation mechanism is closely related to the existence of defects such as dislocations,grain boundaries and cracks.The structure and state of these defects change with the change of strain loading form,strain rate and temperature,so as to improve the properties of metal materials.In the experiment,it is challenging to directly detect the micro defect structure and its dynamic evolution process in the material.At the same time,it is difficult to obtain the data matching with the experimental atomic scale by traditional computational simulation.By tracking the evolution of local time average crystal density field,the phase field crystal method solves the evolution process of atomic scale,naturally describes the dislocation and grain boundary structure,and can achieve a long diffusion time scale that cannot be achieved by molecular dynamics simulation.Therefore,it is very important to study the effect of microstructure on polycrystalline plastic deformation mechanism by phase field crystal method.In this paper,the grain boundary type and grain boundary structure of face centered cubic（111）crystal plane are studied by using single-mode phase field crystal model.Under uniaxial tensile strain loading,the dynamic movement and interaction between polycrystalline plastic deformation dislocation and grain boundary are studied,with emphasis on the dependence of polycrystalline microstructure on temperature in the process of plastic deformation;The nucleation and propagation of polycrystalline cracks were studied under biaxial tensile strain,and the effects of grain boundary type and temperature on the nucleation and propagation of polycrystalline cracks were revealed.Based on the atomic evolution diagram and the change of free energy of the system,the following conclusions are drawn:（1）Taking the two-dimensional triangular phase as the research object,according to the different orientation of the two grains,the essence is the difference of atomic arrangement,forming different types of grain boundaries.When the grain orientation difference is 0～2°,the two grains merge due to the small difference of atomic arrangement;The orientation difference is 2～20°,forming a small angle grain boundary composed of multiple dislocations;The orientation difference is 20～40°,forming continuous large angle grain boundaries;Grain boundaries composed of dislocations are formed when the orientation difference is 40～60°;Moreover,the type of grain boundary is only related to the poor grain orientation,and the grain size can only determine the length of grain boundary.（2）Under uniaxial tensile strain,polycrystalline plastic deformation is divided into four stages.In the first stage,adjacent different number dislocations merge and annihilate,in the second stage,the grain boundary absorbs adjacent dislocations,in the third stage,the grain boundary folds produce cusps to release dislocations,and in the fourth stage,the grain boundary rupture produces dislocations,and the dislocations continue to move under the action of stress.Temperature determines the movement mode of dislocation.At high temperature =-0.25,dislocation moves faster,and climbing and sliding coexist.At low temperature =-0.40,dislocation moves slowly and can only slip.In addition,the interaction mode between grain boundary and dislocation is closely related to temperature.The attraction of grain boundary to dislocation at high temperature is greater than that between different dislocations.（3）Under biaxial tensile strain,polycrystalline microcracks mainly nucleate on grain boundary triple junction and large Angle grain boundary,and their propagation modes include intergranular propagation,transgranular propagation and passivation.Cracks with small grain boundaries nucleate at the dislocation,and the initial propagation direction starts along the opposite direction of the dislocation line.During the propagation process,dislocation is often released at the crack tip,which leads to the crack passivation and stop propagation.The cracks with large Angle grain boundary nucleate directly on the grain boundary,forming small cracks first,and then gradually coalesce into large cracks or large cracks.The nucleation and propagation of polycrystalline cracks can be inhibited by low temperature.In this paper,the phase field crystal method is used to study the influence of microstructure on the deformation mechanism of polycrystalline metal materials,and reveal the deformation mechanism of microstructure on the atomic scale,which is helpful to realize the required mechanical,physical and chemical properties by adjusting the conditions of stress,strain or temperature,so as to further design new materials,new structures and new applications.