Dislocation Structure Evolution Of Premelted Polycrystalline Materials

The research on the evolution of premelting of polycrystalline materials is gradually increasing.The premelting of grain boundaries can change the macroscopic properties of polycrystalline materials,which has attracted the attention of scientists.However,the grain boundary is only a few atomic layers thick,and its performance can be improved by precisely controlling the microstructure of the material.Not only can the microscopic defects of the crystal material be clearly understood,but also the influence of defects on its performance can be understood.So in order to observe these subtle microscopic phenomena,people study the pre-melted microstructures through experiments and computer simulations.Due to the limited experimental conditions,there are many external influence factors,and the success rate is so high that it is difficult to observe the changes in details under the existing experimental conditions.Therefore,researchers study the evolutionary characteristics of micro-structures of materials at the microscopic level through experimental means is not the only means,but also need to use computer simulation to solve some problems that cannot be solved experimentally.There are many practical calculation methods,such as molecular dynamics method,Monte Carlo method,phase field method and so on.Compared with other methods,the crystal phase-field method not only describes the atomic-scale structure,but also has a large diffusion time scale.Polycrystalline metal materials are the majority of materials used today and are subjected to a variety of effects during processing,such as extrusion deformation,high temperatures,and the like.These changes affect the internal structure of the material.Therefore,the crystal phase field method is used to study pure material polycrystalline materials such as Cu,to study the tilted grain boundaries under the solid-liquid coexistence region,the structural transformation mechanism of the premelted region,and the dislocation motion process under stress.The phenomenon,the study of the pre-melting region and the elastic interaction of the wrong parts,etc.,draw the following conclusions:(1)For the simulation of low-angle grain boundaries at different temperatures,it can be seen that when the system temperature is at a low temperature,the angle between the two sets of types of dislocation Berg vectors at grain boundaries is 60 degrees,and each pair of dislocation edges The dislocation BW vector angle is also 60 degrees.For the application of no strain,the lattice dislocations around the grain boundary melt at high temperatures;the closer the pre-melting temperature is to the melting point,the more pre-melting is,and the more pronounced lattice atom softening occurs in this region.(2)Pre-melting the samples with 0,2 degree orientation angles,and applying external force,it was found that lattice dislocations at the grain boundaries slide,and the slip surface changes due to local stress changes,resulting in the formation of subgrains In the world and new grains,the old and new grains attract and eventually annihilate,making the four crystals become complete single crystals.(3)Pre-molten samples with high or low orientation angles of 0,4 degrees,external forces applied,and dislocation interactions of different types occurring at a certain stage,the initiation,nucleation,and proliferation of dislocation pairs occurred,dislocations.The slipping,climbing,and annihilation of pairings causes the change of direction of the dislocational Bohsch vector.The essence of this stage is the softening of the atomic lattice inside the premelting zone and the tight dislocations that are close to each other.Slip and twist occur,causing different types of dislocation reactions to occur within dislocations.There are two slip directions in the atoms of the premelting zone.These are closely related to the movement of the atom dislocation,temperature,and strain.(4)During the evolution of the dislocation configuration,dislocation proliferation,and quenching process at high temperature strain,it is found that the local grain boundary premelts at a temperature close to the solid-liquid coexistence point,and the softened crystal phase domain under strain is prone to occur.Proliferation makes the dislocations easy to rotate and annihilate after slipping.When a variable speed is applied to a polycrystalline material sample,it can be found that the slippage of dislocations at the grain boundaries requires the work done by the grain boundary to do work,so that the dislocation movement occurs first at the grain boundaries,and the larger the rate,the greater the number of climbs.fast.(5)After the strain rate is changed,the rate of grain boundary migration and dislocation movement becomes faster,resulting in a higher dislocation density inside the grain of the material,which is beneficial to grain refinement.