Study On The Evolution Mechanism Of The Microstructure Of Metal Materials

In recent years,the research on metal materials,especially the microstructure evolution of metal materials,is the focus of attention.Metal material is an important material basis for the progress of human society,its development can significantly improve social productivity and improve people’s living standards.With the development of the times,people’s requirements for the performance of metal materials are getting higher and higher.The results show that the properties of metal materials are closely related to their microstructure,and the microstructure of metals,especially the micro defects,has a very important influence on the properties of metal materials.If we can get a clear understanding of the factors that affect the performance of the micro defects,we can control the microstructure of the material to improve the performance of the metal.At present,there are two ways to study the microstructure of materials:one is the experimental observation,the other is the computer simulation.Because of the harsh conditions and high cost of the experimental observations,it is not possible to study the microstructural evolution of materials at micro and nano level only by experimental methods,at this time,the computer simulation is particularly important.With the rapid development of computer and computing technology,a lot of practical methods have been developed,For example,Molecular Dynamics method,Cellular Automata method,Phase Field method,etc.Among them,the crystal phase field method is one of the most powerful computational methods because of its unique physical thought.In this paper,the influence of the evolution of the microstructure of metal materials on the properties of the material is studied by using the phase field crystal method,make innovative work in the following several aspects:(1)Using advanced phase field crystal method,describe solid crystals by periodic symmetry atomic density,the behavior and characteristics of defects in the heteroepitaxial growth process are reflected in the diffusion time scale and the atomic space scale,it is found that the atoms’ number of epitaxial layers is closely related to the inclination of the substrate;(2)Based on the density functional theory,a dimensionless free energy functional model with direct correlation function is proposed,because without discussion the anisotropic solid-liquid interface,ignore related items,Suppose in a crystal,the interatomic potential of all particles is bonded between any pair of particles,the nearest neighbor atom interaction potential of PFC model is derived,when the external strain of the system changes along the tensile direction,the energy of the system is rewritten,and the improved PFC model is obtained,based on the modified PFC model,the evolution of the initiation and propagation of the crack in the system in uniaxial tension with different pre strain level and different atomic column inclination angle is studied;(3)Using the improved PFC model,observe and study at the applied strain the initial grain boundary pre melting(GBPM)region was grown into a larger area of the softened crystal phase(SCPD),also observed the interaction of dislocation,including dislocation on the proliferation,dislocations on the rotation and their annihilation.t was also observed that the shape of SCPD changed with the increase of applied strain.Through systematic research and exploration,the main results and conclusions are as follows:(1)The study on the evolution of the interface morphology of the epitaxial process on a certain substrate shows that,in the process of evolution,the smaller the mismatch,the greater the free energy during the evolution,the final free energy is also larger.When the substrate is flat,the free energy of the system increases with the increase of the inclination angle(from 2°to 5°),and the number of atoms in the epitaxial layer increases.If we want to have more atoms in the epitaxial layer,the inclination of the substrate can be increased appropriately.When the curvature radius of the substrate is convex,with the increase of the inclination angle(from 2°to 5°),at the end of the simulation,the free energy of the system is changed from large to small,and the number of atoms in the epitaxial layer is reduced.If we want to have more atoms in the epitaxial layer,The inclination of the substrate can be reduced appropriately.(2)The crystal phase field method was used to study at the different initial conditions,the propagation behavior of micro cracks in uniaxial tension of Y axis,it is found that most of the samples begin to crack when the strain reaches 0.138,different initial conditions lead to a slight difference in the crack initiation strain.Samples stretched along the X axis,present extended passivation extension feature,samples stretched along the Y axis,there are long cleavage crack expansion at later stage.In all cases,the stress-strain curve increases at first,Crack extension after the tensile strain reaches 0.138,the corresponding curve reaches the peak value.Then the curve decreased until the end of the simulation,the stress-strain curve shows a convex peak.In this paper,the stress-strain curve is consistent with the experimental curve.(3)The study on the configuration and evolution of the grain boundary dislocation shows that:For the high temperature pre melted samples which are not near the solid-liquid coexistence temperature,when the external strain is applied,the lattice dislocation at the grain boundary will slip,sub grain boundaries of dislocations with opposite Burgers vectors move opposite,at last,the sub grain boundaries encounter and annihilation,the twin change into a complete single crystal.High temperature pre melted samples near the solid-liquid coexistence temperature,sub boundary generated in the strain move opposite,when close to a certain distance,the dislocation dipole pair is formed,the 2 dislocations pre melting regions of the dipoles begin to expand,connect and merge,an approximate rod like region is formed.Different types of dislocation interactions occur in this process,appeared initiation,nucleation and multiplication of dislocations,slip and annihilation of dislocation pairs,thus,the direction of the Burgers vector of the dislocation is changed and the dislocation type exchange.Then,the two transformation occurs in the sub grain boundary structure,that is,the dislocation vector direction of sub grain boundaries changes,then,the sub grain boundaries are reversed,finally,the sub grain boundary of the other column returned,interaction,resulting in dislocation decomposition,the sub grain boundary is transformed into a "zigzag" grain boundary,and then disappears,the migration process of the whole grain boundary ends.(4)Under the high temperature strain,the study that the evolution of the dislocations configuration and the dislocations’ proliferation and annihilation process shows:Local GBPM occurs at a temperature close to the solid-liquid coexistence point,SCPD is easy to proliferate under strain,and the dislocation is easy to rotate and annihilate.The SCPDs connection and merge with each other in the strain is increased,but they do not annihilate or disappear.The nature of the interaction between two SCPDs is the relative sliding of atomic array layer(AAL)in SCPD,this makes the applied strain induced dislocations’ configuration change and the dislocation generation and annihilation.The evolution of dislocation configuration in SCPD and the microscopic mechanism controlled by cycle dynamic recovery(CDRV)under high temperature plastic deformation can be well revealed by the improved KM model.