Multi-scale Characterization And Finite Element Simulation Of The Fracture Mechanism Of Face-centered Cubic Metal

As an important structural material,metals have a wide range of applications in engineering structures.The failure of metals in the service process directly affects the safety of the engineering structure,so the prevention of damage failure is of great significance.Ductile fracture is one of the main forms of metal material damage.The study of fracture mechanism has important engineering significance and academic value.It has been a hot research topic of domestic and foreign scholars for a long time.In this study,the ductile fractures of three typical face-centered cubic metals are studied.The multi-scale characterization technique is used to analyze the whole process of ductile fracture.At the same time,based on computational mechanics,the influence of stress state on different stages of fracture process is analyzed by finite element method.According to previous research results,our study divides ductile fracture into three stages:nucleation,growth and coalesence.For the three kinds of face-centered cubic metal Al-Cu alloys,single crystals copper and high-entropy alloys,the nucleation,growth and coalesence of the void during ductile fracture process were characterized by transmission electron microscopy（TEM）and scanning electron microscopy（SEM）.The microstructure changes accompanying the growth process,statistical analysis of the three stages,and the relationship between stress and microstructure changes at different stages of ductile fractures were calculated and analyzed by means of finite element analysis.By comparing the calculation analysis with the experimental test results,the micro-fracture mechanism caused by the void of the face-centered cubic metal is revealed.This research consists of three parts:（1）Study on the nucleation behavior of Al-Cu alloy under complex stress conditions.Previous studies on void nucleation have focused on alloys containing a circular,elliptical or approximately circular or elliptical second phase.The second phase of the Al-Cu alloy used in this study isθphase（Al₂Cu）.The distribution of this phase in the Al matrix affects the nucleation and growth behavior of the cavity under load.In this study,the uniaxial and biaxial tensile tests of Al-Cu alloy were carried out,and the laws of cavity nucleation under different axial stress ratios were studied by means of in-situ TEM and SEM.From the results of in-situ TEM and static SEM,the positions of void nucleation can be roughly divided into two categories:void nucleation in the nucleation of the matrix and void at the interface between the matrix and theθphase.The statistical results further show that under different axial stress ratios,the volume of the void nucleus at the interface of the matrix/θphase is larger than the void nucleus inside the matrix.As the biaxial stress ratio increases,the proportion of void nucleation at the interface of the matrix/θphase increases,but the total number of voids in the nucleation decreases.Based on the experimental results,a representative volumetric cell model with lamella particles was established,different proportions of biaxial loads were applied to the volume cells to simulate the experimental process.The parameters were studied by adjusting the geometric parameters of the model（second phase particle volume fraction）and the material parameters（base and second phase particle modulus and hardening coefficient）.The finite element numerical simulation results show that the critical stress of the nucleation at the interface is smaller than the critical stress of the nucleation inside the matrix when the biaxial stress ratio is greater than 0.7.As the hardening coefficient of the matrix increases and the volume fraction of the second phase increases,the nucleation at the interface and the critical stress in the nucleation of the matrix increase simultaneously.When the model parameters are close to the experimental Al-Cu alloy,the critical stress of the void nucleation at the interface is much smaller than inside matrix.This conclusion is consistent with the experimental results.（2）Three typical face-centered cubic metal metals,Al-Cu alloy,single crystal copper and CrMnFeCoNi high-entropy alloy were studied by in-situ TEM.The void growth and coalesence process during ductile fracture process.Three main modes of void coalesence were observed experimentally:neck coalesence,shear coalesence,and coalesence caused by deformed twins.The different stacking fault energy of the material leads to the different void coalesence mode:the Al-Cu alloy has a high stacking fault energy,and void coalesence is mainly the necking and shearing mode;Single crystal copper has higher stacking fault energy than CrMnFeCoNi high entropy alloy.but the fault energy of them is smaller than that of Al-Cu alloy,so the void coalesence mode is coalesence by deformation twins.This mode is rarely found in previous studies.The experimental results are compared with the classic McClintock model and the Brown-Embury model.The comparison results show that before the void coalesence by necking,especially in the initial stage of the void growth,the growth of the void can be described by the McClintock model,and the McClintock model predicts the coalesence of the void well;the Brown-Embury model needs to be properly modified to match the experimental data.Based on the results of in-situ experiments,this study also established a finite element model to simulate ductile fracture crack propagation.Through numerical simulation,the Lode parameters and the stress triaxiality T during the void growth and coalesence are obtained.The results show that the shear stress can drive the non-spherical void nucleation and void coalesence by the shear mode（the shear band is formed on the two voids and nano twins）.（3）The crack propagation process and microstructure changes of CrMnFeCoNi high-entropy alloy at different stress states were studied.Combined with the in-situ TEM and SEM,the shaped samples with different stress states were designed.The microstructure changes during the ductile fracture process of high-entropy alloys were characterized from atomic to micron scale.The experimental results show that the in-situ TEM tensile samples produce different microstructures in different regions.This phenomenon is caused by different local stress conditions.When the plane average stress of the deformation region of the sample reaches the amorphous to form an average stress,and the critical shear stress of the twin formation does not reach,the region is dominated by the formation of amorphous structure.On the contrary,this region is dominated by the formation of deformation twins.At the same time,the results of in-situ SEM show the influence of deformation twins on different stages of high-entropy ductile fracture process and the effect of high temperature on ductile fracture of high-entropy alloy.In this paper,the whole process of face-centered cubic metal ductile fracture is covered,and the mechanism of different stages of fracture is discussed in depth.It can provide theoretical guidance for improving the ductility and damage resistance of face-centered cubic metal structure materials,and develop new high-performance metal materials.Provide theoretical support.