Crystal Plasticity Damage Model Of High Entropy Alloys Based On Dislocation Density

The concept of high entropy alloy（HEAs）is considered as a breakthrough in the traditional alloy design concept,which provides a new idea and direction for the design of new materials from the perspective of changing mixing entropy or configuration entropy.At the same time,with its unique design concept,HEAs has greatly expanded the development space of metal materials and developed a variety of alloys with excellent properties.From the point of view of materials science,the characteristics of high strength,high plasticity,good wear resistance and excellent corrosion resistance of HEAs are closely related to their unique microstructure,and the study of microstructure evolution has important guiding significance for the design and development of materials and performance optimization.From the point of view of mechanics,it is necessary to establish a multi-scale constitutive model that can relate the microstructure evolution and macroscopic mechanical properties of metal materials,so as to overcome the limitation of traditional computational models that lack description of micro mechanism when simulating metal plastic deformation.In this paper,based on the crystal plasticity theory,a multi-scale crystal plasticity finite element model（CPFEM）based on dislocation density was established.Mechanical properties,damage behavior and microstruc-ture evolution of HEAs under different application conditions were predicted by CPFEM.The main work is as follows:（1）Based on the traditional crystal plasticity theory,the internal state variable of dislocation density and continuum damage were introduced by modifying the flow criterion and hardening model respectively,and the evolution characteristics of the damage and dislocation density models were analyzed.At the same time,the basic theory of crystal plasticity and its solution method in finite element were introduced,and the numerical solution process of the improved model in user material subroutine（UMAT）was derived in detail.（2）The quasi-static tensile properties,necking behavior and microstructure evolution of Ni Co Cr Fe were studied in detail by combining experiments and CPFEM.The results show that CPFEM with dislocation density and damage can accurately describe the quasi-static mechanical behavior of Ni Co Cr Fe and reasonably capture the shape and size of the necking region.The length and width of the necking region are 7%and 23%different from the predicted results,respectively,but within a reasonable range.Both experiments and CPFEM show that after tensile deformation,Ni Co Cr Fe appears strong<111>//RD and weak<100>//RD fiber texture and forms Cube,R-Cube_（RD）,S,Cu,Brass,A and Goss texture distribution characteristic mainly.During the tensile process of the microscopic polycrystalline model,the damage is easily formed at the grain boundaries where the dislocations are piled up and the stress is concentrated,which manifests as an intergranular damage mechanism.At the same time,the formation of damage will change the stress distribution in its nearby region and provide conditions for further evolution,that is,the softening effect of the damage point will transfer the stress it bears to the nearby region,making the surrounding grains in a state of high stress,thereby promoting damage growth and expansion.（3）The rate-sensitive dislocation dynamic annihilation coefficient k₂ and the adiabatic temperature rise equation were introduced into the crystal plasticity model,and the thermal-mechanical coupling calculation of metal materials under impact load was realized.The dynamic shear mechanical behavior and microstructure evolution of Co Cr Ni Si_0.3 were studied in detail by combining the experimental techniques of split hopkinson pressure bar and electron backscatter diffraction.The results show that both experiments and CPFEM show that the damage first forms at the corner point of the hat-shaped shear specimen,and with the increase of deformation,the damage expands rapidly from the corner point along the shear direction,gradually forming a slender adiabatic shear band（ASB）.CPFEM considering strain rate effect and adiabatic temperature risecan accurately describe the dynamic shear mechanical response of Co Cr Ni Si_0.3,the shape of dynamic shear deformation and the evolution of ASB,and reasonably predict the change of strain rate and temperature in the shear region of the hat-shaped specimen.In addition,the fiber texture of<101>//SD and<001>,<111>//SPN are formed in the shear region during Co Cr Ni Si_0.3 shear deformation.The orientation distribution function（ODF）figures show that the texture distribution gradually approachesα-fiber from dispersion and tends to flow alongα-fiber to R-Goss texture.