Multi-level Crystal Plasticity Model Of High Entropy Alloy Considering Strain Rates Effect

With excellent strength and toughness,great wear,radiation and corrosion resistance as well as other comprehensive properties,high-entropy alloys(HEAs)have become the most widely studied nowadays and will also occupy an important position in future industrial applications,its unique alloy design strategy makes HEAs own some structural characteristics such as: the serious lattice distortion,crystal defect spread slowly,chemical short-range order(CSRO)and so on.From the angle of material science,these characteristics are used to account for the source of excellent performance,but the emergence of new micro mechanism must lead to traditional material model and numerical calculation method is difficult to apply in engineering simulation of HEAs.The multi-scale computation model combining microscopic mechanism and macroscopic mechanical response information need to be developed for future alloy service condition(high strain rate,high and low temperature load,shear failure and so on).Crystal plasticity finite element theory(CPFE)based on the physical properties of crystallography is used to describe the microplastic deformation mechanism in continuum mechanics method,and it is a suitable tool to study the matching relationship between excellent properties of alloys and the of microstructure evolution.In the present paper,a strong strain-rate-sensitive face-centered cubic Co Cr Fe Ni HEA producing more nano-twin under high strain rate loading is taken as studying material.Nanotwin formation mechanism and the interaction relationship between twin and dislocation are validated through analyzing and researching the mechanical response and deformation mechanism of HEAs under the dynamic,quasi-static shear loading.The multi-level crystal plasticity model and ABAQUS User MATerial subroutine(UMAT)are coded on account of the balance relationship between dislocation multiplication and annihilate.The mechanical properties,texture characteristics,twin evolution and adiabatic temperature rise of Co Cr Fe Ni HEAs upon dynamic and quasi-static shear are simulated and analyzed systematically via using macro and micro corresponding Taylor-crystal plasticity finite element model(T-CPFEM)and representative volume element-crystal plasticity finite element model(RVE-CPFEM).Specific research contents are as follows:Firstly,the mechanical properties and microstructure evolution of Co Cr Fe Ni HEAs under room temperature quasi-static and dynamic shear is tested through the design shear specimen size and using split-hopkinson tensile bars(SHTB)as shear test tools.The research results show that the alloy also presents the strain rate sensitivity generated during tension and compression loading and has the high work hardening capacity in the process of dynamic loading,this is because the combined action between the short obstacle related to thermal activation mechanism and nano-twin activation.In addition,by comparing the higher geometrical necessary dislocation density(GND)and the smaller grain width to length ratio in the dynamic samples,it is found that the alloy breaks through the ductile-plastic compatibility problem,which lays a foundation for the application of extreme conditions.Additionally,on the premise of Taylor polycrystalline homogenization assumption,the phenomenological CPFEM,which can be used to simulate the mechanical response of HEAs in the process of deformation,is developed on account of the strain rate sensitive solid solution strength description method,forest dislocation density evolution rule,the interaction between dislocation and twin.In addition,the variate: dislocation density and twin volume fraction(TVF)are taken as status updates variable.By comparing the quasi-static and dynamic tensile tests of Co Cr Fe Ni HEAs,the influences of dislocation and twinning on the macroscopic mechanical response,the characteristics of texture evolution and the updating law of the integral number of twins are analyzed by means of single element model verification.Final,the multi-element finite element models are used to simulate the uniaxial tension at various strain rates.It is proved that the stable and efficient phenomenological crystal plasticity model can be used to calculate the macro-mechanical response and micro-mechanism evolution of Co Cr Fe Ni HEAs.Finally,according to dislocation multiplication methods: Frank-Read source and cross-slip mechanism,and the principle of dislocation annihilation and recovery process as well as the conversion relationship between mobile and immobile dislocation,the dynamic dislocation annihilation and recovery parameters are constructed systematically to describe the high strain rates and different temperature loading process and further the dislocation hardening model representing the long range obstacle resistance(LRO)of HEA is built.The method for describing Labusch solid solution strength is established on account of the resistance effect of CSRO on dislocation slip.Meanwhile,based on the experiment of nano-twin nucleation mechanism for HEA,the twin nucleation and thickening mechanism in combination with the cross slip and three layers of stack fault form twin from the grain boundary are used as the foundation,and are verified the reasonability based on energy balance equation.In addition,the flow model for nano-twin generation of HEA is first time built via considering the influence of the impact process for twinning.Twin resistance is indicated by the effect of dislocation and existing twins on the nucleation of new twins.The mean free path method(MFP)coupling slip,twin and grain size is employed to describe the relationship between dislocation and twin.Rotating model: predominant twin reorientation(PTR)method is used as rotate twin orientation.Emphasizing on the elastic stiffness matrix with twin generated and the influence of the adiabatic temperature rise on dislocation activation and the formation of twin,a multi-scale thermally activated CPFEM is established by using the finite element method and Taylor's homogenization hypothesis.In order to achieve the scale matching of macroscopic and microscopic scales,RVE method and the modeling process of intra-grain and grain boundaries corresponding to the experimental grain size and dislocation density distribution are used,and the mechanical properties and deformation mechanism evolution of Co Cr Fe Ni HEAs under quasi-static and dynamic shear loading are simulated by ABAQUS.Through detailed comparison of finite element calculation results and experimental data,it is concluded that:(1)Under the same equivalent plastic strain,the change of strain rates is not significantly affect for dislocation multiplication,but has a promoting effect for the process of twin,namely the increasing for yield strength and the initial flow stress under high strain rates originate from the strain rate sensitive solid solution.(2)High work hardening performance under high strain rate is mainly attributed to the generation of twin.In particular,the adiabatic temperature rise under high strain rate loading can cause the material to become soft,such changes have played an important role in balancing work hardening rate of alloy.Finally,in conclusion,it can be concluded that the thermally activated CPFEM developed in this study can provide guidance and reference for the performance analysis and engineering simulation of HEAs in extreme environments.