Indentation Theory Of High Entropy Alloy Based On Strength Criterion And Strain Gradient

At present,CoCrFeNiMn high-entropy alloy(HEA)has been used in various engineering fields due to its many excellent properties.For example,HEAs are often used as magnetron sputtering targets to adapt to the application of materials at high and low temperature environments;To solve the problem of the energy crisis,more and more researches have been conducted on the ion-irradiation of HEAs,and the effect of irradiation on the material is usually on the micron scale.However,these specimens cannot be tested by conventional equipment.The micro-indentation test is often used to study the mechanical properties of materials.Due to the existence of size effect in micro-indentation testing,many indentation theories are produced to solve the size effect.However,these theories are derived based on the J₂ criterion and the ideal elastic-plastic hypothesis,which will lead to a big deviation between the test results and the engineering application.In order to solve this problem,this study mainly carried out the four aspects,as follows:(1)Lots of experimental studies have been carried out on CoCrFeNiMn HEA by uniaxial tension,uniaxial compression,composite compression-shear and composite tension-shear test.The results show that the strength of the material is not sensitive to the strain rate in the quasi-static range and shows obvious asymmetry between tensile yield strength and compressive yield strength.In this study,the Arcan test device was improved to achieve the composite tension-shear test,and stress-strain curves of different calculation methods were obtained by digital image correlation technology.The differences between the mean strain of the gage section and the true strain of the neck segment were discussed.(2)Based on the experimental results under different stress states the relationship between hydrostatic pressure and lode angle is decoupled,and the initial yield function of CoCrFeNiMn HEA was established.The yield criterion includes the effect of tension and compression asymmetry.The physical significance and the value range of material parameters in the function were analyzed and calibrated.Then,the evolution form of theoretical subsequent yield surface is deduced,based on the hypothesis of associated flow and the asymmetric strengthening method.There is a good agreement between the experimental yield surface and the theoretical yield surface for four equivalent strains.Therefore,the correctness of the yield criterion was verified.(3)This section derived the expression of the length-scale parameter,and a relationship is built between plastic deformation,deformation gradient,and statistically stored dislocation,geometrically necessary dislocation.Furthermore,a theoretical model is established based on yield criterion,hardening function,Taylor model,and deformation gradient theory,which can be used to describe the size effect of HEA under indentation.Compared with the Nix Gao model,this theoretical model takes into account the strength criterion,the hardening mechanism,the interaction between the two types of dislocation,and the influence of the elastic deformation under the indenter.Finally,the accuracy of this model is verified by the micro-indentation of the different angle indenters.The results show that the proposed theoretical model has more accurate predictive ability than the Nix Gao model.(4)The effect of macroscopic pre-compression on the indentation hardness of CoCrFeNiMn HEA was studied by using the proposed theoretical model.The HEA was pre-compressed with different degrees of plastic deformation at room temperature,600?,800?,and 1000?,respectively,and then its micro-indentation hardness was measured.The results show that the pre-compression temperature and the degree of plastic deformation have a significant influence on the indentation hardness,especially when the temperature reaches the dynamic recrystallization temperature of the material.This research method establishes the relationship between macroscopic plastic deformation and micro-indentation hardness,and the theoretical model provides a basis for the quantification of this relationship.