Effect Of Dynamic Pre-compression On Micro-scale Indentation Hardness Of CoCrFeNiMn High-entropy Alloy

High-entropy alloys have excellent mechanical properties,such as high hardness,high strength,high electrical resistivity,excellent wear resistance,excellent magnetic properties,high-temperature mechanical and oxidation resistance,etc.which also makes high-entropy alloys more recent research hotspots of the year.Although there have been a lot of studies on the CoCrFeNiMn high-entropy alloy,the mechanical response of the alloy under high temperature dynamic is still unclear,and further research is needed to explain the high temperature dynamic mechanical behavior of the alloy.In addition,high-entropy alloys have broad application prospects in the micro-scale field.Understanding the micro-mechanism of high-entropy alloys has also become the research focus of high-entropy alloys.Based on this,this paper studies the effect of pre-compression on the micro-scale indentation hardness of alloys.The content and conclusion are as follows:(1)The CoCrFeNiMn high-entropy alloy was prepared by the high-vacuum arc melting method,and the crystal structure of the alloy was analyzed by X-ray diffraction analysis(XRD).The test showed that the alloy has a single face-centered cubic(FCC)crystal structure;using X-ray Energy Dispersive Spectrometer(EDS)tested the element composition and percentage content of the alloy,and the results showed that the mass fractions of the main elements were:Mn17.2%,Co17.9%,Cr19.0%,Fe20.1%,Ni25.9%;Using electron backscatter diffraction experiment(EBSD),the alloy crystal grain size is about a few micrometers to more than ten micrometers.(2)In the range of quasi-static compressive strain rate of 2×10^-4s^-1?2×10^-2s^-1 and dynamic compressive strain rate of 2800s^-1?5200s^-1,the CoCrFeNiMn high-entropy alloy exhibits obvious The strain rate effect and work hardening effect.Under quasi-static conditions,the alloy's strain rate sensitivity value is m=0.028.Compared with quasi-static conditions,the alloy exhibits higher strain rate sensitivity under dynamic high strain rate conditions,and the yield strength increases with strain rate.It increases,and the work hardening behavior of the alloy also shows obvious differences with the change of strain rate.The alloy has higher temperature sensitivity.Compared with room temperature,the yield strength of the alloy is lower than room temperature at 400-1000?,and decreases with the increase of temperature.It is worth noting that the alloy is at 800?1000?,the flow stress decreases greatly,and this phenomenon is caused by the influence of adiabatic temperature rise and dynamic recrystallization of the alloy.In addition,the plastic strains generated by dynamic pre-compression specimens at room temperature are 12%,23%,40%,and 47%,respectively.The plastic strains generated by dynamic pre-compression at high temperature are 45%(400°C),51%(600°C),61%(800?),72%(1000?).(3)Using electron backscatter diffraction(EBSD)technology to characterize the microstructure of the pre-compressed sample in the axial and radial compression planes,and use the Johnson-Cook constitutive equation considering adiabatic temperature rise and dynamic recrystallization to test the sample the kind of plastic deformation is analyzed.The results show that the grain size of the CoCrFeNiMn high-entropy alloy does not change significantly when the temperature is lower than 800°C.When the deformation temperature reaches 800°C,the alloy undergoes dynamic recrystallization and the grain size increases significantly.When the deformation temperature increases to 1000°C The recrystallized structure is almost completely formed.The Johnson-Cook constitutive equation considering the adiabatic temperature rise and dynamic recrystallization has better consistency with the dynamic experimental results of the CoCrFeNiMn high-entropy alloy,which further verifies the influence of adiabatic temperature rise and dynamic recrystallization on the deformation behavior of the material.(4)Axial and radial micro-indentation tests were carried out on the pre-compressed specimens in chapter 3.Based on the macro-scale strengthening function,combined with the deformation gradient theory of micro-scale indentation and considering the elasticity during the indentation test deformation,a theoretical model is established to describe the micro-indentation scale effect of high-entropy alloys,and the model is used to explore the effect of macro-precompression on the micro-indentation hardness of CoCrFeNiMn high-entropy alloys.The results show that:different plasticity under macro-precompression deformation has a significant effect on the micro-scale indentation hardness of the alloy.Compared with axial compression,the hardness H₀ that eliminates the scale effect under radial compression is greater.This research method establishes the relationship between the macroscopic plastic deformation and the micro-scale indentation hardness,and also provides a new idea for the realization of the micro-scale indentation test to determine the research method of the internal plastic deformation of the material.