Dynamic Mechanical Behavior Of Fe₄₀Mn₂₀Cr₂₀Ni₂₀ High-Entropy Alloy At Room And Cryogenic Temperature

Different from common solid solution alloys,high entropy alloys（HEAs）are usually composed of elements with equal atomic ratio or nearly equal atomic ratio,and there are no solvent atoms and solute atoms.Therefore,HEAs have the comprehensive properties,such as excellent low-temperature properties,good fatigue and fracture properties,outstanding corrosion and wear resistance and good thermal stability.These excellent comprehensive properties make it a strong competitor for the next generation of structural materials.In order to further expand the application of HEAs in extreme service environment,it is imperative to study its mechanical response and microstructure evolution at low temperature,high strain rate and their coupling.However,most studies on dynamic deformation of HEAs focus on the field of compression,and there is little research on dynamic tension,especially at low temperature.Therefore,it is a far-reaching work to reveal the unique mechanical response of HEAs in the field of low-temperature dynamic tension,introduce environmental parameters such as temperature and strain rate,and deeply explore the deformation mechanism of materials under extreme service conditions.In this study,Fe₄₀Mn₂₀Cr₂₀Ni₂₀ HEA is selected as the research object.Firstly,the effects of thermomechanical treatment on the mechanical properties and microstructure of the current alloy are studied.Secondly,the samples with good room temperature quasi-static mechanical properties are subjected to room temperature,low temperature and high temperature,quasi-static and dynamic tensile tests respectively to study the effects of temperature and strain rate on the mechanical behavior,The main results of the study are as follows:（1）Hot-rolled plate of Fe₄₀Mn₂₀Cr₂₀Ni₂₀ HEAs is a single-phase FCC structure.After 80%cold rolling and 800℃annealing,the yield strength of the alloy under quasi-static tension is398 MPa,the ultimate tensile strength is 679 MPa and the plasticity holds 32.6%.Compared with hot-rolled plate,σphase and M₂₃C₆ precipitate from FCC matrix in CR800 samples,and the grain size heterostructure is introduced through non-uniform deformation,which greatly improves the strength of the alloy.（2）At 298 K,when the strain rate is increased from 10^-4/s to 3000/s,the strength and plasticity are improved at the same time,meeting the dynamic characteristics of"the higher the strain rate,the stronger the strength and toughness".The activation of deformation twins under high strain rate and the deformation sub-grains induced by short-range orders structure lead to high strain rate sensitivity（0.3978）.The stacking fault energy of the alloy is determined by thermodynamic calculation and molecular dynamics simulation,which provides a theoretical basis for the emergence of deformation twins.Finally,J-C and Z-A models are used to predict the experimental results.（3）When the temperature drops to 77 K,the stacking fault energy decreases accordingly.The deformation nano-twins become the main deformation mechanism.Compared with the high-density primary deformation twins at low temperature,the activation of secondary twins at low temperature quasi-static state leads to the increase of plasticity by 18.7%.Based on the classical four strengthening mechanisms and Taylor model,the yield strength prediction model and flow stress prediction model related to temperature and strain rate are established respectively.The prediction results are in good agreement with the experimental data.（4）In the quasi-static high-temperature tension at different temperatures,with the increase of temperature,the strength and plasticity decrease at the same time,but the strain rate sensitivity increases gradually,and the softening resistance of the material increases.Superplasticity of the alloy is not achieved at high temperature,which is mainly due toσphase precipitates dynamically during the tensile process,and the distribution density increases,resulting in the embrittlement of the alloy.