Study On Strain Rate/Temperature Dependent Mechanical Properties And Deformation Mechanism Of CrCoNi Based Medium-Entropy Alloys

Medium/high entropy alloys（MEAs）break through the traditional alloy design concept,which was based on one or two metal elements,and design alloys from the perspective of mixing entropy or configuration entropy.Due to its unique design concept,medium/high entropy alloys show excellent comprehensive mechanical properties compared with traditional alloys,such as:high strength,high hardness,high wear resistance,high oxidation resistance,high temperature resistance,corrosion resistance,etc.Excellent mechanical properties make medium/high-entropy alloys potentially used in defense equipment,aerospace,nuclear reactors and other fields.The structural materials will inevitably experience complex plastic deformation and microstructural evolution processes in extreme environments.Therefore,optimizing the mechanical properties of alloys and studying their mechanical behaviors and deformation mechanisms is an important foundation to promote high-entropy alloys toward applications.In this paper,based on the study of deformation mechanism,the composition and microstructure of the CrCoNi MEA were optimized.The quasi-static and dynamic loading with a wide strain rate range were performed on the CrCoNi and the optimized MEAs by quasi-static Instron universal testing machine and the Split Hopkinson Tensile Bar for the study of tensile mechanical behavior.The temperature effect and strain rate effect under quasi-static and dynamic tensile conditions were studied respectively.The deformed samples were microscopically characterized to study the deformation mechanism.Finally,based on the microscopic deformation mechanism and dislocation dynamics,a unified microstructural evolution constitutive relation was established.The main research contents are as follows:（1）Based on the deformation mechanism of the CrCoNi MEA,its mechanical properties were optimized in two ways.1)The CrCoNiSi_x MEAs system was designed by addingSi element.The addition ofSi increases the lattice distortion and reduces the stacking fault energy（SFE）,which causes denser deformation twins（DTs）and the transformation from a face-centered cubic（FCC）to a hexagonal close-packed（HCP）structure during the plastic deformation process.This promoted higher strain-hardening and plastic deformation capabilities,thereby breaking through the constraints of strength and ductility and improving strength and plasticity at the same time.Compared with the CrCoNi MEA,the tensile strength and fracture elongation of CrCoNiSi_0.3 MEA was increased from 790 MPa to 960 MPa,and from 58%to 92%,respectively,which is superior to most high strength-ductility alloys;2)The CrCoNiSi_xAl_x medium-entropy alloy system was designed,and a precipitation-strengthened structure was formed in the CrCoNiSi_0.2Al_0.2 medium-entropy alloy.The yield strength and ultimate tensile strength reached 850 MPa and 1287 MPa,respectively,and the elongation remained at 23±5%,which achieving a substantial increase in strength while maintaining a considerable elongation.Analyze its precipitation strengthening mechanism and find that Orowan strengthening mechanism is more conducive to the improvement of strain-hardening and strength.（2）The effects of temperature,grain size and strain rate on the quasi-static tensile mechanical behavior of CrCoNi and CrCoNiSi_0.3 MEAs were investigated.It was found that both CrCoNi and CrCoNiSi_0.3 MEAs showed a substantial increase in strength at cryogenic temperature,their yield strength reached 600 MPa and 980 MPa,respectively.This is because the increases of the density of deformation twins and FCC→HCP phase transition due to the reduction of SFE at cryogenic temperature,which greatly improved the strain-hardening ability.In CrCoNiSi_0.3 MEA,the elongation decreases due to the formation of excessive HCP phase.In the study of the grain size effect,it is found that the CrCoNiSi_0.3 MEA had a larger lattice friction stress and a stronger grain size effect than the CrCoNi MEA,and the Hall-Petch relationship of the CrCoNi and CrCoNiSi_0.3 MEAs were established;In the study of strain rate effect,it was found that both CrCoNi and CrCoNiSi_0.3 MEAs showed higher strain rate sensitivity in the quasi-static loading range,and the CrCoNiSi_0.3 was higher and reached 0.0235.（3）The dynamic tensile mechanical behaviors of CrCoNi and CrCoNiSi_0.3 MEAs under a wide range of strain rates(1200 s^-1～5000 s^-1)loading were investigated.Combined with the quasi-static tensile mechanical behavior,it is found that,the CrCoNi MEA exhibits a simultaneous increase in strength and ductility with the increase of tensile strain rate,and the CrCoNiSi_0.3 MEA exhibits an increase in strength and a decrease in ductility.As the strain rate increased from 1×10^-3 s^-1 to 5000 s^-1,the yield strength of CrCoNi and CrCoNiSi_0.3 MEAs increased from 360 MPa to 1150 MPa and from 450 MPa to 1600 MPa,respectively,the engineering fracture strain increased from 57%to 90%and decreased from 92%to 66%,respectively.The strain rate sensitivities reached 0.357 and 0.406,respectively.Compared with quasi-static tensile,the deformation mechanism of the CrCoNi MEA was the increase of twinning density,the appearance of cross twins,the appearance of FCC→HCP phase transition and the formation of nanocrystals with the increase of strain rate.The deformation mechanism of CrCoNiSi_0.3 MEA was the increase of twinning and cross twinning density and the increase of FCC→HCP phase transition.CrCoNiSi_0.3 MEA have higher twinning density and HCP volume fraction generated by phase transformation than the CrCoNi MEA,which led to stronger strain-hardening ability and higher strength,but the plentiful HCP phase leads to the decrease of ductility.（4）The mechanical behavior and deformation mechanism of the CrCoNi and CrCoNiSi_0.3MEAs were studied under cryogenic temperature and dynamic coupled loading.Compared with dynamic tensile at room temperature,both two alloys exhibit strength enhancement and ductility reduction under dynamic tensile at cryogenic temperature.Its yield strength reaches980 MPa and 1320 MPa under the 2100 s^-1 strain rate at cryogenic temperature,respectively.The CrCoNi and CrCoNiSi_0.3 MEAs showed strong temperature sensitivity and reached 1.428and 1.4773,respectively.This is due to the fact that both alloys show an increase in twin density and an increase in HCP phase compared to dynamic tensile at room temperature,which improves the strain-hardening ability,but leads to the decrease of ductility due to the increased twin thickness.（5）Precipitation-strengthening structures with two different precipitation volume fractions were achieved in the CrCoNiSi_0.3 MEA by adjusting the annealing temperature.The quasi-static and dynamic mechanical behavior and deformation mechanism of precipitation-strengthened CrCoNiSi_0.3 MEAs were studied.It was found that,compared with the FCC single-phase structure,the yield strength of the CrCoNiSi_0.3 MEA with larger precipitation volume fraction under quasi-static and 5000 s^-1 strain rate dynamic tension reached 806 MPa and 2280 MPa,respectively,and the elongation maintained at 24%and 13%,respectively,and the strength further increased under low temperature dynamic tensile.The strain rate sensitivity and temperature sensitivity of the alloy decreased with the increase of the precipitate volume fraction.Its precipitation strengthening mechanism is Orowan bypass mechanism.Compared with the FCC single-phase structure,the deformation mechanism of precipitation strengthening is the increase of dislocation density,high-density dislocation stacking and the formation of dislocation loops near the precipitation.The dislocation density and dislocation loop thickness increase with the increase of the strain rate,and further increase at cryogenic temperature.（6）Based on the dislocation dynamics and the above microscopic deformation mechanism,the evolution mechanism of dislocation loops was proposed,and a unified plastic flow microstructural evolution constitutive model including the influence of the precipitation phase was established to describe the strain rate and temperature dependent mechanical behavior of the CrCoNi and CrCoNiSi_0.3 MEAs.