Effect Of Carbon Doping On Microstructure,Mechanical Properties And Tribological Properties Of Co₂₁Cr₂₁Fe₂₁Ni₂₁V_14.5 High-entropy Alloy

High entropy alloys（HEA）has high strength,toughness and stability due to its multiple principal elements and adjustable component ratio,which shows great potential in the field of tribology.In particular,transition metal alloys represented by CoCrFeNiV have excellent wear resistance and high-temperature stability due to its face-centered cubic（FCC）and sigma（σ）phase intermetallic compounds with a tetragonal structure.However,the single V powder is expensive and the ductility of the material is severely reduced after the formation of the compound.At the same time,it is easy to produce serious elemental segregation by the traditional casting and melting preparation method.To address the above problems,this thesis is aimed at improving the microstructure and mechanical properties by introducing C elements to reduce the V content to form interstitial carbides,while reducing the material cost.The 1.5 at.%carbon-doped Co₂₁Cr₂₁Fe₂₁Ni₂₁V_14.5C_1.5HEA was prepared by powder metallurgy.The effect of C doping on the microstructure and mechanical properties of the alloy was systematically investigated.The wear mechanism and the evolution of oxidized glaze layer in the friction and wear process under harsh environment were investigated,and the results are as follows:（1）Composition design,preparation and phase structure of the alloy.Isoatomic ratio CoCrFeNiV HEA（C0）and non-isoatomic ratio Co₂₁Cr₂₁Fe₂₁Ni₂₁V_14.5C_1.5HEA were prepared by powder metallurgy,and the sintering temperatures of the carbon-doped alloy were set to 1000（C10）and 1200℃（C12）according to thermodynamic calculations.The phase structure,except FCC andσphases,formed M₇C₃and M₂₃C₆interstitial carbides and VC phase was generated in situ at 1200℃.The average size of carbides was about 350 nm,partially located at grain boundaries.Due to the low stacking fault energy of the multi-principal alloy,twins were formed in the FCC phase with low stacking fault energy.（2）Effect of carbon doping on the mechanical properties of the HEA.The alloy’s resistance to deformation is improved by grain boundary and twin boundary strengthening and carbide pegging,which gives it excellent work-hardening ability.A certain amount of V causes the carbides to assume irregular shapes and improves hardness and wear resistance.the compressive strength and compressive plastic strain of C10 and C12 high-entropy alloys were 808±17 MPa,7.27±0.28%and 969±21 MPa,8.43±0.36%,respectively.The compressive strengths were both higher than those of the C0 alloy,but the compressive plastic strains were slightly lower（727±15 MPa,8.89±0.38%）.（3）The effect of carbon doping on the tribological properties of the alloy in air and vacuum environments.Since carbon doping leads to an increase in both hardness and yield strength of the alloy,it makes the activation and proliferation of dislocations difficult and therefore difficult to occur delamination wear,but M₇C₃carbides lead to an increased tendency of exfoliation with increasing load during friction,which triggers three-body wear.Therefore,the main wear mechanisms of C12 alloy are abrasive wear,adhesive wear and oxidation wear.Compared to the air environment,the limitation of vacuum leads to less lubricating oxides in sliding friction due to frictional chemical reactions,resulting in severe adhesive wear.（4）The effect of carbon doping on the tribological properties of the alloy in high temperature environment.The together act of VC,M₂₃C₆and M₇C₃interstitial carbides andσ-phase play a role in wear resistance and self-lubrication at 500～800℃;while at600～800℃ with the increase in the rate of diffusion of alloying elements at higher temperatures,the tribochemical reactions out of the alloy,including those like Cr₂O₃,NiCr₂O₄,NiO and other compounds with high-temperature stability and oxidation resistance to form a thin and compact oxidation glaze layer with high-temperature self-lubricating properties,resulting in that C12 alloy has better tribological behavior than C0 alloy.The wear mechanism of C10 alloy is mainly abrasive wear,adhesive wear,fatigue wear and oxidation wear due to the lower high temperature softening hardness and the large number of pin whisker-like brittle oxides V₂O₅generated on the surface.