Effect Of C And Mo Microalloying On Microstructure And Mechanical Properties Of CoCrFeNi System High Entropy Alloys

High-entropy alloy is a single-phase solid solution alloy consisting of four or more elements in equimolar ratio or non-equimolar ratio.Not only different from traditional alloys in design concept,but also superior to traditional alloys in performance,thus showing high strength,high plasticity,high hardness,corrosion resistance and other performance characteristics.Generally,single-phase FCC high-entropy alloys are difficult to apply because of their poor strength.Recently,a new cobalt-rich high-entropy alloy has been developed.Its elongation can reach91.4%,but its strength is not satisfactory.Traditional cobalt-based alloys have been widely used to develop blades,gas turbines and metal orthopedic implants due to their superior mechanical properties and corrosion resistance.Therefore,single-phase FCC cobalt-rich high-entropy alloys with high strength and high plasticity were developed.It has broad development prospects in the application field of high-entropy alloys.The composition of Co₄₅Cr₂₅Fe₁₅Ni₁₅Mo₅ high-entropy alloy was regulated by micro-adding C and Mo,and optimizes it by different rolling processes.The microstructure was characterized by X-ray diffraction analysis（XRD）,scanning electron microscopy（SEM）with energy dispersive spectrometer（EDS）,transmission electron microscopy（TEM）,electron backscatter diffraction（EBSD）.The effects of C and Mo elements on the microstructure and mechanical properties of Co₄₅Cr₂₅Fe₁₅Ni₁₅Mo₅ high entropy alloy,and the effects of different rolling processes on the microstructure and mechanical properties of CoCrFeNiMoC high entropy alloy,and the strengthening and toughening mechanism were studied.The result is as follows:The as-cast[Co₄₀Cr₂₅Fe₁₅Ni₁₅Mo₅]_100-xC_xhigh-entropy alloys are single-phase FCC structures.With the increase of carbon content,the lattice distortion of the alloy increases first and then decreases.When the solid solution limit of C atoms in the Co₄₀Cr₂₅Fe₁₅Ni₁₅Mo₅ high-entropy alloy is less than 1 at.%,the interstitial solid solution strengthening effect is significantly enhanced.With the increase of carbon content,the grain size of the alloy decreases first and then increases,and the precipitates gradually increase.The yield strength and tensile strength of the alloy gradually increased with increasing carbon content,while the elongation first increased and then decreased.When 0.5 at.%carbon was added,the yield strength and elongation of the alloy increased to 406MPa and 63%,respectively.When C element is added,the deformation mechanism of the alloy is dominated by the transformation-induced plastic deformation,and the transformation twin-induced plastic deformation is dominated,so that the strength and toughness of the alloy are simultaneously improved.The as-cast[Co₄₀Cr₂₅（Fe Ni）_35-yMo_y]_100-xC_x high-entropy alloys are single-phase FCC structures.When Mo is microalloyed,the lattice constant of the alloy increases first and then decreases with the increase of molybdenum content.The lattice constants of the alloys after microalloying of C and Mo increased gradually with the increase of molybdenum content.There are dendrites in the grains of each alloy structure,the primary dendrite and the secondary dendrite are enriched with Mo element,and a point-like Mo-rich precipitate appears at the root of the dendrite.When Mo alloying,with the increase of molybdenum content,the yield strength gradually increased,and the elongation showed a continuous downward trend.After C and Mo microalloying,the yield strength showed the same trend with the increase of molybdenum content,and the strength was greatly improved compared with Mo alloying.When C and Mo are alloyed,the yield strength,tensile strength and elongation of[Co₄₀Cr₂₅Fe_15.5Ni_15.5Mo₄]_99.5C_0.5 high-entropy alloy are 371MPa,833MPa,and 73%,respectively,exhibiting excellent strength and toughness.After hot rolling process,the dendrite segregation of[Co₄₀Cr₂₅（Fe Ni）_35-yMo_y]_100-xC_x high-entropy alloy is eliminated,the grain size is significantly reduced,and the alloy microstructure is composed of completely recrystallized grains and unrecrystallized grains.[Co₄₀Cr₂₅Fe_15.5Ni_15.5Mo₄]_99.5C_0.5 high entropy alloy was rolled at 1100°C for 80%,and the grain size was about 16μm.The yield strength,tensile strength and elongation were 757 MPa,1186MPa and 69%,respectively.Compared with the as-cast alloy,the strength increased by 386 MPa and 353 MPa,respectively,while the elongation decreased by only 4%,exhibiting excellent combination of strength and plasticity.The deformation mechanism is dominated by twin-induced plastic deformation,and the strengthening mechanism is dominated by fine grain strengthening and solid solution strengthening.After annealing at 800°C for 6 min for the cold rolled[Co₄₀Cr₂₅Fe_15.5Ni_15.5Mo₄]_99.5C_0.5 high entropy alloy,the microstructure of the alloy is composed of incompletely recovered deformed grains and fine recrystallized grains.The yield strength of the alloy is 1055 MPa and the elongation is still 37%.The inhomogeneous structure makes the alloy exhibit excellent mechanical properties.When the annealing temperature is1100°C,the alloy consists of fully recrystallized equiaxed grains with uniform grain size,which makes the elongation of the alloy up to 85%.