Study On The Microstructure And Micromechanical Behavior Of CoCrNi-based High-entropy Alloys

High entropy alloys,as a new type of structural material,have unique microstructures and excellent comprehensive properties,such as high strength and toughness,friction and wear resistance,corrosion resistance,radiation resistance,and high-temperature resistance.It has promising potential for applications in transportation,aviation and aerospace,attracting the attention of a growing number of researchers.High entropy alloys with FCC structures have received widespread attention due to their excellent plasticity and moderate strength.In recent years,among the many high entropy alloys with FCC structure,the CoCrNi alloy has received a great deal of attention as it shows a good strength and ductility match at both room and low temperatures.However,the microstructure evolution and micromechanical behavior of CoCrNi-based high entropy alloys have not been clear and lack of systematic research.Meanwhile,the low yield strength problem of the CoCrNi casting alloys needs to be solved.Hence,this paper focuses on the CoCrNi-based high-entropy alloys,using synchrotron X-ray diffraction technology combined with microstructure characterization methods to investigate the changes of microstructure and mechanical properties and to reveal the microstructure evolution and micromechanical behavior of alloys during tensile deformation.First.the microstructure evolution and micromechanical behavior of Fe20Co30Cr25Ni25 and Fe20Co30Cr30Ni20 alloys during room temperature tensile deformation were systematically studied.It was found that two alloys maintained a single-phase FCC crystal structure after rolling and annealing,and the mechanical properties at room temperature increased with the Cr content.The martensitic phase transformation occurred during the tensile deformation of the Fe20Co30Cr30Ni20 high-entropy alloy,with critical stress of about 555 MPa.The generated long stripshaped nanoscale ε-martensite and the γ matrix followed the orientation relationship of {111}γ//(0001)ε,<110>γ//[11 2 0]ε.The synergistic combination of the TRIP effect and TWIP effect made Fe20Co30Cr30Ni20 high entropy alloy have sustained strain hardening ability during the tensile process,and achieve high strength and large plasticity.Second,the microstructure evolution and micromechanical behavior of Fe20Co30Cr25Ni25 high entropy alloy with partial recrystallization microstructures during room temperature tensile deformation were studied.It was found that the partially recrystallized alloy had a combination of high yield strength and moderate plasticity(yield strength～943 MPa,plasticity～18.9%).No obvious martensitic transformation was found during the tensile deformation of the partially recrystallized alloy.The nano-twinning produced during the deformation can promote the strain-hardening ability of the alloy.The grains orientated with[200]parallel to the loading direction yield before the macro-yielding,and the partially recrystallized alloy had a higher initial dislocation density,and the dislocation density increased with the macroscopic strain.In addition,the casting CoCrNiAl dual-phase high entropy alloy was developed.The effect of Al on the microstructure and mechanical behavior of CoCrNiAl high entropy alloy were systematically studied.It was found that the volume fraction of BCC phase in CoCrNiAl alloy gradually increased with the Al content,the alloy from a dual-phase structure to a lamellar eutectic structure,the yield strength and tensile strength of the alloy improved,and the fracture elongation degenerated.The yield strength of(Co20Cr30Ni50)85Al15 casting high entropy alloy is～542 MPa,the tensile strength is～1025 MPa,the uniform elongation is～30%and the product of strength and elongation of 30.75·GPa%.During the tensile deformation,the slip bands in the L12 phase,the phase transition from B2 structure to BCT structure,the deformation twins in the B2 phase and the precipitation strengthening effect of nanoparticles in the B2 phase significantly improved the work hardening ability of the alloy,resulting in the elongation of the alloy nearly twice that of the common eutectic high entropy alloys.At last,high strength and large ductility(Co20Cr30Ni50)85Al15 dual-phase high entropy alloy was developed,and the effect of the heat treatment process on its microstructure and mechanical behavior was systematically studied.After annealing at 700℃ for 1h,the phase composition was L21 plus B2,and the partially recrystallized microstructures were obtained.The yield strength of the alloy is the best(～1468 MPa),but the plasticity is low(～12.3%).After annealing at 900℃ for 1h,the phase composition was 112 plus B2,and the large recrystallized grains were obtained.The alloy has the lowest yield strength(-987 MPa),but the best plasticity(～27.90%).After annealing at 750℃ for 1h,the phase composition of the alloy was L12 plus B2 plus a small amount of σ,and a recrystallized structure with fine grains was obtained.The optimal matching of strength and plasticity was achieved(tensile strength～1689 MPa,fracture elongation～24.6%).During the tensile deformation,the martensitic transformation from the B2 structure to the BCT structure can promote the strain-hardening ability of the alloy,thus greatly improving the uniform elongation and optimizing the strength-plasticity combination of the alloy.To sum up,this dissertation investigated the influences of composition and heat treatment process on the microstructure evolution and micromechanical behavior of CoCrNi-based high entropy alloys,which provides a theoretical basis and experimental support for an in-depth understanding of the deformation mechanism of the alloy,achieving strong but ductile alloys,and developing highperformance FCC structured high entropy alloys.