Microstructure And Mechanical Properties Of Entropy Alloy In FCC Structure Enhanced By L1₂ Precipitated Phase

Multi-component high entropy alloy(HEA)is a hot research direction in recent years.Simple phases with some unique alloy properties can be obtained by adjusting the composition.They are key materials for core thermal components of high-end equipment in the fields of aerospace,energy,and other fields,and have potential engineering application prospects.Among them,the face-centered cubic(FCC)mesentropic alloy(MEA)has considerable ductility,good corrosion resistance and radiation resistance.However,their strength is relatively low.Therefore,the strategy of strengthening the ductile FCC matrix phase is usually adopted to design HEAs with excellent performance.Among various strengthening methods,precipitation strengthening is a very effective mechanism to control the balance between plasticity and strength,pushing the strength of alloys to new level.This paper chooses CoCrNi,CoFeNi two kinds of the FCC matrix,by adding Al,Ti and V L12 phase formation elements,combining mechanical deformation,such as rolling,annealing,aging and heat treatment,deposition phase shape,volume ratio,matrix microstructure.Adjusting and obtaining a typical two-phase structure "FCC+L12",which the FCC phase is the matrix phase,and the L12 phase is the precipitated particles Then the two alloy are tensiled at room temperature to study deformation mechanisms.The results show that:(1)In order to explore the microstructure and properties of(CoFeNi)82Ti5V8Al5 alloy,this paper mainly studied the samples of three different heat treatment and rolling processes,namely complete recrystallization(CRA),double aging(CAA),double rolling(CRCA).The three alloys were composed of " FCC+L12" two-phase.In the CRA condition,the microstructure of the samples is polycrystalline,and a large number of annealing twins.The grain size is～16 μm,and high density L12 precipitated particles(～18 nm)are uniformly distributed within the grains.While CAA and CRCA alloys show heterogeneous structure,including recrystallization zone and non-recrystallization zone.The average grain size in the recrystallization zone of CAA samples is about～1.4 and there is a coarse L12 phase at the recrystallization boundary,about-200 nm,accompanied by a few small size spherical particles about～62 nm.High density L12 nanoparticles were uniformly distributed in the non-recrystallized area,with an average particle size of～54 nm.The grain size in the recrystallization zone of CRCA samples is about～0.7 μm,and there are high density spherical particles of about～22 nm L12 precipitates uniformly distributed within the grains,and high density spherical L12 precipitates of～46 nm are also distributed in the non-recrystallization zone.The combination of the heterogeneous structure and the precipitated phase can greatly improve the strength of the alloy.The tensile test results show that the yield strength of the CRCA alloy is as high as～1500 MPa,the ultimate tensile strength is～1747 MPa,and the elongation is～16%.Compared with CRA and CAA alloys,CRCA alloy shows ultra-high strength.(2)For the(Co2Ni2Cr)85Al7.5Ti7.5(A17.5Ti7.5)alloy,the solid solution aging alloy and the sample annealed at 1110℃(3min)and then aged at 800℃(1h)after solid solution and rolling 65%were studied respectively.Both of them have a typical ’FCC+L12’ dual-phase structure.and L12 particles are precipitated in the FCC matrix(the precipitated particles are identified as L1 2 structure mainly by SEM,XRD and TEM analysis of the rolled alloy).Forthe solution-aged alloy,the effect of aging temperature on the formation of particles and the peak temperature of aging were studied.The sample aged at 700℃ for 5h was the most obvious which the precipitation-strengthened L12 particles had the best synergistic effect of strength and plasticity,and its yield strength increased from～765 MPa to～950 MPa.For the as-rolled Al7.5Ti7.5 alloy.the mechanical properties of the Co2Ni2Cr matrix alloy without Ti and Al were compared.The yield strength of the Co2Ni2Cr alloy was～208 MPa,the tensile strength was about～640 MPa,and the elongation was as high as～66%.The Al7.5Ti7.5 alloy has a yield strength of～1125 MPa,an ultimate tensile strength of～1540 MPa,and an elongation of～38%due to the precipitation of high-density L12 precipitated particles.Compared with the Co2Ni2Cr alloy without precipitation strengthening,the yield strength is increased by-917 MPa(441%),the ultimate tensile strength is increased by～900 MPa(141%),and the elongation is maintained.