Effects Of Nb On The Microstructures And Mechanical Properties Of Al_xCoCrFeNi High Entropy Alloys

High entropy alloys are a new type of alloys designed and developed on the basis of containing multiple main elements.This complex element compositions don't form a complex phase structure like traditional alloys,which are often simple face-centered cubic(FCC)or body-centered cubic(BCC),and high entropy alloys generally have excellent mechanical properties such as high strength,high hardness and high wear resistance.However,high entropy alloys also have the problem of mismatch between strength and plasticity,that is,single phase BCC alloys generally have higher strength and lower plasticity;on the contrary,the alloys with single-phase FCC generally have good plasticity,and lower strength,which seriously restricts there applications in the industrial fields.Therefore,in this thesis,single-phase FCC Al_0.3Co Cr Fe Ni,FCC+BCC dual-phase Al_0.7Co Cr Fe Ni and single-phase BCC Al Co Cr Fe Ni were studied for Nb alloying(Al_xCoCrFeNi Nb_y,x=0.3,0.7 and 1;y=0?1).High entropy alloy rods and sheets were prepared by vacuum arc furnace and suspension melting furnace,respectively.The sheets were rolled at different temperatures with different deformations,and then annealed at different temperatures.The phase compositions and microstructures were observed by XRD,OM,SEM and EBSD.Meanwhile the microhardness,compressive mechanical properties,tensile mechanical properties and wear resistance at room temperature were tested.The evolution on the microstructures and mechanical properties of Al_0.3Co Cr Fe Ni Nb_y alloy with the Nb content,rolling process parameters and annealing temperature were investigated in detail.The better Nb content and the suitable rolling process and annealing process were determined,and the strengthening mechanism and wear mechanism were discussed.It will provide certain theoretical guidance and technical support for the compositional design of new high entropy alloys with high performance and broaden the application field of high entropy alloys.The following main conclusions are obtained:(1)Addition of Nb can change the phase composition of Al_xCoCrFeNi alloy rods and form the Laves phase in the alloy.With increasing the Nb content,the alloy microstructure transform from hypoeutectic to hypereutectic,and the Laves phase changes from the initial interdendritic phase to the dendritic phase.(2)With the increase in the Nb content,the hardness of the hardness of the alloy rods with three phase structures increases,and the single-phase FCC system has the largest increase.The compressive strength first increases and then decreases,but it decreases gradually for single-phase BCC system.The compressive yield strength gradually increases and the plasticity gradually decreases.For single-phase FCC system,Al_0.3Co Cr Fe Ni Nb_0.5alloy close to eutectic point has excellent comprehensive mechanical properties,and its compressive strength UCS and yield strength CYS are 2397 MPa and 1579 MPa,respectively.For FCC+BCC dual-phase system,Al_0.7Co Cr Fe Ni Nb_0.1alloy has the best comprehensive compression performance,and its UCS and CYS are 2635 MPa and 1755MPa,respectively.For single-phase BCC system,the comprehensive compression performance of Al Co Cr Fe Ni is the best,UCS and CYS are 3350 MPa and 1421 MPa,respectively.(3)The addition of a small amount of Nb(y=0.1)can significantly refine the coarse equiaxed grains in the original Al_0.3Co Cr Fe Ni alloy sheet,and the average grain size decreases from 152 to 31?m.After homogenization at 1100°C for 36 h,the original Laves phase of Al_0.3Co Cr Fe Ni Nb_0.1 alloy sheet has completely dissolved and the rolling property has been significantly improved.Rolling deformation makes the grains of the alloy sheets be elongated along rolling direction and produces Copper and Brass textures.With increasing the deformation,the strength of Brass texture increases,while that of Copper texture decreases or disappears.With increasing the deformation,the strength increasesfor the Al0.3Co Cr Fe Ni alloy sheet,but it decreases for the Al_0.3Co Cr Fe Ni Nb_0.1 alloy sheet rolled at400°C.Both alloy sheets rolleds at room temperature with deformation of 80%exhibit obtained excellent mechanical properties.The tensile strength UTS,yield strength YS and elongation EL of Al_0.3Co Cr Fe Ni alloy are 1276 MPa,1125 MPa and 2.5%,respectively.The UTS,YS and EL of Al_0.3Co Cr Fe Ni Nb_0.1 alloy increase to 1403 MPa,1265 MPa and 5.0%,respectively.The wear resistance of Al_0.3Co Cr Fe Ni Nb_0.1 alloy is better than that of Al_0.3Co Cr Fe Ni alloy,and the wear resistance is improved after rolling deformation.(4)After alloy sheets rolled at room temperature with deformation of 80%are annealed at 700°C for 1 h,the comprehensive tensile mechanical properties are significantly improved.The UTS,YS and EL of Al_0.3Co Cr Fe Ni alloy are 1421 MPa,1330 MPa and 16.0%,respectively.The UTS,YS and EL of Al_0.3Co Cr Fe Ni Nb_0.1 alloy are 1728 MPa,1610 MPa and 4.0%,respectively.As the annealing temperature increases to 900°C.The recrystallization of the alloy sheets occurs at the place where the deformation storage energy is high.The plasticity of the alloy sheets is improved significantly,and the strength decreases.