Composition Design,Microstructure And Mechanical Properties Of CoCrFeNi-Based Precipitation Strengthened High-Entropy Alloys

High entropy alloys（HEAs）have received wide attention from researchers since first proposed in 2004.The CoCrFeNi-based face-cen-tered cubic（FCC）HEAs,as the most classical of them,have excellent duc-tility.However,the low strength of this series HEAs limits the application in practical engineering.Due to the slow diffusion effect,the comprehen-sive performance of FCC HEAs can be well improved by precipitation strengthening mechanism.The development of nanoprecipitation strength-ened HEAs has become one of the promising orientations in recent years.At present,such alloys mainly use the method of adding alloying elements to introduce the second phase,which needs to combine lots of theoretical calculations for the design and preparation of different system alloys and lacks universality.For the difficult issue in composition design of precipitation strength-ened HEAs,this paper proposes a new strategy for the composition design of precipitation strengthened HEAs based on the diffusion couple method,introducing Al and Ti,Cu and Ti,and（Al,Cu and Ti）into CoCrFeNi-based HEAs,respectively.Five diffusion couples,CoCrFeNiTi-CoCrFeNiX（X is Al or Cu）,CoCr_0.5FeNiX_0.5Y-CoCr_0.5FeNiX_0.5Z（X,Y and Z are any one of Al,Cu and Ti）,were developed.According to the change of diffusion layer concentration in the diffusion couples,five new precipitation strengthened HEAs were designed respectively.After the series mechani-cal heat treatment,the yield strength and ultimate tensile strength of the five alloys after aging at 700℃ for 8 h were found to be significantly higher than the solid solution state,Co₂₄Cr_20.9Fe_25.1Ni₂₄Al₃Ti₃ alloy was 851 MPa and 1281 MPa,with elongation of 30%;Co_22.8Cr_19.0Fe_23.5Ni_24.7Cu₅Ti₅ alloy was 1020 MPa and 1339 MPa,with elongation of 19%;Co₂₀Cr₁₆Fe_32.5Ni_17.5Al₄Cu₃Ti₇ alloy was 1004 MPa and 1508 MPa,with elongation of 26%;Co_22.3Cr_11.8Fe_31.7Ni_20.8Al₆Cu_3.7Ti_3.7 alloy was 772 MPa and1017MPa,withelongation of 21%;Co_18.6Cr_14.5Fe_38.4Ni_18.7Al_1.1Cu_3.2Ti_5.5 alloy was 895 MPa and 1286 MPa,with elongation of 25%,respectively.Of them,the mechanical properties of Co_22.8Cr_19.0Fe_23.5Ni_24.7Cu₅Ti₅ and Co₂₀Cr₁₆Fe_32.5Ni_17.5Al₄Cu₃Ti₇ alloys were significantly better than those of the precipitation strengthened HEAs reported so far.The heat treatment process of Co₂₄Cr_20.9Fe_25.1Ni₂₄Al₃Ti₃ al-loy also showed that the alloy had better mechanical properties after mul-tiple aging compared to single temperature aging,and the annealing time of 1100℃ before aging had a significant effect on the mechanical proper-ties.Further observation on the microstructures of the five alloys indicated that the developed precipitation strengthened HEAs after heat treatment all had FCC-structured matrix+L1₂-structured precipitates,with precipitates of about ten nanometers in size,uniformly distributed on the matrix.The lattice constants all showed a pattern of decreasing from the solid solution state,aging at 600℃,aging at 700℃ and aging at 800℃ in sequence,and the average grain size was between 3-16μm.The fractures of all five HEAs contain quasi-intelligible fracture and tough nest fracture,which are mixed brittle-tough fracture mechanisms.Co_22.3Cr_11.8Fe_31.7Ni_20.8Al₆Cu_3.7Ti_3.7 alloy and Co_18.6Cr_14.5Fe_38.4Ni_18.7Al_1.1Cu_3.2Ti_5.5alloy have a significantly larger proportion of quasi-intelligible fracture compared to the other three alloys.Meanwhile,the investigation of the deformation organization for Co₂₄Cr_20.9Fe_25.1Ni₂₄Al₃Ti₃and Co_22.8Cr_19.0Fe_23.5Ni_24.7Cu₅Ti₅ indicated that dislocation and lamellar dislocation were the main deformation mecha-nisms of these two alloys.