Investigation On Microstructure And Properties Of FeCoNiCrB Eutectic High Entropy Alloys

Single-phase high-entropy alloys have attracted widespread attention due to their high strength or high plasticity.However,the preparation of high-entropy alloys with both high strength and high plasticity still remains challenges.In recent years,eutectic high-entropy alloys with two-phase structures have been proposed based on the guiding concept of eutectic alloys.By combining the hard,brittle phase with the ductile phase,eutectic high-entropy alloys can achieve the balance of strength and ductility,which opens a new windowfor the design and preparation of high entropy alloys.The fluxing methods using molten oxides can make the alloy melts with the eutectic composition obtain a large degree of undercooling and then can refine the crystal grains and improve the performance of the alloy,which have been applied to the preparation of advanced alloys.In this thesis,the FeCoNiCrB eutectic high-entropy alloys with large undercooling and network structure were prepared by the fluxing methods,the phase structures,microscopic morphology,and mechanical properties were systematically studied.The main findings of this thesis are as follows:1.The"pseudo-binary"eutectic high-entropy alloy[（FeCoNi）_1-xCr_x]₈₃B₁₇（x=0-0.25）with a network structure was successfully prepared by the fluxing method.Experiments show that the alloy exhibits uneven stratification when x is between 0.19 and 0.25.The upper and lower layers of the alloy ingot are composed of gross Cr-rich phase and Ni-rich phase,respectively.The alloy is brittle.when the content of x is reduced to 0.18,The alloy melt has a large undercooling degree（～350℃）,the alloy morphology and structure transform into a uniform two-phase（tetragonal Cr₂B phase and face-centered cubic Fe Ni₃ phase）network-like eutectic structure,and the alloy has good compressive plasticity.When x is reduced 0.17 to 0.12,the morphology and two-phase structure of the alloys are consistent with x=0.18,and the compressive plasticity of the alloy could be further improved.When x drops to 0.10,the alloy morphology changes from a network-like eutectic structure to an uneven structure composed of network-like regions and island-like regions.At this time,a third phase(face-centered cubic Fe₂₃B₆ phase)precipitate out.When x=0.05,the alloy morphology transforms into an island-like structure.When x=0,the alloy morphology transforms into an island-like structure dotted with different sizes（Cr₂B phase disappears）.Through comprehensive mechanical properties,it is found that when x=0.15,the ratio of brittle phase to the ductile phase of the alloy is 0.58:0.42,and the compressive mechanical properties of the alloy are the best among its vicinity compositions.The compressive yield strength is 1332MPa,and the compressive fracture strength is 1902MPa.The plasticicty is 17.5%,and Vickers hardness is 517HV.2.By adjusting the boron content,the two-phase ratios can be adjusted to achieve the optimization of the mechanical properties of the alloy.By reducing the B content,based on the glass melt coating method,successfully controlled the content of brittle phase in the[（FeCoNi）_0.85Cr_0.15]_100-yB_y（y=10-17）alloy.Experiments show that as the B content decreases,the melting point of the alloy rises slightly,and the degree of subcooling increases during the cooling process.When y=15 and 12,the degree of subcooling can reach～370℃and～390℃,respectively.When the boron content decreases from 17 at.%to 10 at.%,as the brittle phase content of the alloy decreases,the compressive yield strength and microhardness also decrease,and the compressive plasticity gradually increases.The experimental results show that the alloy composition with the best comprehensive mechanical properties is[（FeCoNi）_0.85Cr_0.15]₈₅B₁₅,the ratio of brittle phase to plasticity（volume ratio）is 0.48:0.52,the compressive yield strength is 1159MPa,and the compressive fracture strength is 2254MPa.The compression plasticity exceeds 55%.Moreover,through room temperature rolling（50%reduction rate）and high temperature annealing（950°C and 1000°C）for 10h to eliminate defects such as internal holes and optimize two-phase local composition in the alloy,the good tensile properties of the y=12alloy can be obtained in room temperature,respectively:（1）heat treatment of 950℃alloy yield strength is 508MPa,fracture strength is 670MPa and plasticity is 12%;（2）heat treatment of1000℃alloy’s yield strength is 435MPa,fracture strength is 645MPa and plasticity is 20%.In summary,the fluxing method can be used to achieve a deep undercooling for FeCoNiCrB eutectic high-entropy alloy melts of a large mass.By fine-tuning the composition of the alloy,the optimization of the two ratios of the brittleness/ductile phase is further realized,and the mechanical properties could be improved.The research results are helpful to provide new ideas for the development and preparation of new eutectic high-entropy alloys that possess both strength and toughness.