Study On Microstructure,Mechanical And Corrosion Resistance Properties Of Fe_xNi_65-xCr₂₀Al₁₀Nb₅ High-entropy Alloy

High entropy alloy systems composed of transition group metal elements show a broad application prospect due to many excellent properties and have received wide attention from researchers.For Fe-Ni-Cr-Al-Nb alloy system,the content of Ni element in it has a decisive influence on the corrosion resistance of this alloy in seawater and Pb-Bi environments.Based on this,this thesis designs and prepares FexNi65xCr20Al10Nb5(x=30,35,40,45)high entropy alloy,and uses analytical tests such as Xray diffractometer,scanning electron microscope,EDS energy spectrometer,transmission electron microscope,X-ray photoelectron spectroscopy,Vickers hardness tester,universal mechanical testing machine,stagnant lead-bismuth eutectic(LBE)corrosion device and electrochemical workstation to systematically study the effects of Fe/Ni molar ratio,heat treatment process on the microstructure,mechanical properties and corrosion resistance of the high-entropy alloy were systematically investigated,and the following conclusions were drawn:(1)With the increase of Fe element and decrease of Ni element,the crystal structure of dendritic phase of cast FexNi65-xCr20Al10Nb5 high-entropy alloy changes from FCC to BCC,the volume fraction of FCC phase becomes less and less,and the volume fraction of BCC phase becomes more and more,and when x=45,the dendritic organization consists of a single BCC phase.(2)Fe30Ni35 and Fe35Ni30 high-entropy alloys with FCC-dominated dendritic structure exhibit lower yield strength and excellent ductility,and plastic deformation can reach 22%.For Fe40Ni25 and Fe45Ni20 high-entropy alloys with BCC-dominated dendritic phase structure,the yield strength is higher than that of Fe30Ni35 and Fe35Ni30 high-entropy alloys,but the ductility is poorer,which is mainly attributed to the transformation of the alloy crystal structure and the second-phase strengthening effect.(3)By analyzing the corrosion resistance of FexNi65-xCr20AI10Nb5 alloy in 3.5 wt%NaCl solution,it was found that the corrosion resistance of the alloy was closely related to the volume fraction of FCC phase and BCC phase,and the corrosion resistance was good when the volume fraction of FCC phase was high,thus the Fe35Ni30 high-entropy alloy showed the best corrosion resistance.The Fe45Ni20 high-entropy alloy with a single BCC structure in the dendrite organization is more susceptible to chloride ion attack and has the worst corrosion resistance in 3.5 wt%NaCl solution.Therefore,the corrosion resistance of the FexNi65-xCr20Al10Nb5 high-entropy alloy in 3.5 wt%NaCl solution can be improved by appropriately increasing the Fe element content in the alloy and introducing a small amount of BCC phase,thus forming a FCC+BCC dualphase structure.The reduction of Ni content in the alloy can reduce the dissolution corrosion of the substrate in the liquid lead-bismuth alloy,and induce the formation of a dense aluminum oxide film with better corrosion resistance on the surface of the substrate,thus improving the corrosion resistance of the alloy to lead-bismuth.(4)For the Fe35Ni30Cr20Al10Nb5 high-entropy alloy with good overall performance for heat treatment,the alloy consists of dendritic phases(FCC and B2-NiAl)and interdendritic phases(Laves and B2-NiAl)when the annealing temperature is 700℃;when annealed at 800℃ and 900℃,some of the dendritic phases decompose into Laves and B2-NiAl,and the grain size of the alloy becomes larger.(5)The Vickers hardness,compressive strength and elongation of Fe35Ni30Cr20Al10Nb5 high-entropy alloy were 522.6 HV,1247.7 MPa and 9.3%,respectively,after annealing at 700℃.After annealing at 800℃,the plasticity of the alloy was improved,the strength was slightly reduced,and the best overall mechanical properties were attributed to the decomposition of the dendritic phase in the alloy.With the further increase of annealing temperature to 900℃,a large number of FCC phases in the alloy decomposed,and the residual FCC phases worked together with the newly generated ordered B2 phases,and the inconsistency of the two phases in the deformation process caused the decrease of the comprehensive mechanical properties.This shows that the mechanical properties of Fe35Ni30Cr20Al10Nb5 high-entropy alloy are related to the decomposition degree of FCC phase,and the decomposition of moderate amount can strengthen the mechanical properties of the alloy,while the serious decomposition degree will deteriorate the comprehensive mechanical properties of the alloy.(6)The dynamic potential polarization curves of Fe35Ni30Cr20Al10Nb5 highentropy alloy show that the best corrosion resistance is achieved at an annealing temperature of 900℃,followed by 800℃ annealed alloy,cast alloy and 700℃annealed alloy.Further analysis shows that the dendritic phase rich in Nb elements is more resistant to chloride ions and improves the corrosion resistance of the alloy.The results of XPS analysis showed that the Nb-rich dendritic phase can form NbO2 with better corrosion resistance,which is more resistant to chloride ions and improves the corrosion resistance of the alloy.