Preparation Of High Entropy Alloys By Molten Salt Electrodeoxidization And Hot Pressing Sintering Process And Their Properties

As a new kind of multiprincipal element alloy,high entropy alloys(HEAs)break the concept of traditional alloy with one or two major elements.Due to its unique structure and excellent performance,HEAs have a great potential application value in the fields of high strength structural materials,corrosion and high temperature resistant materials,high performance capacitor and catalytic materials.Compared with the traditional HEA preparation process,the electrodeoxidation process in molten salt has the advantages of short process,low cost and low energy consumption,because the metals and alloys can be obtained directly from its metal oxides in one step.As a common and mature technology,vacuum hot pressing sintering technology(VHPS)has been widely used in the material forming field.Therefore,the combined process of molten salt electrodeoxidation and VHPS(FFC-VHPS)was proposed in this paper.Conventional HEAs,HEAs base wear-resistant materials,refractory HEAs,refractory HEAs base ultra-high temperature resistant composite materials were prepared by FFC-VHPS.The alloying mechanism in the electrodeoxidation process was explored and the properties of HEAs were optimized to provide a research basis for the low-cost and short process preparation of HEAs and a novel idea for the preparation of HEAs matrix composites in extreme environments,such as ultra-high temperature and high wear resistance.The main research contents and results of this paper were as follows:(1)Single phase FCC CoCrFeNi HEA was prepared by direct electrodeoxidation of the mixed metal oxide precursor in molten salt.The microstructure of the alloy was aggregates combined by many nodular particles.The cathodic reduction process obeys a3PI propagation mechanism and the electrodeoxidation process includes the rapid deoxidization and the deep deoxidization process.The current efficiency was proportional to the porosity of the precursor and inversely proportional to the electrolysis time.The results show that Ni O and Co O were easier to electroreduction than Cr2O3 and Fe2O3,and the reduced Feaggregates into dendritic structures.Alloying process begins with the formation of the metal,with Ni CoCralloy formed first,then nonequlatomic CoCrFeNi alloy,finally equlatomic CoCrFeNi alloy.(2)A comparative study was carried out on the preparation of bulk CoCrFeNi alloy by FFC-VHPS,mechanical alloying combined VHPS(MA-VHPS)and arc melting method.The result shows that Cr7C3 was formed between the cathodic C deposited andCrduring VHPS process,which make the hardness of the alloy prepared by FFC-VHPS was better than that of MA-VHPS,and was about twice than that of arc melting method.The grain size of the alloy was less than 50μm due to the low processing temperature of VHPS process.The tensile strength of FFC-VHPS and MA-VHPS alloy was better than that of prepared by arc melting method.The corrosion resistance of CoCrFeNi HEA in 3.5 wt.%Na Cl was better than that in 1 M KOH,and the corrosion resistance in 0.5 M H2SO4 solution was the worst.(3)The properties of CoCrFeNi HEA were optimized by adding different metal elements.The effects of Al,Ti and V on the phase,microstructure and properties of CoCrFeNi HEA were studied.Furtherly,the effect of Ti on the phase,microstructure and properties of Al CoCrFeNi Tix HEA were studied.The result shows that with the increase of Al contents,the phase of the alloys transformed from FCC(Al0-Al0.5)to FCC+BCC(Al0.7-Al0.8)and,finally,to BCC(Al0.9-Al2.0),the tensile strength and hardness of the HEA increase significantly,but the plasticity decreases gradually.Add Al has no obvious effect on the corrosion resistance of the HEA in 3.5 wt.%Na Cl,but weakens the corrosion resistance in 1 M KOH and 0.5 M H2SO4.CoCrFeNi Vx and CoCrFeNi Tix HEA(x=0,0.5,1,1.5)was single-phase FCC structures.The addition of V can reduceCrsegregation and refine grain.The increase of V can significantly improve the hardness of CoCrFeNi Vx alloy,but the high V content decreases the tensile strength and plasticity.The increase of V also improves the corrosion resistance of the HEA in 3.5 wt.%Na Cl,1 M KOH and 0.5 M H2SO4.The addition of Ti can reduceCrsegregation and significantly increase the hardness of CoCrFeNi Tix HEA,but the high Ti content reduces the tensile strength and ductility of the alloy.With the increase of Ti content,the corrosion resistance of the alloy increased significantly in 0.5 M H2SO4 and increased slightly in 1 M KOH.CoCrFeNi Ti0.5 alloy has the best corrosion resistance in3.5 wt.%Na Cl.AlCrFeNi Tix alloy was mainly composed of B2 phase and A2 phase,and the surface forms a ceramic coating containing Ti N and TCP phase.With the increase of Ti,the hardness of the alloy increases significantly,but the tensile strength and plasticity decrease.Due to the formation and gradual increase of Ti N,the wear mechanism changed from both of predominantly delamination and accompanied oxidative wear to single delamination wear.The ceramic coating was the main reason for the high hardness and wear resistance of the alloy.(4)ZrHfNbTa HEA with BCC structure was successfully prepared by direct electrodeoxidation of its mixed metal oxides.The results show that cathodic C deposited not only form carbides with metal elements,but also wraps around the edge of alloy particles to form amorphous phases.VZrHfNbTa HEA powders with BCC structure were prepared by adding V element,and VZrHfNbTa-C1.6 high entropy carbide(HEC)was formed on the surface of the alloy after VHPS.The hardness of the HEC surface was 1534 HV.Furtherly,(VZrHfNbTa)C HEC nanoparticles with FCC structure were prepared by adding V and C element,and the particle size was less than100 nm.The supercapacitive behavior and catalytic properties of VZrHfNbTa and(VZrHfNbTa)C powders were evaluated in a three-terminal electrochemical cell.The specific capacity of(VZrHfNbTa)C was 50 F/g at the scan rate of 100 m V/s,and the capacity retention rate was 89%after 2500 cycles at 5 A/g in 1 M KOH solution,which were better than the specific capacity of 14 F/g and the capacity retention rate of 84%of VZrHfNbTa.C can improve the kinetics of catalytic reaction,increases the number and activity of active sites,which makes the catalytic performance of(VZrHfNbTa)C significantly better than that of VZrHfNbTa.(5)VZrHfNbTa refractory HEA basedHfcoating composite was prepared by FFC-VHPS process.The results show that the addition of C can improve the deoxidization efficiency of theHfO2 precursor.The mixed powders ofHfandHfC prepared by direct electrodeoxidation of theHfO2 precursor system has 1.26 wt.%oxygen content and 23.87%current efficiency,and the energy consumption was 13.62k W·h/kg,about a quarter of that of the Kroll method.The coating thickness of VZrHfNbTa basedHfcoating composite prepared by VHPS was 100～150μm.Zr in the HEA matrix was diffused in the coating and the diffusion distance was about 60μm.After high temperature oxidation,the weight gain of coated sample was 56.56 mg/cm~2,which was smaller than that of uncoated sample(98.09 mg/cm~2).Cracks appeared on the surface of the uncoated sample and oxidation powders appeared in the surface and inner sample.The surface of the coating sample was oxidized and sintered,and the denseHfO2 protection layer was formed.Except for V,other elements were diffused obviously in the coating.The oxygen content in the uncoated sample was higher than that in the coated sample.The surface hardness of the uncoated sample decreased significantly after oxidation,but the coated sample changed little.