Composition Design,preparation And Properties Of Fe-Co Based High-entropy Alloys

With the increase of temperature,the mechanical properties and soft magnetic properties of traditional Fe-Co based soft magnetic materials generally deteriorate.The soft-magnetic amorphous materials are not easy to be cold processed due to their strong composition sensitivity and brittleness.The high-entropy alloy(HEA)as a novel material has unique phase structure and great comprehensive properties,including excellent mechanical properties,good wear resistance,corrosion resistance and soft magnetic properties.The HEA overcomes the component complex and performance weakness of traditional alloy,which is expected to possess a wide application prospect in aerospace,marine or other extreme environmental conditions.In this project,the(Fe_0.5Co_0.5)₇₅B₂₁Ta₄HEA was used as the research object.Through the composition design,the refractory elements of Zr and Mo with different atomic radius and lattice configuration were selected as additional elements to obtain the new Fe-Co based HEA.The effects of element addition and treatment process on the phase structure,magnetic properties,corrosion resistance and microhardness of the HEA were investigated.Through effective structure modulation,the saturation magnetization(M_s)and corrosion resistance in seawater solution of HEA were improved,and the soft-magnetic HEA with excellent comprehensive properties have been acquired.The results are as follows:1.Two kind of(Fe_0.5Co_0.5)₇₅B₂₁Ta₄HEAs obtained by arc melting and subsequent cryogenic treatment have the similar phase structures,which are mainly composed of(Fe Ta B)₁₃Co₄,body centered cubic(BCC)solid solution and Ta-B-rich intermetallics.The components of HEA fabricated by copper mold suction casting become more complex,which contain dual solid solutions with FCC structures(FCC1+FCC2),besides(Fe Ta B)₁₃Co₄and Ta-B-rich intermetallics.The results show that the faster cooling rate can inhibit the precipitation of BCC phase,which is beneficial to the formation of FCC phase with higher density.The arc-melted(Fe_0.5Co_0.5)₇₅B₂₁Ta₄HEA has cellular dendrite morphology,and the sample after cryogenic treatment presents typical dendrite morphology.The magnetic test shows that the coercivity(H_c)values of the three samples are less than 13 Oe,showing typical soft-magnetic behavior.The highest M_sof the suction-cast sample is 118.48 emu/g,which is attributed to the precipitation of FCC phase with higher atomic packing density induced by rapid cooling.The FCC phase has more ferromagnetic elements and larger total magnetic moment per unit volume.The results of electrochemical corrosion in seawater solution show that the arc-melted HEA has the most positive corrosion potential(E_corr),minimum corrosion current density(i_corr)and passivation current density(i_pass),and relatively wide passivation region(?E_P),showing the relatively excellent comprehensive corrosion resistance.According to the hardness test,the cryogenic treatment significantly improves the microhardness of the arc-melted sample(1153 HV).It is mainly due to the increased size of the Ta-B hard-phase caused by the cryogenic treatment,and the obvious lattice distortion of the matrix caused by the Fe segregation.It further shows that proper cryogenic treatment has a positive effect on the microhardness of the HEA.2.The(Fe_0.5Co_0.5)₇₀B₂₁Ta₄Mo₅HEA was designed by adding Mo with BCC structure.The results show that two kind of(Fe_0.5Co_0.5)₇₀B₂₁Ta₄Mo₅HEAs obtained by arc melting and subsequent cryogenic treatment have the similar phase structures,which are composed of dual solid solutions with BCC structures and(Co,Fe)_20.8(Ta,Mo)_2.18B₆intermetallic compound.The phase component of suction-cast HEA becomes more complex,possessing FCC solid solution and?phases,besides retaining BCC1 and(Co,Fe)_20.8(Ta,Mo)_2.18B₆intermetallic phases.It presents that the suction-cast sample shows typical dendrite morphology,and two types of HEAs obtained by arc melting and through subsequent cryogenic treatment show like"core-shell"structure.The H_cvalues of three alloys are less than 37 Oe,exhibiting typical soft-magnetic behavior.After cryogenic treatment,the sample has the highest M_svalue of110.6 emu/g,which is significantly increased by 13%compared with the arc-melted sample.This can be explained that the cryogenic treatment leads the structural relaxation,further releases the local internal stress and reduces the pinning effect,promoting the movement of domain wall.The results of electrochemical corrosion in seawater solution display that the alloy after cryogenic treatment has the most positive E_corr(-0.929 V)and smallest i_corr(3.817×10^-5m A/mm²),indicating a good general corrosion resistance.The width of?E_Pof suction-cast sample is the largest,which is 1.0 V,indicating that the passive film is more stable.The microhardness of the sample after cryogenic treatment is 1061 HV,which is 2.9%higher than that of the arc-melted sample.It also reveals that proper cryogenic treatment has a positive effect on the mircorhardness of the HEAs.3.The(Fe_0.5Co_0.5)₇₅B₁₁Ta₄Zr₁₀HEA was designed by adding a close-packed hexagonal Zr element with large atomic radius.The sample prepared by arc melting is mainly composed of double solid solutions with BCC1+BCC2 structures,(Fe Ta BZr)₁₃Co₄and Ta-Zr-B-rich intermetallic compounds.The phase structure of the suction-cast samples prepared retains(Fe Ta BZr)₁₃Co₄and Ta-Zr-B-rich phases,which also inhibit the formation of BCC solid solution.All three alloys show typical dendrite morphologies.The magnetic properties of the three samples exhibit typical soft-magnetic behavior.The highest M_sand the lowest H_cof arc-melted sample is 82.97 emu/g and 73.72 Oe,respectively.According to electrochemical corrosion results,the sample after cryogenic treatment possesses lower i_corrcompared with the arc-melted sample,indicating the sluggish anodic activation corrosion process.The?E_pof the suction-cast sample is wider,presenting the formation of the stable passive film.The microhardness of the sample after cryogenic treatment reaches 901 HV,which is 3.37%higher than that of arc melted HEA.4.According to above results,the element addition and different treatment process have obvious impact on the microstructures and properties of the Fe-Co-based HEA systems.The effect of cryogenic treatment on improving the soft-magnetic properties of the arc-melted(Fe_0.5Co_0.5)₇₀B₂₁Ta₄Mo₅HEA is the most obvious,which is characterized by the significantly increased M_s.Moreover,it is also beneficial to enhance the microhardness of all HEAs.The(Fe_0.5Co_0.5)₇₅B₂₁Ta₄HEAs prepared by arc-melting,cryogenic treatment and suction-cast have the best comprehensive corrosion resistance,high mircohardness and soft-magnetic properties,respectively.