Microstructure,Thermal Stability And Corrosion Resistance Of Soft Magnetic Fe-based Amorphous Alloys

Fe-based metallic glasses?MGs?with good soft magnetic properties are used in engineering materials such as transformers,motors and sensors.Although the soft magnetic Fe-based MGs have significant advantages over traditional soft-magnetic materials,there are still many challenges,for example,the high brittleness and low thermal stability.Moreover,the comprehensive magnetic properties still need to be further improved.In this paper,the Fe-based amorphous ribbons were prepared by melt-spinning technique.The effects of elemental additions?Nb,Mo,Ta,Zr and NbNi?on the microstructure,thermal stability,hardness,magnetic properties and corrosion resistance in the seawater solution of Fe-based amorphous ribbons have been systematically investigated by XRD,FESEM,TEM,DSC,microhardness tester,alternating gradient magnetometer and electrochemical workstation.Moreover,the correlation between microstructures and properties was discussed.In addition,we also studied the effects of cryogenic and annealing treatment on the microstructures,thermal stability,and properties of the soft magnetic Fe-based MGs.We expect that the soft magnetic Fe-based MGs with excellent comprehensive properties would be developed through the exploration of elemental additions and different treatment processes.The main results are as follows:1.The(Fe_0.8Co_0.2)₇₀B₂₅M₅?M=Nb,Mo,Ta and Zr?amorphous ribbons were prepared by melt-spinning.The amorphous ribbons with Nb and Ta addition exhibit the larger supercooled liquid region??T_x?,suggesting the high glass-forming ability?GFA?.The amorphous ribbons with Ta and Zr addition show the high onset crystallization temperature?T_x=612 and 560??,which are higher than most of the soft magnetic Fe-based amorphous materials,indicating the high thermal stability.The as-spun(Fe_0.8Co_0.2)₇₀B₂₅M₅ MG system has the high microhardness above 850 HV.The coercivity?H_c?values of the amorphous ribbons with Nb,Ta and Zr addition are less than 0.85 Oe,indicating the typical soft magnetic behavior.The amorphous ribbon with Ta addition enhances the magnetic exchange with the matrix and possesses the high saturation magnetization?M_s?of 122 emu/g,exhibiting good soft magnetic properties.The amorphous ribbon with Nb addition shows the most positive corrosion potential(E_corr)and wide passive region??E?,presenting the optimal corrosion resistance in the seawater solution.2.The?Fe-Co?₇₅B₂₁M₄?M=Nb,Ta,NbNi,Mo and Zr?amorphous ribbons were prepared by melt-spinning.The?T_x values of as-spun(Fe_0.7Co_0.3)₇₅B₂₁Nb₄ and(Fe_0.7Co_0.3)₇₅B₂₁Ta₄ amorphous ribbons are 44?and 35?,respectively,showing the high GFA.The amorphous ribbon with Zr addition reveal the high T_g?562??and exhibits the good thermal stability.Compared with(Fe_0.8Co_0.2)₇₀B₂₅M₅ amorphous ribbons,the contents of Fe and Co are increased and the content of B element is decreased.The decreased average distance between nearest-neighbor atoms enhances the interatomic bonding force,leading to the high microhardness,which is greater than 970 HV.The(Fe_0.8Co_0.8)₇₅B₂₁Nb₄ amorphous ribbon possesses the higher M_s of 206 emu/g,revealing the excellent soft magnetic properties.Nb addition is beneficial to reduce the Fe-Fe atom spacing.In addition,the increased ferromagnetic element and the decreased B contents are beneficial to increase the M_s and optimize the soft magnetic properties.The amorphous ribbon with NbNi addition shows the most positive E_corrorr and wider?E,presenting the good corrosion resistance in the seawater solution.3.The as-spun(Fe_0.7Co_0.15Ni_0.15)₇₅B₂₁Nb₄?Z3?,(Fe_0.7Co_0.3)₇₅B₂₁Ta₄?Z2?and(Fe_0.8Co_0.2)₇₅B₂₁Nb₄?M1?amorphous ribbons were annealed at<T_g region,the annealed samples exhibit the low H_c due to the structural relaxation,stress release and atomic diffusion.Annealing at T_g-T_x and>T_x regions,the?-<Fe,Co>phase and Fe-B compound sequentially precipitate from the amorphous matrix,leading to magneto-crystalline anisotropy,further increasing H_c.Annealing at different regions of<T_x,a certain amount of 30-50 nm?-<Fe,Co>phases are dispersed in the amorphous matrix,which generate the strong magnetic exchange with the amorphous matrix and enhance the M_s.Annealing at>T_x region,the precipitation of the Fe-B compound reduces the M_s.The nucleation activation energy?E_x?is smaller than the growth activation energy?E_p?of the as-spun Z3 amorphous ribbon,indicating that the growth of crystal needs to overcome higher energy barriers during crystallization.So nucleation is easier to perform.Compared with as-spun M1 amorphous ribbon,the 477??<T_g?annealed sample shows the most positive E_corrorr and wider?E,indicating the better corrosion resistance in the seawater solution.4.The as-spun Z2 and M1 amorphous ribbons were treated for cryogenic treatment?CT?at-20 and-70?.CT maintains the amorphous characteristics for the Z2 amorphous ribbon,which occurs structural relaxation and releases internal stress.However,CT?-20??induces crystallization of the as-spun M1 amorphous ribbon.The Fe₂₃B₆ compounds are precipitated from the amorphous matrix.CT improves the T_x value,especially the-70?CT samples show better thermal stability.According to the potentiodynamic polarization curves,CT reduces the corrosion resistance in the seawater solution for the amorphous ribbons.Compared with the as-spun Z2 amorphous ribbons,the exothermic enthalpy??H?of CT samples reduces slightly,leading to a decline in excess free volumes,which correspond to low density of the domain-wall pinning sites.Therefore,the CT samples exhibit low H_c.The M_s value of-20?CT sample is about one time higher than the as-spun one,indicating the good soft magnetic properties.However,the precipitation of the Fe₂₃B₆ compounds from the amorphous matrix reduces the M_s value.