Effects Of Magnetic Field And Stress Field Heat Treatments On The Microstructure And Soft Magnetic Properties Of Nanocrystalline FeSiBCuNb Alloys

The development of electronic components in the direction of high performance,miniaturization and high power puts forward higher requirements for the performance of soft magnetic materials.Fe-based nanocrystalline alloys with high permeability（μ）,high saturation magnetic induction（B_s）,very low coercivity（H_c）and magnetostrictive coefficient（λ_s）have been widely used in reactors,transformers,sensors,filters,transformers and other devices.Among them,nanocrystalline FeSiBCuNb alloy has highμ,low power consumption（P_s）and good frequency characteristics,which makes it the most potential soft magnetic material for high frequency and high power density devices.The nanocrystalline alloy prepared by conventional heat treatment has poor application characteristics in medium and high frequency.Magnetic heat treatment has become a common process to optimize its magnetic properties.However,the effects of different magnetic heat treatments on the crystallization mechanism of the alloy need to be further clarified.In this paper,the effects of ordinary vacuum heat treatment,transverse magnetic field heat treatment and longitudinal magnetic field heat treatment on the microstructure,magnetic domain structure and soft magnetic properties of Si-rich Fe_73.5Si_15.5B₇Cu₁Nb₃ alloy have been studied,and the industrialization technology has been developed.In addition,the effect of thermal-thermal coupling treatment on the crystallization behavior and magnetic properties of the alloy was also studied by hot isostatic pressing.The main research contents and results are as follows:The effect of ordinary annealing on the microstructure and magnetic properties of Fe_73.5Si_15.5B₇Cu₁Nb₃ alloy has been studied.It is found that the alloy has high amorphous formation ability and good thermal stability.The thermal crystallization mechanism of Fe_73.5Si_15.5B₇Cu₁Nb₃ alloy is the combination of chemical concentration fluctuation and heterogeneous nucleation.The effect of annealing temperature on microstructure and properties was analyzed.It was found that the annealing temperature at 560℃promoted the formation of high volume fraction,fine and evenly distributed nanocrystals,and good magnetic properties were obtained.With the increase of annealing temperature,the saturation magnetic induction intensity first increases and then decreases,and reaches the maximum of135.4emu/g at 550℃.The coercivity first decreased and then increased,and the minimum coercivity was 0.56A/m at 560℃.The inductance and impedance increase first and then decrease,and reach the maximum at 560℃,which are 12.2μH（100k Hz）and 10.6Ω（100k Hz）,respectively.The effect of transverse magnetic field annealing on the microstructure,magnetic structure and magnetic properties of Fe_73.5Si_15.5B₇Cu₁Nb₃ alloy has been studied.With the increase of annealing time（10-70 min）,transverse magnetic annealing promotes the optimal growth ofα-Fe（Si）grains.After annealing for 30min,the average grain size（～14.2 nm）of the samples is only slightly larger than that of the samples after vacuum heat treatment（～12.6nm）.Although the grain size after vacuum heat treatment is smaller,the grain size distribution after transverse magnetic annealing is more uniform and highly oriented,forming a uniform nanocrystal structure.The results show that the magnetic field mainly affects the growth direction of the crystal.When annealing for 10-30 min,with the increase of annealing time,the ordered arrangement of Fe-Fe atom pairs induced by transverse magnetic annealing promotes the distribution of randomly oriented magnetic domains along the magnetic field direction,forming uniaxial anisotropy,which can effectively regulate the magnetization mechanism,achieve uniform magnetic domain structure,and obtain flat hysteresis loops.However,as the annealing temperature was further increased（50-70 min）,the inhomogeneous fringed magnetic domain edges and domains with pinning sites reappeared in the sample.Compared with vacuum heat treatment,the Fe_73.5Si_15.5B₇Cu₁Nb₃ nanocrystalline alloy induced by transverse magnetic annealing for 30 min exhibits superior high-frequency characteristics.With the increase of annealing time,the coercivity and loss firstly decreased and then increased,and reached the minimum at 30min,the coercivity was 0.32A/m,and the loss was28.61W/kg（0.2T,100k Hz）.The inductance and impedance increase first and then decrease,and reach the maximum at 30min,the inductance is 2.8μH（0.3V,1MHz）,the impedance is15.65Ω（100k Hz）,35.4Ω（1MHz）.The transverse magnetic annealing temperature was further optimized under the premise of keeping the optimal annealing time unchanged at 30min.The experimental results show that low temperature transverse magnetic annealing only induces a large number ofΑ-Fe nanoparticles precipitation,but has no effect on the growth rate of nanocrystalline grains.At440℃and 460℃,the grain size is 14.2nm and 14.3nm,respectively.Meanwhile,at low transverse magnetic annealing temperatures 440℃and 460℃,the magnetic domains have smooth edges,broad stripes and 180°magnetic domain walls without any branches,indicating low pinning effect and uniform magnetic domain structure.With the increase of transverse magnetic annealing temperature,the coercivity,loss and rectangle ratio all decrease first and then increase,reaching the minimum at 440℃,the coercivity is 0.3A/m,the loss is26.76W/kg（0.2T,100k Hz）,and the rectangle ratio is 0.09.The effect of longitudinal magnetic field annealing treatment on the microstructure,magnetic structure and magnetic properties of Fe_73.5Si_15.5B₇Cu₁Nb₃ alloy has been studied.Compared with ordinary annealed samples,longitudinal magnetic field annealing can effectively improve the magnetic domain structure of strips,reduce the pinning effect,and obtain stripe domain walls with smooth edges and few branches.In addition,under the longitudinal magnetic field,the width of the magnetic domain increases gradually with the increase of annealing time.With the increase of longitudinal magnetic annealing time,the saturation magnetic induction intensity first increased and then decreased,and the maximum saturation magnetic induction intensity was 135emu/g at 30min.Both the coercivity and the loss decreased first and then increased,and the minimum was found at 30min,the coercivity was 0.2A/m,and the loss was 94.88W/kg（0.2T,100k Hz）,and rectangular ratio of 0.976.Compared with ordinary annealing,longitudinal magnetic annealing can obtain higher rectangular ratio and lower coercivity,but the decrease of permeability energy is more obvious,only 1240.The longitudinal magnetic annealing temperature was further optimized by keeping the optimal annealing time unchanged at 30min.With the increase of the longitudinal magnetic annealing temperature（400℃-490℃）,the coercivity and loss first decreased and then increased,and reached the minimum at 430℃,the coercivity was 0.21A/m,the loss was89.74W/kg（0.2T,100k Hz）,and the permeability increased to 2562.5.The magnetic core obtained a larger inductance of 1.64μH（0.3V,100k Hz）,a larger impedance of 2.13Ω（100k Hz）,and a higher rectangle ratio of 0.982.In this paper,the microstructure and properties of the amorphous alloy are regulated by the coupling of temperature field and stress field using the hot isostatic pressing process,and the pressure（10,20,40,80,120 MPa）is changed at 560℃.The effects of hot isostatic pressing on the microstructure,magnetic domain structure and soft magnetic properties of Fe_73.5Si_15.5B₇Cu₁Nb₃ alloy were studied.The coupling of isotropic pressure field and temperature field generated by hot isostatic pressure promotes the dissolution of Si atoms in Fe cells to form Fe（Si）solid solution,which leads to the increase of crystal plane spacing.Meanwhile,HIP promotes the precipitation of a large number of Cu clusters in the amorphous matrix,which is directly related to the nucleation mechanism ofα-Fe（Si）phase.During crystallization,these high density Cu clusters act as nucleation sites ofα-Fe（Si）grains to achieve high volume fraction,uniform and ultra-fine grain microstructure.In addition,the isotropic compressive stress field affects the balance between induced anisotropy and average magnetic crystal anisotropy,which effectively controls the magnetic structure evolution and magnetization mechanism.Compared with ordinary heat-treated samples,the hot isostatic pressing makes the Si rich Fe_73.5Si_15.5B₇Cu₁Nb₃ strip exhibit excellent comprehensive soft magnetic properties in the whole frequency range.When the pressure is 10MPa,the minimum loss is 36.89W/kg（0.2T,100k Hz）,the minimum coercivity is 4.04A/m,and the maximum permeability is 32380.The above results indicate that the magnetic field heat treatment and stress heat treatment have an important effect on the soft magnetic properties of the microstructure of FeSiBCuNb soft magnetic alloy.Compared with conventional vacuum annealing,transverse magnetic field annealing can increase the inductance and impedance value,reduce the coercivity,loss and Br/Bs,while longitudinal magnetic field annealing can increase the coercivity,loss and B_r/B_s,reduce the inductance and impedance value.Therefore,the amorphous materials with better comprehensive properties can be obtained by adjusting the direction of external magnetic field reasonably.In this paper,the soft magnetic properties of FeSiBCuNb alloy are regulated by optimizing the magneto-thermal and thermal-thermal coupling processing technology,which provides guidance for promoting the high-frequency engineering application of nanocrystalline soft magnetic materials.