Structure And Magnetic Properties Of Fe-B-(Cu,Hf) Nanocrystalline Alloys

Fe-based nanocrystalline alloys have excellent soft magnetic properties,such as high saturation magnetic flux density（B_s）,low coercivity（H_c）,high permeability,low core loss,which have been widely used as core materials in high-frequency transformers,current transformers and reactors.However,owing to the low amorphous forming ability（AFA）and thermal stability of the precursors,the high-B_s nanocrystalline alloys have a poor manufacturability.Improving the manufacturability while maintaining the high B_s of the nanocrystalline alloys is of great significance to the development and application of the nanocrystalline alloys.Ternary Fe-B-Cu and Fe-B-ETM（ETM=Nb,Zr,Hf）nanocrystalline alloys developed based on binary Fe-B alloys have attracted extensive attention because of their high B_s and good magnetic softness.Nevertheless,the Fe-B-Cu nanocrystalline alloys are sensitive to annealing process,and the precursors of the Fe-B-ETM nanocrystalline alloy ribbons are prone to surface crystallization,which increase the preparation difficulty of the nanocrystalline alloys under industrial conditions.Microalloying is an effective way to adjust the AFA and crystallization behavior of the amorphous alloys,which is expected to regulate the soft magnetic properties and manufacturability of the above-mentioned ternary nanocrystalline alloys while maintaining the high B_s.The correlation between the type and amount of alloying elements and the precursor structure of the above-mentioned ternary alloys,and the effect of micro alloy on crystallized structure,magnetic properties and manufacturability are not clear.The relevant influence mechanism should be clarified.Therefore,it is necessary to make a systemic investigation and analysis,which can provide technical supports and theoretical guidelines for the development and industrial production and application of the high-B_s nanocrystalline alloys.In this work,the composition dependence of the melt-spun structure,crystallized structure and soft magnetic properties was systematically investigated for the Fe-B-Cu alloys.The effects of addition of early transition metal elements（ETM=Zr,Nb,Mo,Hf,Ta,W）on the structure and magnetic properties of the Fe-B-Cu alloys before and after annealing were studied.The correlations of the composition and ribbon thickness with the melt-spun structure,nanocrystalline structure and magnetic properties were established.The composition dependence of the melt-spun and nanocrystalline structures and magnetic properties of the Fe-B-Hf alloys was investigated.The effects of the alloying element（P,Zr,Y and Cu）on the surface crystallization behavior,uniformity of the nanocrystalline structure and magnetic properties of the alloys were studied as well.Based on the analysis of the crystallization behavior and microstructure characterization of the alloys,the mechanism of the refinement of the nanocrystalline structure and the improvement of magnetic softness was clarified.The mechanism of inhibiting surface crystallization of the ribbons based on addition of the alloying elements was revealed.The main research results are summarized as follows:（1）The melt-spun alloys form a fully amorphous structure in the composition ranges of（13–14 at.%B,1.0 at.%Cu）and（15–18 at.%B,1.0–1.3 at.%Cu）,respectively.After annealing at a low heating rate（H_r）,the averageα-Fe grain size(D_α-Fe)of the alloys is more than 27 nm,and the H_c is higher than 70 A/m.The melt-spun alloys precipitate high number-density（N_d）α-Fe nanoparticles in the composition ranges of（13–17 at.%B,1.7 at.%Cu）and（16–17 at.%B,2.0 at.%Cu）,respectively.Under the low-H_r annealing,a fine nanocrystalline structure is formed,and the excellent soft magnetic properties are obtained.For the alloy with 16 at.%B and 1.7 at.%Cu,the D_α-Fe and H_c are 16.2 nm,14.3 A/m,respectively,and the B_s is as high as 1.82 T.The formation of fine and uniform nanocrystalline structure for the high B and Cu alloys is attributed to the strong competitive growth effect among high-N_d pre-existing nanoparticles to inhibit the uneven growth of the grains during annealing.（2）The ETM addition into a Fe₈₆B₁₃Cu₁ amorphous alloy refines the crystallized structure,improves the magnetic softness and widens the annealing temperature window(ΔT_oa).The addition effect is significant when the ETM content is≥5at.%,especially for Zr,Nb,Hf and Ta elements.The significant improvement of the crystallization activation energy of the alloy contributed by 5 at%ETM addition restrains the over growth of theα-Fe grains.For a Fe_85.3B₁₃Cu_1.7 alloy with high-N_dpre-existingα-Fe particles,addition of only 2 at.%ETM can refineα-Fe grains and improve magnetic softness andΔT_oa of the nanocrystalline alloys.Addition of 2 at.%Nb,Mo,Ta and W with moderate atomic radii significantly reduces the size of the pre-existing particles in the melt-spun alloys but maintains the high N_d.After annealing,the D_α-Fe and H_c of the annealed alloys are lowered from 18.1 nm and 32.6 A/m to～16 nm and～10 A/m,respectively,effective permeability（μ_e）at 1 k Hz is increased from5400 to 15000,theΔT_oa is widened from 30 to 90 K,and B_s is maintained at 1.75–1.78 T.The formation of the fine nanocrystalline structure for Fe_83.3B₁₃Cu_1.7ETM₂ alloys is mainly due to the combined effects of the competitive growth of the pre-existingα-Fe particles and ETM-inhibited atomic diffusion.（3）Changing the ribbon thickness can control the structure of precursors and improve the structure and soft magnetic properties of Fe-B-Cu-Nb nanocrystalline alloys.Increasing the thickness of a Fe_82.3B₁₃Cu_1.7Nb₃ alloy from 15 to 23–32μm changes the structure of the melt-spun ribbon from a single amorphous phase to a composite of high-N_dα-Fe nanoparticles embedded in the amorphous matrix.After annealing,the ribbons with the thicknesses of 23–32μm obtain a fine nanostructure and superior soft magnetic properties.The nanocrystalline ribbon with the thickness of 23μm possesses more uniform structure and better magnetic softness,and the D_α-Fe,H_c,μ_e at 1 k Hz andΔT_oa is 15.8 nm,9.6 A/m,15000and 120 K,respectively,which are superior to those of 31.4 nm,93.2 A/m,1700 and 90 K,respectively,for the ribbon with 15μm.Changing the ribbon thickness induces the precipitation of the high-N_dα-Fe nanoparticles by regulating the cooling rate,thus promoting the formation of a uniform fine nanocrystalline structure.（4）An appropriate substitution of P for B or Zr for Hf can enhance the AFA of the Fe-B-Hf alloys and thus inhibit the surface crystallization,as well as improve the structure and soft magnetic properties of the annealed alloys.Doping of 0.1–0.2 at.%Y into the Fe-B-Hf alloy enhances the AFA by reducing oxygen impurity content in the alloys,and promotes the formation of the fully amorphous melt-spun ribbons.In consequence,the uniformity of the crystallized structure of the alloys is increased,and the H_c is lowered.Minor addition of Cu into the Fe-B-Hf alloys inhibits the precipitation of coarse textureα-Fe grains in the ribbon surface by increasing the quantity of heterogeneous nucleation site,and thus refines the structure and improves the magnetic softness of the nanocrystalline alloys.For the Fe-B-Hf-Cu nanocrystalline alloys in the composition region of 1 at.%Cu,4–9 at.%B and 4–6 at.%Hf,the D_α-Fe and H_c can be lowered to 11.5 nm and 5.7 A/m,respectively,and the B_s is around 1.76 T.