Preparation And Properties Of Soft Magnetic Fe-Co-based Amorphous Alloys

Broadband-gap semiconductor materials promote the high frequency,high efficiency and miniaturization of power electronic devices,and at the same time put forward higher requirements on the performance of matching soft-magnetic materials,such as high saturation magnetization intensity and low core loss.Compared with the traditional soft-magnetic materials such as silicon steel,ferrite and Permalloy,Fe-Co-based amorphous and nanocrystalline alloys not only have excellent soft-magnetic properties,but also have advantages in thermal stability and corrosion resistance,showing excellent comprehensive performance and a wide application prospect in power electronics.Based on the principle of component design of soft-magnetic Fe-Co-based amorphous/nanocrystalline alloys,two quaternary Fe-Co-based amorphous alloys of(Fe_0.5Co_0.5)₇₅B₂₁Ta₄ and(Fe_0.8Co_0.2)₇₅B₂₁Ta₄ with low iron content were designed and fabricated by melt spinning technique.The structural relaxation,nanocrystallization and full crystallization of Fe-Co-based amorphous alloys induced by appropriate annealing treatment and cryogenic treatment were studied.The microstructure,thermal stability,soft-magnetic properties and corrosion resistance of Fe-Co-based alloy ribbons were investigated more systematically by means of XRD,FESEM,TEM,DSC,VSM,magneto-optical Kerr microscopy,PPMS,electrochemical workstation and microhardness tester.The correlation between structure and properties was initially explored with a view to developing multi-domain coupled,soft magnetic Fe-Co-based amorphous alloys with excellent comprehensive properties.The main research results are shown as follows:1.Preparation and property characterization of(Fe_0.5Co_0.5)₇₅B₂₁Ta₄ amorphous alloy ribbons:The as-spun ribbons shows excellent soft-magnetic properties with a coercivity（H_c）of0.22 Oe,a saturation magnetization（M_s）of 127 emu/g,and a Curie temperature（T_c）of 642 K.The magneto-optical Kerr microscopy test results show that the main sources of H_c are the inhomogeneous internal stress field and the shape anisotropy due to the uneven surface of the ribbon.The thermal crystallization process of this alloy ribbon is as follows:amorphous phase→structural relaxation→α-Fe（Co）+amorphous phase→α-Fe（Co）+（Fe,Co,Ta）₂₃B₆+amorphous phase→α-Fe（Co）+（Fe,Co,Ta）₂₃B₆.The DSC results indicate that the as-spun and relaxed amorphous alloys exhibit two-step crystallization behavior.In contrast,both 760 K and 820 K-annealed samples exhibit one-step crystallization behavior.Despite the increase in H_c,the 820 K-annealed sample obtaines the highest M_s value of 134 emu/g and has the ability to reach the saturation magnetization state more rapidly.It indicates that the appropriate annealing treatment leads to the optimize precipitation of bothα-Fe（Co）and（Fe,Co,Ta）₂₃B₆ nanocrystallines,which is beneficial for the enhanced soft-magnetic properties.Combining the evolution of magnetic domains in the 820K-annealed sample with the phase analysis,it is clear that the ferromagnetic exchange coupling between the nanocrystalline and the amorphous matrix cannot effectively counteract the magnetic crystal anisotropy of theα-Fe（Co）phase.In addition,with the large precipitation of the hard-magnetic（Fe,Co,Ta）₂₃B₆ phase,both of above are the main reason for the increase of H_c for the thermal crystallization samples.According to the results of the potentiodynamic polarization curves measured in 3.5 wt.%Na Cl solution,the annealed samples all exhibite a more positive corrosion potential(E_corr)compared to the as-spun samples,and the samples with more crystallization products show a wider passive region,indicating that proper annealing treatment is beneficial to improve the corrosion resistance.The cryogenic treatment does not affect the amorphous nature of the as-spun ribbons.Based on the DSC curves and crystallization activation energy calculations,it is known that the thermal stability of the cryogenic treated samples is reduced,but the crystallization temperature interval is increased.The nucleation and growth of the primary phase of the 77 K-cryogenic treated sample is easier from the amorphous matrix than that of the as-spun sample.The cryogenic treated and as-spun samples have similar M_s and H_c values,but the 77 K treated samples show the ability to reach the saturation magnetization state efficiently.The microhardness values of both treated samples are greater than 1030 HV.Among them,the 253K-cryogenic treated sample shows the largest microhardness value of 1160 HV.In addition,the cryogenic treatment is beneficial to increase the passive region width and improves the pitting resistance of the samples.2.Preparation and characterization of(Fe_0.8Co_0.2)₇₅B₂₁Ta₄ amorphous alloy ribbons:This as-spun alloy ribbons shows superior soft magnetic properties with H_c of 0.03 Oe,M_sof 134 emu/g,and T_c of 686 K.The thermal crystallization process is similar to that of(Fe_0.5Co_0.5)₇₅B₂₁Ta₄,but this as-spun and annealed sample exhibit one-step crystallization behavior.700 K-annealed relaxation state sample shows lower onset crystallization temperature（805 K）than the as-spun sample（809 K）and reduced thermal stability.The 870 K-annealed sample has the highest M_s value（140 emu/g）compared to the as-spun sample,despite the increase in H_c.In addition,the 700 K structural relaxation state sample shows the ability to reach the saturated magnetization state efficiently,indicating minimal pinning of the magnetization process to the magnetic domain wall rotation.The electrochemical corrosion results show that the annealed state samples all exhibit more positive E_corr compared to the as-spun ribbon,indicating that appropriate annealing treatment can improve the general corrosion resistance of the sample.Based on the DSC test results and crystallization activation energy calculations,it is clear that the cryogenic treatment reduces the thermal stability of the amorphous phase.The H_c of the cryogenic treated samples increased slightly but shows the ability to reach the saturation magnetization state more efficiently during the magnetization process,thus demonstrating the effectiveness of the cryogenic treatment in improving the soft magnetic properties of the as-spun amorphous alloys.Compared with the as-spun samples,the cryogenic treated samples shows smaller passive current density,indicating that it is easier to form a passive film on the surface after anodic activation.Based on the morphology after electrochemical corrosion at a constant voltage of-0.5 V,the passivation film of the 77 K-cryogenic treated samples is more susceptible to pitting corrosion.