Study On The Microstructure And Magnetic Properties Of Nanocomposite Nd₂Fe₁₄B/α-Fe Permanent Magnetic Materials

Nanocomposite magnetic materials consisting of a hard magnetic phase exchange coupled to a soft magnetic phase have attracted considerable attention because of their unusually high remanence, high energy products and low cost.In this dissertation, the Nd₂Fe₁₄B/a-Fe-type nanocomposite magnets have been prepared by melt spinning and subsequent crystallization treatment. The effects of alloy compositions on the microstructure, magnetic properties, thermal stability and inoxidizability have been investigated systemically. Special attention has been paid to the crystallization behavior and crystallization kinetics of the nanocomposite magnets, as well as the correlations among the phase constitution, microstructure and the thermal stability. The influences of manufacture procedure on the microstructure and magnetic properties have also been studied, and the differences between the induction-melt spinning equipment and arc-melt spinning equipment for the samples preparation have been discussed. X-ray diffraction (XRD), differential scanning calorimeter (DSC), thermo gravimetric analysis (TGA), transmission electron microscope (TEM), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), optical microscope (OM) and vibrating sample magnetometer (VSM) have been employed.The results show that:The additions of Nb and Zr significantly change the crystallization behavior of the amorphous phase, refine the microstructure, thus enhance the exchange coupling between the soft and hard magnetic phases. For Nd₁₀Fe₈₄B₆ alloy, after the Zr addition the crystallization behavior changes from the difficult nucleation and easy growth for the grains to the easy nucleation and difficult growth for the grains. Nd₂Fe₁₄B and a-Fe phases precipitate simultaneously from the amorphous phase, and the formation of Nd₃Fe₆₂B₁₄ metastable phase is completely prevented. For the (Nd_0.4Pr_0.6)_8.5Fe_85.5B₆ alloy, the combined Nb and Zr additions increase the precipitating temperature of a-Fe, avoid the formation of metastable phase, and the crystallization behavior changes from a three-step to a single-step process, thusrefine the grain size and improve the magnetic properties.The thermal stability of coercivity can be improved through increasing the microstructure-dependent parameter ï¿¡ or decreasing Neff. Nb and Zr additions make the grains more homogeneous and regular, leading to the decrease of Neff, thus improve the thermal stability. However, the Curie temperature of NdaFe^B decreases with doping Nb and Zr;therefore, over additions of Nb and Zr are detrimental to the thermal stability of the magnets.For NdxFe93_xB6Zri (x=9^ll) alloys, the volume fraction of NdaFenB increases with increasing Nd content, and the samples with x=10 exhibit the optimization magnetic properties and thermal stability. For NdioFegg-xZriBx (x=5⁸) alloys, the remanence and the thermal stability of remanence are deteriorated with increasing B content. The coercivity and the thermal stability of coercivity improve firstly with increasing B content, and then are deteriorated with the further increasing B. The Fe3B phase is also found for the high B content. The energy product reaches the maximum at x=6.The Co substitution for Fe can increase the remanence, but decrease the intrinsic coercivity of the magnets. Over addition of Co leads to the grain coarsening and the formation of Nd2(Fe,Co)i7 phase, thus deteriorates the magnetic properties. Co addition enhances the Curie temperature of Nd2Fei4B and cc-Fe, which is helpful to improve the thermal stability of remanence. The samples with 2at% Co content exhibit the best thermal stability of iHc. The Dy addition can increase the intrinsic coercivity, but decrease the remanence. Dy addition or combined Dy and Co additions effectively improve the thermal stability of Nd2(Fe,Zr)i4B/a-Fe nanocomposite magnets. The effect of combined Dy and Co additions is better than that of sole Dy addition.Additions of Zr, Nb and Co can significantly increase the inoxidizability of the magnets, the combined Zr and Co additions exhibit greater improvement in inoxidizability than other additions. In terms of inoxidizability, NdioFe84B6