Preparation Of Nanocrystalline NdFeB Magnets By Reactive Milling And Desorption-recombination Sintering

Nanocrystalline magnetic materials have attracted considerable attention in recent years because of their potential high remanence, coercivity, and maximum energy product. Nanocrystalline magnetic powders are usually synthesized by melt-spinning and mechanical alloying, and these powders are generally used for making bonded magnets. The bonded magnets have the advantage of easy processing, high dimension precision and good mechanical properties. However, the low density and the existence of polymer binder for the bonded magnets reduce the magnetic properties, leading to the low values of (BH)max. To solve this problem, a new technique, which combines mechanically activated disproportionation and desorption-recombination sintering, has been proposed to obtain nanocrystalline highly-densified NdFeB magnets. The mechanically activated hydrogenation– disproportionation of as-cast NdFeB alloy, the cold compaction process of as-disproportionated NdFeB alloy powders, and the desorption- recombination in situ sintering of as-disproportionated NdFeB alloy powder compacts were investigated systematically. The effect of processing parameters on the microstructures and magnetic properties of the nanocrystalline NdFeB magnets was studied. The production process of nanocrystalline anisotropic NdFeB magnets was investigated as well. The effect of processing parameters on the microstructure anisotropy and magnetic properties of the anisotropic magnets was studied, and the coercivity mechanism of such nanocrystalline NdFeB magnets was also analyzedMicrostructural changes of as-cast NdFeB alloy during mechanical milling in hydrogen were characterized by XRD, M?ssbauer spectra and TEM analysis. The results showed that,since the disproportionation of Nd2Fe14B phase needs much higher activation energy than that for hydrogenation, the characteristics of the reaction between NdFeB alloy and hydrogen during milling could be interpreted as follows: the hydrogenation occurred much earlier than disproportionation; after hydrogenation, a further milling was needed to promote the energy level of the hydrogenated alloy so that the disproportionation reaction was activated; the kinetics of hydrogenation is much faster than that of disproportionation.The aspects of the thermodynamics and kinetics for the mechanically activated disproportionation were investigated, and the kinetic equation was derived. The results revealed that the disproportionation kinetics was affected by the rotation speed, ball to powder mass ratio, hydrogen pressure, and composition of the alloy.The cold compactability of the as-disproportionated NdFeB powder was investigated, and the results showed that the nano-structured as-disproportionated NdFeB alloy powder had good cold compactability, they could be cold pressed into highly-densified compacts by conventional die pressing. After cold pressing at 1400MPa, the relative density of the compacts can achieve above 0.9. The cold deformation process of the compacts was also studied, and it was shown that a strong {110} crystallographic texture of theα-Fe was formed after cold upsetting.The desorption–recombination behavior of as-disproportionated NdFeB alloy during vacuum sintering was investigated by the means of XRD, M?ssbauer spectra, and TEM analysis, and the effect of processing parameters on the microstructure of magnets was also analyzed. The results showed that nanocrystalline NdFeB magnets with average grain size of about 50nm was obtained by vacuum sintering at 780℃for 30 min.The magnetic properties of the nanocrystalline NdFeB magnets obtained at various parameters were measured by VSM. The influences of processing parameters on the microstructure and properties of the magnets were investigated. In the present study, the highest magnetic energy product of the nanocrystalline isotropic Nd16Fe76B8 magnets, was 106.3kJ/m3, the compressive strength and flexural strength of the magnets achieved 301MPa and 126.3MPa, respectively. The magnetic properties of the nanocrystalline anisotropic NdFeB magnet were higher than that of the isotropic NdFeB magnet, which achieved 135.2kJ/m3.The formation of magnetic anisotropy in nanocrystalline anisotropic NdFeB magnets was investigated by means of XRD, TEM and VSM. It was found that the magnetic anisotropy of the magnets was attributed to the seeding effect of the {110} crystal texture of theα-Fe phase in the as-deformed alloy. The mechanism of magnetization and demagnetization for the nanocrystalline NdFeB magnets was discussed. It was found that the coercivity was controlled by the domain wall pinning for the magnets obtained by completed desorption-recombination reaction and controlled by the nucleation-pinning co-working magnetization mechanism for the magnets obtained by uncompleted desorption-recombination reaction.