The Research Of Propersities And Microstructure In Sintered Nd-Fe-B Magnets With Dy Hydride Addition

In this work, melting, strip casting, hydrogen decrepitation and jet milling were applied to prepare Nd-Fe-B main phases powders, and crushing, hydrogen decrepitation and ball milling were applied to prepare Dy, Dy-Ga, and their hydride powders as the adding alloys. Then blending these powders according to a certain proportion, we got the magnets after a series of processing such as pressing, magnetic field orientation, sintering and two steps of annealing. X-ray diffraction, AMT-4 magnetic measurement, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), scanning electron microscope (SEM) fitted with Energy Dispersive X-ray (EDX) were applied to study the influence of magnetic properties and microstructure in sintered Nd-Fe-B magnets with Dy, Dy alloy and their hydride powders addition. The results showed that:The magnets without addition of adding alloys have high remanence and low coercivity, this is because that the composition of the magnets is near the main phase component, the addition of heavy rare earth is low and its distribution in the magnets is uniform because of melting addition, low content of rare earth-rich phases leads to many defects and holes and low density. Main phases are easy to contact with each other, so it promote the magnetic exchange coupling.With the addition of Dy hydride (DyH3, DyF2), the coercivity of the magnets was increased significantly, but the remanence was not decreased rapidly. Optimum coercivity was obtained when 2.0 wt% Dy hydride powders were added as grain boundary additives. It is because the hydrogen desorbed from Dy hydrides during sintering, react with oxygen in grain boundary, and thus improve the wetting ability between matrix phases and intergranular phases and improves the coercivity. Besides, fine dysprosium powders become more active and wrap matrix phases well so that the diffusion of Dy to the surface layer of matrix phases is convenient, decreasing the defects and holes, then dysprosium decreases in intergranular phases and aggregates in surface layer of matrix phases.For the magnets with the addition of Dy-Ga and its hydride powders, we found that Dy-Ga hydride powders can also decrease the content of oxygen in the magnets, and thus improve the magnetic properties. Besides, Dy-Ga hydride powders are finer to wrap the main phases well so that the diffusion of Dy is convenient and the density of the magnet is improved. However, after an excess addition of Dy-Ga hydride powders, the desorbed hydrogen will react with main phases and lower the orientation of the magnets, thus deteriorate the magnetic properties. Besides, the uniform diffusion of Dy in the magnets is another reason that why the magnetic properties are decreased greatly.