Effect of diffusion annealing on the hysteretic properties of sintered Nd-Fe-B magnets

Effect of diffusion annealing on the formation of dysprosium-saturated grain boundaries in sintered Nd-Fe-B magnets has been studied. As a source of diffusion, we used powders of metallic dysprosium, DyCu, DyGa, and Dy3Co alloys and DyH x hydride. Plates ∼1 mm thick made of sintered magnets were molded into powders with a particle size from 3 to 500 μm and subsequently annealed in a vacuum at 750–850°C for 0.5–10 h. After annealing, the microstructure and the elemental composition of grains and grain boundaries were studied by scanning electron microscopy. Under optimum annealing conditions, Dy atoms were shown to diffuse preferentially through grain boundaries of the magnets. The coercive force H c of Nd-Fe-B plates, which were annealed in contact with the DyGa, Dy3Co and DyH x powders and subsequently annealed at 550°C, increases by 4–6 Oe; no substantial decrease in the remanence takes place. The increase in the coercive force depends of the powder particle size and temperature and time of diffusion annealing.