Microstructure and Properties of Die-Upset Nd-Fe-B/Dy $_{2}$ O $_{3}$ Composite Magnets

Fabrication of rare-earth-based permanent magnets with both high electrical resistivity and excellent magnetic properties for the traction motor applications is highly challenging. In this paper, Nd-Fe-B/Dy₂O₃ composite magnets have been fabricated by hot-pressing and die-upset. The influence of Dy₂O₃ filler on the microstructures, electrical resistivity and magnetic properties of the Nd-Fe-B magnets have been investigated by scanning electron microscope, four-probe resistivity meter and hysteresis graph. The results showed that the electrical resistivities of the Nd-Fe-B/Dy₂O₃ composite magnets increased with the increase of Dy₂O₃ amount and reached 1270 cm when the Dy₂O₃ amount increased to 20 wt.%. Whereas the electrical resistivity of the nondoped Nd-Fe-B magnets is 230 cm. The cross-section SEM images of the composite magnets showed that the Dy O phase formed laminated structure and some agglomerate phenomenon. The agglomerate areas became larger with the increase of Dy₂O₃ amount. The residual magnetic flux density and maximum energy product of the Nd-Fe-B/Dy₂O₃ composite magnets decreased due to the nonmagnetic Dy₂O₃ filler. It is interesting to find that coercivity of the Nd-Fe-B/Dy₂O₃ composite magnets increased from 7.8 kOe to 9.64 kOe when the Dy O amount increased from 10 wt.% to 20 wt.%. The reason may be that some of the dysprosium diffused into the Nd-Fe-B matrix and formed (Nd,Dy)₂Fe₁₄B compounds. In the mean time, some of neodymium-rich phase diffused to Dy₂O₃ phase, replaced dysprosium and formed Nd O . This probably happened during the die-upset process at 850°C.