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

摘要

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/Dy $_{2}$O $_{3}$ composite magnets have been fabricated by hot-pressing and die-upset. The influence of Dy $_{2}$O $_{3}$ 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/Dy$_{2}$ O$_{3}$ composite magnets increased with the increase of Dy $_{2}$O $_{3}$ amount and reached 1270 $mu Omega$ cm when the Dy$_{2}$O$_{3}$ amount increased to 20 wt.%. Whereas the electrical resistivity of the nondoped Nd-Fe-B magnets is 230 $mu Omega$ cm. The cross-section SEM images of the composite magnets showed that the Dy$_{2}$ O$_{3}$ phase formed laminated structure and some agglomerate phenomenon. The agglomerate areas became larger with the in- rease of Dy $_{2}$O $_{3}$ amount. The residual magnetic flux density $({rm B}_{rm r})$ and maximum energy product of the Nd-Fe-B/Dy $_{2}$O $_{3}$ composite magnets decreased due to the nonmagnetic Dy$_{2}$ O$_{3}$ filler. It is interesting to find that coercivity of the Nd-Fe-B/Dy$_{2}$O $_{3}$ composite magnets increased from 7.8 kOe to 9.64 kOe when the Dy$_{2}$ O$_{3}$ 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)$_{2}$Fe$_{14}$B compounds. In the mean time, some of neodymium-rich phase diffused to Dy $_{2}$O $_{3}$ phase, replaced dysprosium and formed Nd$_{2}$O$_{3}$. This probably happened during the die-upset process at 850 $^{circ}$C.