Recently, high coercivity Nd-Fe-B permanent magnets have become an indispensable material for the traction motor of hybrid electric vehicles (HEVs) or electric vehicles (EVs). For the high coercivity, heavy rare-earth (HRE) elements such as Dy or Tb are substituted for Nd in Nd-Fe-B magnets. However, the content of HRE elements in Nd-Fe-B magnets needs to be reduced due to their uneven distribution in the earth crust and low output, causing their prices to soar. Developing high coercivity Nd-Fe-B permanent magnets without HRE elements have been drawing a great attention. Actually, the low coercivity of HRE-free Nd?Fe?B magnets can be attributed to insufficient microstructure control, such as grain size and grain boundary. It is reported that grains of a single magnetic domain size(∼300 nm), along with grain boundary phase causing sufficient magnetic decoupling among the grains, are the most desirable microstructure to achieve high coercivity Nd?Fe?B magnets without HRE elements.
In the this study, the ultra-fine grained Nd-Fe-B magnetic material was fabricated by HDDR and hot-deformation process and carried out the grain boundary diffusion process (GBDP) in order to enhance the coercivity by control grain boundary phase without the heavy rare-earth elements.
In the HDDR process, microstructure of the master alloy was affect HDDR powders. Therefore, the master alloy powders must have a single crystalline and uniform distribution of Nd-rich phase for higher magnetic properties of HDDR powder. The higher Nd content and slow dehydrogenation speed could help continuous and uniform Nd-rich grain boundary phase, increasing coercivity of HDDR powders.
In the hot-deformation process, deformation temperature, strain, and strain rate are important parameter for outstanding magnetic properties of magnets. The remanence and coercivity tended to increase and decrease with increasing strain, respectively. At the strain of below 0.5, the remanence increased with decreasing the strain rate, but the tendency was gradually reversed with increasing the strain. This result caused by the prolonged deformation, which induce the squeeze-out of Nd-rich phase from grain boundaries and the formation of coarse Nd-rich phase.
In addition, the coercivity enhancement of Nd-Fe-B magnet by grain boundary diffusion process (GBDP) has been studied with new method. The mixed powders of NdHx and Cu were used as diffusion source instead of the Nd-Cu eutectic alloy powders in order to simplify the GBDP. In the GBDP, the amount of diffused Nd-Cu alloy increased with increasing heat treatment temperature. However, when the same amount of Nd-Cu is diffused, the coercivity increment of the heat-treated magnet at low temperature is higher than that of the heat-treated magnet at high temperature. It is considered that higher temperature can induce the grain growth and increase the
magnetization of the Nd-rich grain boundary phase. From comparison with recently reported results, which Nd-Cu eutectic alloy powder was employed, it is confirmed that GBDP with reaction of NdHx and Cu nanopowders is more efficient.