Nd-Fe-B magnets containing no heavy rare earth (HRE) elements exhibit insufficient coercivity to withstand the demagnetization field at elevated temperatures. The grain boundary diffusion (GBD) process provides the best route to fabricate high-coercive Nd-Fe-B magnets with low consumption of expensive HRE resources. Here we give a special review on the grain boundary diffusion sources and their coating methods. Up to now, various types of grain boundary sources have been developed, starting from the earliest Tb or Dy metal. The HRE-M eutectic alloys were firstly proposed for reducing the cost of the diffusion source. After that, the diffusion sources based on light rare earth and even non rare earth elements have also been proposed, leading to new understanding of GBD. Now, the diffusion sources including inorganic compounds, metals, and alloys have been employed in the industry. At the same time, to coat the diffusion source on the magnets before diffusion treatment, various methods have been developed. Different from the previous review articles for GBD, this review gives an introduction of typical types of diffusion sources and their fabrication approaches. The effects of diffusion source on the microstructure and magnetic properties are summarized briefly. In particular, the principles and applicability of different coating approaches were discussed in detail. It is believed that this review can provide a technical guidance for the industry for designing the diffusion process and products meeting specific requirements.

Ce-containing Nd–Fe–B magnets have been widely used because of their low cost, but the effects of Ce content in the magnets on the grain boundary diffusion (GBD) behavior are still unclear. In this work, Pr–Nd–Ce–Fe–B sintered magnets with various Ce contents were treated by using a Tb–Cu GBD process. The effects of Ce content on the diffusion behavior of Tb and the microstructure evolution of the magnets were systematically investigated. The results indicate that the coercivity of magnets with Ce/TRE (TRE: total rare earth) ratios of 0, 4, 10, 25, and 35 wt% was increased by 68%, 81%, 79%, 42% and 21%, respectively, after Tb–Cu diffusion. The coercivity enhancement increases firstly with the increasing Ce/TRE ratio from 0 to 10 wt% then decreases with the further increase of Ce content in the magnet. For the magnets with low Ce content, an increase of Ce content lowers the melting point of the RE-rich GB phase, which is beneficial to the diffusion of Tb. In the magnets with high Ce contents, a large amount of REFe₂ phase formed in GBs, which acts as an obstacle to Tb diffusion. The formation of the REFe₂ phase not only blocks the liquid diffusion channel but also consumes Tb. Meanwhile, Tb, Nd, and Pr elements can enter into the REFe₂ phase during the GBD treatment, leading to the transition of the REFe₂ phase from paramagnetic to ferromagnetic, which is not beneficial to magnetic isolation and coercivity enhancement. This study thus provides a deep understanding of the role of Ce in the diffusion of HRE, which could provide a reference for the industrial development of the GBD process.