Correlation between Ce content and Tb–Cu grain boundary diffusion efficiency in Ce-containing Nd–Fe–B magnets

Abstract

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.