Coercivity enhancement of NdCeFeB magnet by two-step grain boundary diffusion with Pr70Cu10Al15Ga5 and DyH

Nd-Ce-Fe-B sintered magnets with Ce-substitution amounts of 30 wt% were treated by one- and two-step grain boundary diffusion processes, in which DyHx and low-melting-point Pr70Cu10Al15Ga5 (at%) alloys were designed as the diffusion sources and diffused into the magnets according to the different steps and orders. Results showed that the increase of coercivity was only 2.7 kOe when introducing heavy rare-earth Dy by the one-step diffusion of DyHx, reflecting the difficulty of conventional diffusion modification on high Ce content magnets. Note that by the two-step grain boundary diffusion (no matter which diffusion source was diffused preferentially in the first step), the coercivities were significantly enhanced over 18.5 kOe from the original 13.64 kOe, with the obvious improvements of thermal stability. Microstructures and compositions analyses revealed that the grain boundaries were greatly optimized after the diffusion of Pr70Cu10Al15Ga5, supporting favorable diffusion channels for the introduction of Dy in the subsequent second step. By contrast when introducing Dy first and then diffusing the Pr70Cu10Al15Ga5 second, the elements in the low-melting-point alloys could drive the superficial Dy to migrate into the deep interior of the magnets. Thus, both the two types of two-step grain boundary diffusions were conductive to improving the utilization of heavy rare-earth Dy in high Ce content magnets. Besides, dynamic domain evolution analyses demonstrated the positive effect of diffusion on anti-demagnetization. This work provided a foundation for the development of high Ce content magnets with high performances.