Effect of intergranular phase proportion in sintered Nd-Fe-B magnets on subsequent grain boundary diffusion using Tb-Cu alloy

Magnetic properties and microstructures of sintered Nd-Fe-B magnets with different Nd-Cu powders addition (0, 2.7, 4.5, 6.2 and 8 wt.%) and their corresponding diffused magnets using Tb-Cu alloys were studied systematically. The results showed that the enhancement of coercivity after diffusion was positively correlated with the intergranular phase proportion of the base substrates, and the relatively larger coercivity increments with 8.69 and 9.16 kOe were obtained at the addition of 6.2 and 8 wt.% respectively, while the coercivity increments only reached 3.54 kOe and 4.00 kOe for the diffused magnets prepared by the substrates with 0 and 2.7 wt.% Nd-Cu powders addition. The further microstructure analyses demonstrated that the higher coercivities were mainly attributed to the rather deeper diffusion depths of Tb elements, and the Nd-rich grain boundary phases in the base substrates were beneficial to construct the effective diffusion channels for subsequent diffusion process. Meanwhile, just due to the lack of efficient diffusion channels in the substrates with lower intergranular phase proportion, a large amount of Tb atoms accumulated on the interface between diffusion source and base magnet, forming a Tb-Fe transition layer and significantly deteriorating the utilization of heavy rare earths. This work supplemented insight and experimental foundation for the base substrate design in the preparation of diffused magnet with high performance. • Diffusion effect for Tb elements in different magnets were studied systematically. • Coercivity enhancement of 9.1 kOe was achieved by constructing diffusion channels. • The element metallurgical behavior in the diffusion couple interface was analyzed. • The in-situ observation for the intergranular phase melting process was performed.