Evolution of Tb-shell homogeneity in Nd-Fe-B sintered magnets by long-term Tb grain boundary diffusion

Grain boundary diffusion process has been applied to effectively facilitate the coercivity enhancement of Nd-Fe-B magnets by the formation of core-shell structure with less heavy rare earth consumption. However, the excessively thick Tb-rich shells and asymmetric shells formed near the shallow diffused surface, not only have minor contribution to the coercivity enhancement but consume more Tb resources. In this work, the influences of diffusion time on coercivity changes and the evolution of asymmetric to symmetric Tb-rich shell have been investigated for the Tb-diffused Nd-Fe-B magnets. With prolonging the diffusion time at 890 ℃, the coercivity rapidly increases from 17.66 kOe to 24.41 kOe when the diffusion time is less than 18 h and keeps nearly unchanged after a longer diffusion time due to the trade-off effect of Tb-rich shell homogeneity and grain growth. Besides, magnetic domain structure and its dynamic evolution of different core-shell structures in Nd-Fe-B magnet further illustrate the positive effects of homogeneous symmetric Tb-rich shell on coercivity enhancement, which is directly observed by in-situ Lorentz TEM and magneto-optical Kerr microscope, and further proved by micromagnetic simulation. This work thus provides a new insight for tuning the Tb-rich shell distribution in the Nd-Fe-B sintered magnets with high performance by grain boundary diffusion.