Tb diffusion induced microstructural evolution and magnetic responses of multi-main-phase Nd-Dy-Fe-Ga-B magnet

Grain-boundary diffusion process (GBDP) emerges as a crucial technology of high-coercivity Nd-Fe-B permanent magnets for the efficient utilization of heavy rare-earths (HREs) Tb and Dy. However, besides the formation of preferred Tb/Dy-rich shell surrounding Tb/Dy-lean core (shell/core structure), the anti-shell/core structure also occurs and causes excessive Tb/Dy consumption. In present work, to gain a deeper verification on the evolution of shell/core and anti-shell/core structure, the multi-main-phase (MMP) Nd-Dy-Fe-Ga-B base magnets were subjected to Tb diffusion over a wide range of duration from 0 h to 36 h. The time-dependent formation of shell/core and anti-shell/core structure with corresponding magnetic responses was unveiled. Before the depletion of deposited Tb film (diffusion time <12 h), the depth of Tb diffusion increases along with prolonged diffusion time. The magnetic hardening of Tb-rich shell/core main phase grains and exchange decoupling of adjacent grains by RE 6 Fe 13 Ga intergranular phase jointly contribute to continual coercivity increment from 16.9 to 27.5 kOe with slightly reduced remanence. After the depletion of deposited Tb film (diffusion time ≥12 h), excessive duration yields the opposite Tb diffusion trend from Tb-rich grains to Tb-lean GB with the formation of anti-shell/core structured grains in near-surface region, resulting in the coercivity of 28.6 kOe with drastically decreased remanence of 12.8 kG in 36 h Tb diffused magnet. These magnetic responses with the microstructural features can be reproduced by the micromagnetic simulation. Above findings offer more understandings on the anti-shell/core structure and also provide new insights into further improving the utilization efficiency of HREs.