High coercivity enhancement in Nd-Fe-B sintered magnets via two-step grain boundary diffusion process with constructed diffusion channels

The grain boundary diffusion process (GBDP) effectively enhances coercivity in sintered Nd-Fe-B magnets; however, inadequate diffusion pathways in low-coercivity commercial magnets severely hinder elemental diffusion. In this work, we propose a two-step GBDP strategy utilizing a Pr 68 Cu 32 alloy followed by a Pr 55 Tb 15 Cu 15 Ga 15 alloy to overcome this fundamental limitation. The Pr 68 Cu 32 alloy diffusion effectively reconstructs continuous and widened grain boundaries, thereby enabling remarkable Tb diffusion depth and uniformity in the subsequent Tb diffusion. As a result, the coercivity is significantly enhanced from 883.56 kA/m to 2040.15 kA/m, with excellent thermal stability ( β =−0.488 %/K). Crucially, the method remains highly effective in thick magnets, with coercivity increases of 921.77 kA/m, 804.76 kA/m, and 710.83 kA/m achieved in 8 mm, 10 mm, and 15 mm magnets, respectively. Microstructural and elemental analyses confirm a substantially improved Tb diffusion coefficient and the formation of a more uniform Tb-rich shell with higher Tb concentration in the two-step GBDP magnet. This work elucidates the mechanism of the two-step GBDP and provides a strategy for fabricating Nd-Fe-B magnets with substantially enhanced coercivity.