Tailoring the Diffusion Depth and Microstructure of Nd-Fe-B Magnets by Low-Pressure Grain Boundary Diffusion Process

A novel grain boundary diffusion process with low-pressure technology was employed to develop Nd-Fe-B sintered magnet samples as thick as 2mm. The uniform and successive Nd-rich phase in the low-pressure diffused magnets promotes Tb atoms to permeate inside the magnets, and the diffusion depth increases from 390 μm to 880 μm. Diffusion coefficients ( D ) of vacuum grain boundary diffusion process (VGBDP) and low-pressure grain boundary diffusion process (LPGBDP) were acquired by computation, rising from 2.90 × 10 -13 m 2 /s to 2.95 × 10 -12 m 2 /s, respectively, revealing that the diffusion coefficient of LPGBDP is an order of magnitude higher than VGBDP. The increasing magnetocrystalline anisotropic field (122.2kOe) and the optimized microstructure jointly determine the improvement of coercivity, which increases significantly from 17.47kOe to 27.73kOe by low-pressure thermal diffusion treatment, 58.7% higher than the original magnet. Meanwhile, the distribution of homogeneous grain boundary phase and the embellishment of Tb for the grain phase were mainly responsible for the excellent temperature stability in the range of 20°C-200°C. This work proposes a viable method for preparing the RE-Fe-B magnets with slight heavy rare elements and high comprehensive performance