Tailoring the diffusion depth and microstructure of RE-Fe-B magnets by low-pressure grain boundary diffusion process

Abstract

A novel grain boundary diffusion process (GBDP) with low-pressure technology was employed to develop RE-Fe-B sintered magnets with a thickness of 2 mm. The gas pressure promotes Tb atoms to permeate the deeper of the magnets due to the uniform and continuous RE-rich phase, increasing the diffusion depth from 345 µm to 880 µm. According to the results acquired by computation, diffusion coefficients (D) of vacuum grain boundary diffusion process (VGBDP) and low-pressure grain boundary diffusion process (LPGBDP) rose from 2.27 × 10−13 m2/s to 2.95 × 10−12 m2/s, respectively. The increasing magnetocrystalline anisotropic field (122.2 kOe) and the optimized microstructure jointly determine the improvement of coercivity, which increases significantly from 17.47 kOe to 27.73 kOe by low-pressure diffusion treatment, 58.7% higher than the original magnet. Meanwhile, the homogeneous grain boundary phase and the thicker Tb-shell were mainly responsible for the excellent temperature stability in 20°C-200°C. This work proposes a viable method for preparing the RE-Fe-B magnets with slight HREE and high comprehensive performance.