Unusual surface microstructural evolution of Nd–Ce–Fe–B sintered magnets by (Nd, Pr)Hx grain boundary diffusion

Abstract

Usually the grain boundary diffusion process (GBDP) provides a facile approach toward high-coercivity Nd–Fe–B magnets by modulating the surface microstructure, i.e. constructing nonferromagnetic grain boundary layers and forming magnetically hardening shells of 2:14:1 main phase. However, by applying (Nd, Pr)H x GBDP to the 25 wt% Ce-substituted Nd–Ce–Fe–B sintered magnet, it is surprisingly discovered the ultimate coercivity below 12.4 kOe over a wide range of diffusion times. The low coercivity increment level of 1.5 kOe after an optimal 10 h diffusion is restrained by the unusual surface layer, i.e. an apparent Ce segregation at the surface region to form REFe 2 (RE = rare earth) intergranular phase at both triple junctions and grain boundaries, rather than the expected Nd/Pr-rich enrichment at the RE 2 Fe 14 B/RE-rich interface or at the GBs. Prolonging diffusion time from 6 to 20 h exacerbates the formation of REFe 2 phase and witnesses a more pronounced surface layer with a limited diffusion depth of ~80 μm, which is much lower than the reported GBDP Nd–Fe–B (millimeters) or Nd–La–Ce–Fe–B (hundreds of micrometers). The limited diffusion depth may be correlated to the blocked diffusion channel by the massive REFe 2 intergranular phase that replaces the conventional RE-rich phase. Above findings demonstrate that the characteristic REFe 2 phase highly affects the surface microstructural evolution of GBDP Nd–Ce–Fe–B magnet, and highlight future work toward sophisticated engineering of REFe 2 phase. • (Nd, Pr)H x GBDP yields unusual surface microstructure evolution of Nd–Ce–Fe–B sintered magnets. • REFe 2 TJ and GB phase forms preferentially at the diffusion surface. • Massive Ce segregates at surface region, rather than the expected Nd/Pr-rich intergranular phase. • Long-time diffusion of 20 h still restrains the diffusion depth to be below 100 μm. • The coercivity increment level of 1.5 kOe is much lower than other reported GBDP magnets.