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

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)Hx 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 REFe2 (RE = rare earth) intergranular phase at both triple junctions and grain boundaries, rather than the expected Nd/Pr-rich enrichment at the RE2Fe14B/RE-rich interface or at the GBs. Prolonging diffusion time from 6 to 20 h exacerbates the formation of REFe2 phase and witnesses a more pronounced surface layer with a limited diffusion depth of ~80 mu m, 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 REFe2 intergranular phase that replaces the conventional RE-rich phase. Above findings demonstrate that the characteristic REFe2 phase highly affects the surface microstructural evolution of GBDP Nd-Ce-Fe-B magnet, and highlight future work toward sophisticated engineering of REFe2 phase.