Microstructural and compositional characterization of terbium-doped Nd–Fe–B sintered magnets

Anisotropic sintered magnets based on the Nd2Fe14B phase doped with Tb were prepared using a grain-boundary diffusion process (GBDP) in order to enhance their coercivity. A FEGSEM microstructural analysis revealed that these GBDP magnets had a core-shell structure, where thin, Tb-rich, (NdTb)2Fe14B shells are formed on the original matrix Nd2Fe14B grains after diffusion of the Tb. This shell thickness varies from a few tens of nanometres in the middle of the magnet up to a few micrometers near the edge. The exact chemical composition of these shells was determined using EDS and WDS electron-probe microanalyses, which were modified and optimized for submicrometer scale analyses. When analyzing the common Nd–Lα, Tb–Lα and Fe–Kα lines a mutual multiple overlap in the EDS spectra is present and, as a result, an accurate quantitative analysis was only feasible when using WDS. Using this technique we were able to achieve a lateral analytical resolution of 0.4 μm. A further improvement in resolution, down to 0.15 μm, was realized with a dedicated set-up using low-voltage EDS, analyzing the “atypical” low-energy Nd–Mα, Tb–Mα and Fe–Lα lines. Quantitative analyses confirmed that the reaction phase (NdxTb1 − x)2Fe14B is formed after the diffusion of Tb with the equilibrium concentration of Tb being equal to x ≈ 0.5, i.e., with the atomic ratio of Nd/Tb equal to 1/1. We also found that a relatively sharp Tb concentration gradient from the shell to the core occurs within a length of ≈ 0.5 μm, while the Fe concentration remains unchanged. In terms of magnetic properties, the Tb-doping significantly increased coercivity by ≈ 30% while the remanence remained at the same value as in the undoped Nd–Fe–B.