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

• Published in: Materials characterization 2012-05, Vol.67 (Complete), p.27-33
• Main Authors: Samardžija, Zoran, McGuiness, Paul, Soderžnik, Marko, Kobe, Spomenka, Sagawa, Masato
• Format: Article
• Language: English
• Subjects: ANISOTROPY; Applied sciences; BORON ALLOYS; CHEMICAL COMPOSITION; COERCIVE FORCE; Cross-disciplinary physics: materials science; rheology; DIFFUSION; DOPED MATERIALS; EDS; ELECTRON MICROPROBE ANALYSIS; Exact sciences and technology; GRAIN BOUNDARIES; IRON ALLOYS; MAGNETIC MATERIALS; MAGNETIC PROPERTIES; MATERIALS SCIENCE; MATRIX MATERIALS; Metals. Metallurgy; Microanalysis; Microstructure; Nd–Fe–B magnets; NEODYMIUM ALLOYS; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Physics; Powder metallurgy. Composite materials; Production techniques; Sintered metals and alloys. Pseudo alloys. Cermets; Solidification; TERBIUM; WDS; X-RAY SPECTRA
• ISSN: 1044-5803; 1873-4189
• DOI: 10.1016/j.matchar.2012.02.017

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 0.4μm. A further improvement in resolution, down to 0.15μm, 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 ≈0.5μm, 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. ► Nd–Fe–B sintered magnets were doped with Tb using grain-boundary diffusion process. ► A tiny core-shell reaction phase was formed around the Nd2Fe14B matrix grains. ► EDS and WDS analyses confirmed (Nd0.5Tb0.5)2Fe14B equilibrium shell composition. ► Coercivity of Tb-doped Nd–Fe–B increases by 30% without a drop in remanence.