Rare earth elements based permanent magnets are the strongest magnetic materials and have been applied in a range of electronics, energy devices, and traction motors, etc. It has been widely proved that the performance of Nd-Ce-Fe-B magnet is closely related with the microstructure of the material. In particular, the optimization of grain boundaries (GB) is vital for enhancing the magnetic properties of magnets.

This work systematically studies the thermally induced phase and microstructure evolution behavior of (Nd_0.75Ce_0.25)₂Fe₁₄B nanocrystalline sample ranging from RT to 750 °C and establish continued microstructure evolution behavior at different temperature. The reconstruction of GB and elemental diffusion behavior was investigated at the nanoscale. In-situ STEM resolves the rare earth elements and oxygen accumulate at GB area from 200 °C forming a uniform thin amorphous layer (~1 nm) between grains. The dissociation of the primary phase at 350 °C and crystalized as BCC-Fe, Fe₂B, and (Nd_0.75Ce_0.25)₂O₃ phases are driven by thermal activation at a higher temperature. Further, a non-oxygen-involved phase evolution mechanism is observed by ex-situ STEM, where atomic-thin rare earth elements form intergranular films that surrounded the metastable Fe₂₃B₆ phase. Three different Lanthanides enriched intergranular films are separated from the main phase and identified, which is (NdCe)Ox, (NdCe)₂Fe₂₁B₆, and pure (NdCe). This work provides the essential information of phase transformation and microstructure evolution of NdCeFeB-based magnets at high temperatures, and new insights into thermal treatment design and thermal stability considerations.

Byung Oh Jung

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