The formation mechanism of the nanocrystalline cells in Sm2Fe17Nx powder during overnitridation

It is known that an overnitridation process significantly improves the coercivity of coarse Sm-(Fe,M)-N magnet powder (M = Cr, Mn, Ti, V) and also introduces a unique microstructure in the particles wherein nanocrystalline cells are separated by amorphous walls (=frequently called nano-cell like structure). It has however been reported that those nanocrystalline cells rotate spontaneously during overnitridation and thus the final powder product becomes unfortunately isotropic. The phenomena have widely been known for more than two decades but the formation/rotation mechanism of the nano-cell like structure is not well understood yet. In this study, the evolutions of the magnetic properties and the microstructure during overnitridation were investigated in greater detail. The multi-scale microstructural observation technique that combines the recent electron microscopic machines disclosed what was occurring in the particles during overnitridation; it was found that the local nitrogen content and the microstructure in a particle are highly heterogeneous and the spontaneous rotation, surprisingly, do not occur.

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