Structure and chemical compositions of the grain boundary phase in Nd-Fe-B sintered magnets

Abstract

We revisited the microstructure of a commercial Nd-Fe-B sintered magnet using scanning electron microscopy (SEM) and aberration corrected scanning transmission electron microscopy (STEM) in order to clarify the microstructure feature that is relevant to the coercivity. Two types of thin Nd-rich grain boundary (GB) phases were found: one is crystalline and the other is amorphous. The crystalline Nd-rich GB phase form at the flat surface of c-plane of the Nd2Fe14B grains, which contains a higher amount of Nd compared to the amorphous GB phase that form on the grain boundaries nearly perpendicular to the c-planes. This indicates that the intergranular exchange coupling in sintered Nd-Fe-B magnets is anisotropic. Correlative SEM and STEM analysis of triple junction Nd-rich phases revealed five types of Nd-rich phases, i.e., fcc-NdO, hcp-Nd2O3, NdFe4B4, metallic Nd-rich phases with the fcc structure and the Ia3¯ structure. The metallic Nd-rich phases form at the sharp edges of Nd-rich oxide and boride grains, from which thin Nd-rich phases is infiltrated along grain boundaries to form the thin GB phase. Misaligned Nd2Fe14B grains are often observed in direct contact with the Nd2O3 grains.