Transmission electron microscopy study on Nd-rich phase and grain boundary structure of Nd-Fe-B sintered magnets

The crystal structure and morphology of the Nd-rich phase in postannealed sintered Nd-Fe-B hard magnets were investigated by transmission electron microscopy (TEM). The Nd-rich phase at grain boundaries thinner than about 2 nm was found to be amorphous; as the boundary region broadened, it assumed a face-centered cubic (fcc) structure with a≈0.54 nm; eventually, the selected area diffraction pattern exhibited systematically allowed superlattice spots, revealing that the Nd-rich phase belongs to the space group Ia 3- with a≈1.1 nm. The elemental analysis of the Nd-rich phase showed that it contains a considerable amount of oxygen. These findings indicate that the Nd-rich phase with a≈1.1 nm is in fact isostructural to cubic Nd2 O3 (Mn2 O3 structure). We also found Nd-rich fcc precipitates (a≈0.54 nm) of several ten to hundred nm inside the Nd2 Fe14 B grains. Both intergranular and intragrain Nd-rich phases possess simple orientation relationships with the matrix Nd2 Fe14 B phase, such as (111)Nd2Fe14B ∥ (111)Nd-rich [2- 11]Nd2Fe14B ∥ [2- 11]Nd-rich and (010)Nd2Fe14B ∥ (111)Nd-rich [001]Nd2Fe14B ∥ [1- 10]Nd-rich. Our Lorentz TEM observation suggested that the intragrain Nd-rich precipitates frequently disrupt magnetic domain wall arrangements, and that the magnetic interactions across thin amorphous Nd-rich phase are dominated by a simple dipole interaction.