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

Abstract

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 2nm was found to be amorphous; as the boundary region broadened, it assumed a face-centered cubic (fcc) structure with a≈0.54nm; eventually, the selected area diffraction pattern exhibited systematically allowed superlattice spots, revealing that the Nd-rich phase belongs to the space group Ia3¯ with a≈1.1nm. 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≈1.1nm is in fact isostructural to cubic Nd2O3 (Mn2O3 structure). We also found Nd-rich fcc precipitates (a≈0.54nm) of several ten to hundred nm inside the Nd2Fe14B grains. Both intergranular and intragrain Nd-rich phases possess simple orientation relationships with the matrix Nd2Fe14B 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.