High Resolution Electron Microscopy of Grain Boundaries in Sintered Fe₇₇Nd₁₅B₈ Permanent Magnets

Grain boundaries of sintered Fe 77 Nd 15 B 8 permanent magnets with different heat-treatments have been examined by high resolution electron microcopy. A b.c.c. phase with the lattice constant of a=0.29 nm was found at most of the boundaries of Fe 14 Nd 2 B grains. The b.c.c. phase and Fe 14 Nd 2 B grains are joined with smooth interfaces in the samples annealed at 870 K, which have a coercivity as high as 1000 KA/m. In samples quenched from 1350 K with a coercivity of about 500 KA/m, however, thin plates of the b.c.c. phase extend from the interfaces to the inside of the Fe 14 Nd 2 B grains, and deform the lattice spacings of (001) planes of the Fe 14 Nd 2 B matrix. These thin plates are considered to act as nucleation centers of reverse magnetic domains which reduce the coercivity of the specimen.