A Study of Microstructures of Grain Boundaries in Sintered Fe₇₇Nd₁₅B₈ Permanent Magnet by High-Resolution Electron Microscopy

Abstract

The microstructures of the grain boundaries in an Fe 77 Nd 15 B 8 permanent magnet have been examined by high-resolution electron microscopy, in samples quenched from the sintering temperature of 1350 K and in samples quenched and then annealed at 870 K. The coercivities of the quenched and the quenched-and-annealed samples were 500 and 1000 kA/m, respectively. Thin layers of a bcc phase were observed over the surfaces of most of the Fe 14 Nd 2 B grains in both samples. In the quenched sample, the interfaces between the bcc phase and the Fe 14 Nd 2 B grains were not so distinct, and many thin platelets of the bcc phase were found to extend from the interfaces to the inside of the Fe 14 Nd 2 B grains of which the surrounding lattice is compressed locally. In the annealed sample, the platelets disappear completely and the interfaces between the bcc phase and the Fe 14 Nd 2 B grains become very sharp and distinct. It is concluded that the morphology of the bcc phase, particularly the microstructure of its interface with Fe 14 Nd 2 B grains, controls the coercivity mechanism of Fe 77 Nd 15 B 2 .