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

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Abstract
The microstructures of the grain boundaries in an Fe77Nd15B8 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 Fe14Nd2B grains in both samples. In the quenched sample, the interfaces between the bcc phase and the Fe14Nd2B 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 Fe14Nd2B 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 Fe14Nd2B grains become very sharp and distinct. It is concluded that the morphology of the bcc phase, particularly the microstructure of its interface with Fe14Nd2B grains, controls the coercivity mechanism of Fe77Nd15B2.