Microstructure of Fe-Nd-B permanant magnet and magnetic coercivity

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The microstructures of the grain boundaries in an Fe_<77>Nd_<15>B_8Permanent 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_2B grains in both samples. In the quenched sample, the interfaces between the bcc phase and the Fe_<14>Nd_2B 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_2B 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_2B 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_2B grains, controls the coercivity mechanism of Fe_<77>Nd_<15>B_2.Interface microstructures between rhombohedral Sm_2Co_<17>and hexagonal SmCo_5phases in a sintered permanent magnet of Sm(Co_<0.675>Fe_<0.175>Ni_<0.05>Cu_<0.1>)_7have been investigated by edge-on observations with the many-beam imaging technique. In the specimen annealed at 800゜C for 4h which shows a maximum magnetic coercivity, the interface boundaries were coherent accompanying the lattice distortion;the habit planes were parallel to the (101^^-1)_<2:17>and (112^^-1)_<1:5>planes with the orientation relationship of(0001)_<2:17>//(0001)_<1:5>and (101^^-0)_<2:17>//(112^^-0)_<1:5>. In the specimen annealed for 150 h which shows a lower coercivity, the interface boundaries were characterized with regular alignments of interfacial edge dislocatios. The observed interface structures were discussed in connection with the change in magnetic properties.