Microstructure of hot-pressed and die-upset NdFeB magnets

Transmission electron microscopy was used to establish densification, alignment, and magnetization reversal mechanisms in hot-pressed and die-upset NdFeB magnets. Microstructures of these materials reveal two principal phases: Nd2Fe14B grains and a grain boundary phase of approximate composition Nd7Fe3. The grains in the hot-pressed material are polygonal and isotropic, while those in the die-upset material are flat platelets. The alignment of die-upset magnets arises exclusively from the fact that hot deformation produces platelets of Nd2Fe14B grains with their c axes parallel to the stress axis. It is argued that yielding plays an important role in the densification of the hot-pressed samples, and diffusion slip is critical to the alignment. The grain boundary phase appears as a crystalline or noncrystalline phase in the hot-pressed samples but has an fcc structure in the die-upset samples. Magnetic domain walls are shown to be pinned at the grain boundary phase in both cases. A Nd-rich spongy phase is observed to form at the boundaries where two ribbons fuse, but has no significance for the magnetic properties.