Microstructure of hot-pressed and die-upset NdFeB magnets

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Abstract
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.