Microstructure and magnetic properties of NdFeB magnets fabricated by spark plasma sintering using melt-spun powder

Preserving the fine grain structure of melt-spun neodymium-iron-boron (NdFeB) powder during magnet manufacturing is critical to achieving optimal magnetic properties in the final magnets. Spark plasma sintering (SPS) is an advanced powder metallurgy technology that can rapidly heat and densify the material thus preventing excessive grain growth, which is a promising technique for NdFeB magnet manufacturing. In this study, crushed melt-spun powder (Dy-free) was utilized to fabricate NdFeB magnets by SPS at different temperatures (650, 750, and 850 °C). The produced magnets show good coercivities in the range of 1641–1368 kA/m, which are higher or lower than that of the initial powder (1437 kA/m) depending on the sintering temperature. The microstructure of the magnets was characterized by optical microscopy, scanning electron microscopy, transmission electron microscopy, and scanning transmission electron microscopy. Blocky amorphous Nd-rich regions, which have not been reported before, are observed in the SPSed NdFeB magnet. The formation mechanism is explained considering the special sintering conditions during SPS and the microstructural features of the initial powder. The controversial formation mechanism of the fine and coarse two-zone structure frequently observed in SPSed NdFeB magnets is discussed and it is demonstrated that the growth of the initial grains (with difference in size) of the ribbon has caused the formation of the two-zone structure, not the temperature gradient between the particle-particle boundary and the particle interior as suggested by some authors. Lorentz transmission electron microscopy was employed to image the domain walls in the fine magnetic grains, which showed both stripe-like and maze-like domain walls in the grains, indicating isotropic nature of the formed magnets. The initial magnetization curve and the coercivity mechanism were correlated to the microstructure obtained.