Coercivity saturation in Nd-Fe-B sintered magnets treated by grain boundary diffusion process of Tb-Ni-Al alloy

• Published in: Journal of magnetism and magnetic materials Vol. 591; p. 171698
• Main Authors: Itakura, Masaru, Kontani, Hiroki, Akamine, Hiroshi, Fujimura, Kazumasa, Iriyama, Takahiko
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.02.2024
• Subjects: Microstructure; Permanent magnets; Scanning electron microscopy; Scanning transmission electron microscopy; Scanning electron microscopy; Permanent magnets; Microstructure; Scanning transmission electron microscopy
• ISSN: 0304-8853
• DOI: 10.1016/j.jmmm.2023.171698

•Tb grain boundary diffusion-treated Nd-Fe-B magnets show coercivity saturation.•Lattice diffusion forms thin and recrystallization forms thick Tb-rich shells.•Thin Tb-rich shells increase coercivity and thick Tb-rich shells waste Tb.•To use Tb effectively, formation of thick Tb-rich shells must be prevented. Microstructures of Nd-Fe-B sintered magnets treated by a grain boundary diffusion (GBD) process with different Tb coating amounts are investigated by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). Tb-substituted shell structures are formed in the outer portion of all Nd2Fe14B grains in the magnets. However, thick Tb-rich shells are formed on a micrometer scale on Nd2Fe14B grains in the region up to about 0.5 mm from the coating surface of the magnet, and thin Tb-rich shells several tens of nanometers in size are also formed in the inner region. As the Tb coating amount increases, the Tb concentration and thickness of the thick Tb-rich shells increase. At the same time, the Nd replaced by Tb forms Nd-rich grain boundary (GB) phases, and a large amount of those Nd-rich GB phases are discharged to the coating surface. The coercivity enhancement in GBD is mostly due to the formation of the thin Tb-rich shells, whereas the formation of thick Tb-rich shells wastes a large amount of Tb, causing coercivity saturation. Therefore, it is important to suppress the formation of thick Tb-rich shells and form thin Tb-rich shells with a high Tb concentration uniformly throughout the magnets.