Microstructural and magnetic property evolutions with diffusion time in TbHx diffusion processed Nd-Fe-B sintered magnets

Abstract In this article, evaluations of the microstructure and HRE distribution of grain boundary diffused (GBD) Nd-Fe-B sintered magnets with diffusion time (t) and diffusion depth (d) were investigated. The coercivity (Hcj) first increases and then decreases with increasing t; while the remanence (Br) and the maximun energy product ((BH)m) of GBD magnets have an opposite changing trend. A large amount of Tb diffused into the inner of grains, and formed extensive Tb-rich areas in the surface layer at the beginning of GBD process. This suggests that Tb diffuses not only along grain boundaries, but also via grains in the surface layer because of high concentration gradient of Tb. As the concentration gradient of Tb is reduced, Tb diffuses mainly along grain boundaries. The grain boundary acts as the GBD path, but not as a Tb enriched area. There are two coexisting processes, namely, one process is diffusing from the surface to the inside of the magnet, the other is diffusing from the grain boundaries to the shell, then to the inner of grains. The two diffusion processes cause the changes of microstructure and Tb distribution, thus, the magnetic properties of the GBD samples vary correspondingly. PACS: 75.20.En; 75.50.Bb; 75.50.Kj; 75.50.Ww