Microstructure and Magnetic Properties of Sintered Nd-Fe-B Magnets with Grain Boundary Diffusion Tb

Diffusing Tb element through the grain boundary diffusion process (GBDP) to improve the coercivity had gradually become the main production process of commercial high-performance sintered Nd-Fe-B. However, the optimum diffusion conditions and the distribution of the Tb element in the Tb-deposited magnets prepared by the GBDP with the magnetron sputtering method still need further investigation. Understanding the relationship between thermal treatment processes and the magnetic properties of the GBDP magnet will help us to optimize the GBDP and provide the reference basis for practical production. A commercial sintered Nd-Fe-B permanent magnet without Dy or Tb was used as the diffusion substrate. The initial (annealed) magnet was cut into small pieces along the orientation of the magnet. The thin film of Tb metal was deposited on the surface by the magnetron sputtering technology. Subsequently, Tb-deposited samples were exposed to heat treatment in a vacuum at 825~1025℃ for 2~10 h and then annealed at 500℃ for 2 h. The magnetic properties of the magnet were measured by high field hysteresis meter HyMPulse (Metis Instruments). Back scattered electron (BSE) images were obtained by JSM-7800F field emission scanning electron microscope (FE-SEM) equipped with energy dispersive X-ray spectroscopy (EDX) detector for the constituent analysis. The microstructure and magnetic properties of the magnet treated with different diffusion temperatures and time were compared. The higher the diffusion treatment temperature was, the deeper the Tb element could enter into the magnet. So that the coercivity enhanced gradually as the concentration of Tb element diffused into the magnet increased. The shell structure was formed on the surface of main phase grains. Energy dispersive spectrometer (EDS) analysis indicated that Tb element apparently gathered at the shell. That revealed Tb did not diffuse into the interior, but formed the (Nd, Tb)2Fe14B shell in the outer regions of the main phase grain. The thickness of the Tb-rich shell gradually became thinner with the increase of the distance from the magnet surface but gradually thickens with the increase of the diffusion temperature and the extension of time. The average atomic mass of the surface layer was higher than other regions in the diffusion samples. As the surface layer with a thickness of ~40 μm was polished from the diffusion magnet, the remanence gradually recovered to that of the initial magnet, while the coercivity was almost still constant. The squareness of demagnetization curves of the Tb-deposited sample after 2 h diffusion treatments conspicuous lower than that of the untreated one. The magnetic properties of Tb-diffused magnet maintained an upward trend with the extension of diffusion time, but long-time diffusion treatment over 6 h did not contribute significantly to improve the coercivity. The remanence and magnetic energy product of the Tb-deposited sample after diffusion treatment for 10 h was enhanced significantly compared with those of the magnet diffused for 2 h. In addition, the squareness of demagnetization curve was gradually recovered with the extending of the diffusion time, which almost consistent to that of the initial sample. The Nd-rich thin layer distributed continuously among grain boundary was formed in the magnet by long time diffusion treatment. The more continuous and homogeneous distribution of Nd-rich phase reduced the defects in the grain boundary and inhibited the nucleation of reverse magnetization domain. The main phase grains were separated from each other, which effectively reduced the magnetic phase exchange coupling effect between magnetic phase particles. The thickness of Tb-rich shells became thinner with the distance from the deposited surface increasing. EDS analysis showed that the thin shell structures could still be observed at the grain boundary of the center away from the magnet surface. The results confirmed core-shell structures exists on the entire magnet, which means Tb element penetrated the magnet with a thickness of 4 mm. That was the main reason for the increase of remanence and magnetic energy product after diffusion for 10 h. The results showed that 925℃×10 h+500℃×2 h was the best diffusion treatment process. The coercivity could be increased to 1630.9 kA•m-1, that of 50% higher than that of the untreated magnet. At the same time, the remanence and magnetic energy product of the magnet had no decrease obviously and kept high squareness of demagnetization curves. The orientation of magnets was improved after grain boundary diffusion treatment. The microstructure and properties of the GBDP magnets as a function of the diffusion temperature and time were systematically investigated. Tb element diffused via the grain boundary to the inner and formed the distinct core-shell structures near the external of the Nd2Fe14B grains. The removal of the surface layer increased the remanent magnetic polarization and had no effect on the coercivity of the GBDP magnet. Tb element could be diffused to the center of the magnet by the appropriate diffusion treatment process and gathered in the grain boundaries. The Tb-rich shell structure existing within the magnet was the reason why the GBDP magnet holds the high squareness of demagnetization curves and the high remanence. Thereby, the magnetron sputtering method deposited Tb film combined with GBDP was a practicable process to prepare Nd-Fe-B sintered magnets with high coercivity, remanence, and magnetic energy product. © Editorial Office of Chinese Journal of Rare Metals. All right reserved.