Enhancing the coercivity of Nd-Fe-B sintered magnets by consecutive heat treatment–induced formation of Tb-diffused microstructures

다국어 초록 (Multilingual Abstract)

<P><B>Abstract</B></P> <P>Herein, we probed the microstructure of Tb-diffused Nd-Fe-B magnets to investigate the relationship between Tb-diffused area and coercivity enhancement, employing prolonged stepwise heat treatment to ensure sufficient diffusion of Tb in relatively large-size magnets and revealing that this stepwise annealing generated core-shell structures. Quantitative compositional changes pertaining to individual phases of the multiphase system in each heat treatment process were analyzed by constructing ternary diagrams based on electron probe microanalysis compositional maps. During the grain boundary diffusion process, coercivity increased from 15.28 to 24.86 kOe, while only negligible remanence and energy product decreases were concomitantly observed. Microstructure analysis suggested that coercivity was closely related to the concentration and distribution of Tb; more precisely, the abovementioned core-shell structures successfully suppressed the nucleation of reverse domains at Nd-rich phase/main phase interfaces and therefore enhanced magnet coercivity without decreasing remanence and energy product.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We employed grain boundary diffusion to enhance the coercivity using TbH in large-sized magnets. </LI> <LI> The stepwise annealing for large-sized magnets were introduced for enhancing the coercivity. </LI> <LI> The relationship between Tb-diffused area and coercivity was revealed by probing microstructure. </LI> <LI> EPMA investigations including ternary diagram confirmed with the XRD analysis. </LI> <LI> We enhanced coercivity without decreasing their remanence and energy product. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>