Effects of grain boundary diffusion process on magnetic properties enhancement and microstructure evolution of hot-deformed Nd-Fe-B magnets

Grain boundary diffusion (GBD) process is an important approach for producing Nd-Fe-B magnets with high coercivity and high thermal stability. The GBD for hot-deformed Nd-Fe-B magnets with nano-crystalline microstructure is more complicated compared to sintered magnets. Here, we investigated the effects of different GBD methods, i.e., intergranular addition (in-situ GBD 1#), in-situ GBD from magnet surface during hot pressing and hot deformation (in-situ GBD 2#), and conventional GBD, on the mag-netic properties and microstructure of hot deformed magnets. After the treatment by these three GBD approaches using 2 wt% Pr40Tb30Cu30 diffusion source, the coercivity of the hot-deformed magnet in-creases from 1281 to 1567,1412 and 2022 kA/m, respectively. The coercivity enhancement is attributed to the formation of local (Nd,Tb)2Fe14B phase with strong magnetic anisotropy. Reduced grain orientation is found in both in-situ GBD 1# and conventional GBD treated samples mainly due to the local stress state variation and the rotation of platelet grains. Interestingly, the in-situ GBD 2# processed sample has a high orientation at diffusion surface, which may be caused by the modified surface state of the magnet by the diffusion source. Compared with the in-situ GBD processes, the conventional GBD exhibits a higher utilization efficiency of Tb. Since the in-situ GBD is effective to treat thick hot-deformed magnets, further effort should be aimed at enhancing its diffusion efficiency.(c) 2022 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.