The magnetic hardening and exchange-decoupling strengthening of combined diffusion Nd-Fe-B magnet

Microstructure and magnetic properties of single-sided diffused magnet by TbF3 following Pr-Cu-Al alloy were investigated. It was confirmed that the core-shell structure of TbF3 diffused magnets was developed at the depth of about two hundred micrometers from coated-surface and the coercivity enhancement was mainly resulted from magnetic hardening of Tb-rich shells. However, microstructure and magnetic properties indicated obvious gradient characteristics along diffusion depth and Tb element was partially blocked at near surface of magnet. It was also verified that the following diffusion treatment by Pr80Cu10Al10 alloy for TbF3-diffused magnets can obviously promote grain boundary diffusion of Tb element into deeper regions and optimize uniformity of Tbrich shells. Meanwhile, additional Pr-Cu-Al phase was introduced and continuously distributed at intergranluar regions. Therefore, magnetic hardening of Tb-rich shells and exchange-decoupling strengthening of adjacent 2:14:1 grains isolated by intergranluar phases contributed to further coercivity increment Delta Hci of magnet simultaneously. Furthermore, it was demonstrated that the Hci of diffused magnets was enhanced with a reduction of magnet thickness and an increase in Pr80Cu10Al10 source proportion and then close to saturation due to the limitation of diffusion depth.