Coercivity enhancement and microstructure evolution in Nd-Fe-B sintered magnets by grain boundary diffusing Pr-Co-Al-Ga alloys

Non-heavy-rare-earth Pr₆₇Co₁₂Al₉Ga₁₂ (at.%) alloys were designed as the diffusion sources to conduct grain boundary diffusion (GBD) on commercial Nd-Fe-B sintered magnets. The coating amounts of diffusion source alloys were varied from 1.0 wt% to 11.0 wt%. Magnetic property analyses revealed that the coercivities of the diffused magnets gradually improved with the increasing coating amounts of Pr-Co-Al-Ga alloys, accompanied by the corresponding decreases in both the remanences and maximum magnetic energy products. At 11.0 wt% coating, the coercivity significantly increased from 14.35 kOe (original magnet) to 25.19 kOe. Microstructural analysis demonstrated that the significant coercivity enhancement after diffusion primarily resulted from the formation of continuous and clear grain boundaries in the surface layers. Meanwhile, the multi-element content analysis showed that the residual amounts of diffusion-source alloys after GBD increased with the pre-diffusion coating amounts, and both Al and Ga elements in the alloys were more easily diffused into the interiors of the magnets compared to Pr. Besides, diffusion behaviors of Pr-Co-Al-Ga alloys in the whole GBD process both on the surface of the magnet and in the shallow surface-layer inside the magnet were studied systematically by the in-situ confocal laser scanning and element distribution analyses for GBD intermediate states respectively, demonstrating that the Pr-Co-Al-Ga alloys diffused into magnet interiors at temperatures far below that of conventional GBD treatment (∼900 °C) and the typical continuous grain boundaries formed predominantly during 500 °C annealing stage. Complementary micromagnetic simulations further elucidated the effects of grain boundary modification on demagnetization processes. This work provided insights and experimental foundations for advancing non-heavy-rare-earth GBD technology.