Coercivity enhancement of sintered Nd-Fe-B magnets by diffusing TbHx and Tb86Al7Co7 alloy

TbHx and Tb86Al7Co7 alloys were used as diffusion sources to enhance the coercivity of Nd-Fe-B magnets. The coercivity enhancement and microstructure of the magnets were examined after diffusion, and the results showed that the coercivity increased from the original 1525.1 to 2382.4 kA/m and 2274.9 kA/m in Tb86Al7Co7 and TbHx diffused magnets, respectively. This improvement in coercivity was attributed to the formation of a Tb-rich shell and a new NdTb-AlCo-grain-boundary phase. The Tb86Al7Co7 alloy had a higher diffusion efficiency and diffused deeper than the TbHx alloy due to the low melting point of the NdTb-AlCo-grain-boundary phase. The coercivity temperature coefficient of the magnets improved, increasing from the original −0.55%/K to −0.42%/K and −0.44%/K in Tb86Al7Co7 and TbHx diffused magnets, respectively. The Tb86Al7Co7 diffused magnet exhibited better thermal stability than the TbHx diffused magnet, which suggests that Nd-Fe-B magnets could potentially be used at considerably higher temperatures than they are now.