Coercivity and thermal stability enhancement of Nd-Fe-B magnet by grain boundary diffusing Tb-Y-La-Cu alloys

Commercial N55 sintered Nd-Fe-B magnets were processed by grain boundary diffusion (GBD) with Tb-Cu and Tb-Y-La-Cu alloys. The coercivity increased significantly to 1824 and 1901 kA/m for the Tb70Cu30 and Tb52.5Y7La10.5Cu30 (at%) GBD-treated magnets. Notably, when further increasing the proportion of Y and reducing that of Tb to the composition of Tb39.5Y20La10.5Cu30, the coercivity increment was still large and rather similar to that of Tb70Cu30, accompanied by an obvious optimization of thermal stability. Microstructure analyses demonstrated that the remarkable performance enhancement of Tb-Y-La-Cu GBD-treated magnet was mainly attributed to the synergistic effect of Y and La on Tb, in which, the competitive diffusion of Y and Tb effectively inhibited the lattice diffusion of Tb in grains while La tends to combine with oxygen to form La2O3 and reduce the consumption of Tb at the grain boundary oxide, improving the effective function depth for magnetic hardening shell structure.