High coercivity Nd-Fe-B magnet with low Tb concentration by combined diffusion

Commercial N50 Nd-Fe-B magnet was processed by a combined diffusion process with Pr65Cu15Al20, followed with Pr50Tb30Al20 (at %). The coercivity increment from the original 965.6 kA/m to 1672.5 kA/m was achieved in the combined diffusion sample by introducing 0.23 wt% Tb. However, the coercivity of the conventionally diffused sample with similar Tb content by Pr50Tb30Al20 reached only 1386.2 kA/m. Microstructural and compositional analyses revealed that relatively continuous grain boundary phases were first formed in the diffused sample by Pr65Cu15Al20, moreover, appropriate Tb-rich shells around 2:14:1 grains were further formed during the following diffusion by Pr50Tb30Al20 alloy. The combined diffusion process was helpful in promoting Tb diffusing along the grain boundary phase and further forming uniform Tb-rich shells. It provided a promising way of accelerating Tb diffusing and enhancing Tb availability.