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.