Abstract: The grain boundary diffusion (GBD) process is widely used to enhance the coercivity of Nd‒Fe‒B sintered magnets, representing a critical materials technology for advancing electrification revolution. However, it is well known that the enhancement in coercivity achievable through the GBD process appears to reach a limit. To overcome this limitation, it is essential to thoroughly understand the formation of the magnetically hardening matrix shells and the evolution of the grain boundary (GB) phases along the diffusion path. Here we present the microstructural and magnetic evolution of the Pr-Al-rich matrix shells and the GB phases along the diffusion path in the Pr₈₀Al₂₀ GBD processed Nd‒La‒Ce‒Fe‒B magnets. Firstly, the Pr/total rare earth (TRE) ratio remains relatively constant in the Pr-Al-rich matrix shells from the magnet surface to an interior diffusion depth of ∼500 µm. The consistent Pr/TRE ratio identified within the matrix shells means that there is not a sustained increase in the nucleation field near the RE₂Fe₁₄B matrix grain/GB interface. Secondly, from the magnet surface to an interior diffusion depth of ∼500 µm, the antiferromagnetic high-Al δ-type RE‒Fe‒Al phase at the GBs transforms to a ferromagnetic low-Al µ-type or amorphous RE‒Fe‒Al phase. The predominance of the ferromagnetic low-Al µ-type or amorphous RE‒Fe‒Al phase beyond this diffusion depth implies that, apart from this sub-surface region, there is a lack of magnetic isolation of the adjacent ferromagnetic RE₂Fe₁₄B matrix grains through most of the bulk magnet. These factors jointly contribute to constraining or limiting the coercivity enhancement. We discuss how the microstructural origins of the limits to the coercivity enhancement in the GBD processed magnets found here can enable future design approaches for producing thicker Nd‒La‒Ce‒Fe‒B magnets with higher coercivity.

Key words: Abundant La-Ce, PrAl grain boundary diffusion (GBD) process, Pr-Al-rich matrix shells, Antiferromagnetic high-Al δ-type RE-Fe-Al phase, Ferromagnetic low-Al μ-type or amorphous, RE-Fe-Al phase