Chemical Heterogeneity and Intergranular Phases: Governing Tb Grain Boundary Diffusion in Nd–La–Ce–Fe–B Rare Earth Permanent Magnets

ABSTRACT Unraveling the effects of chemical heterogeneity and intergranular phases on heavy rare earth (Dy/Tb) diffusion is crucial for developing high‐performance, cost‐effective grain boundary diffusion (GBD) magnets with high Ce substitution. Herein, a previously unreported selective diffusion behavior governed by chemical heterogeneity is uncovered, along with the demonstration of a blocking effect of the ZrB 2 phase on Tb diffusion. Comprehensive experimental and theoretical analysis reveals that Tb preferentially diffuses toward Nd‐rich grains rather than LaCe‐rich grains, thereby hindering Tb diffusion and weakening the magnetic hardening effect in the LaCe‐rich grains. This accounts for the lower coercivity increment in multi‐main‐phase (MMP) GBD magnets (5.15 kOe) relative to that in single‐main‐phase (SMP) GBD magnets (5.52 kOe). Despite achieving a higher room‐temperature coercivity (12.43 kOe), the MMP GBD magnets exhibit inferior thermal stability due to the weaker demagnetization resistance of LaCe‐rich grains. Furthermore, the ZrB 2 phase exhibits significantly stronger blocking of Tb diffusion than the REFe 2 phase, which results in a non‐uniform Tb distribution on the grain surfaces. These novel findings establish fundamental microstructure design principles that facilitate the high‐efficiency diffusion of heavy rare earth elements, providing a viable pathway to overcome the coercivity bottleneck in GBD magnets with high Ce contents.