Effect of Post-Diffusion Annealing on the Microstructure and Magnetic Properties of Nd-Fe-B Magnets Prepared by Grain Boundary Diffusion Process Using a Pr66Tb10(CoAlGa)24 Alloy

The effects of post-diffusion annealing on the microstructure and coercivity of sintered Nd-Fe-B magnets are investigated using a Pr66Tb10(CoAlGa)24 diffusion source. Grain boundary diffusion increases the coercivity from 8.95 to 23.20 kOe, while subsequent annealing at 500 °C for 2 h further raises it to 24.24 kOe. This additional gain of approximately 4.5% is accompanied by a remanence decrease of 0.08 kGs, giving a total decrease of 0.30 kGs relative to the original magnet. Microstructural characterization reveals changes in elmental distribution and grain boundary morphology after annealing. Tb-enriched shells are observed near the diffusion surface. In the magnet interior, where Tb enrichment is limited, chemically distinct Nd-rich and comparatively Fe-rich intergranular regions are present. Ga-containing RE-Fe regions at a depth of approximately 2 mm are tentatively associated with RE6Fe13Ga-related phases. The observed compositional heterogeneity suggests a possible role for local Fe partitioning in improving magnetic isolation between adjacent grains. These findings suggest that the additional coercivity enhancement involves grain boundary modification in the magnet interior alongside changes in the near-surface Tb-enriched regions, with reduced intergranular exchange coupling as a possible contributing mechanism.