GBD vs Bulk Alloyed NdFeB

Mainrich's factory-verified production case: a 0.35% Tb surface coating took an N45H magnet to N45UH coercivity — Hcj up 50%, from 17.67 to 26.55 kOe — with no Br loss and 75% less Dy than the conventional alloying route. The savings exist because coercivity in NdFeB is determined at grain boundaries, not in grain interiors. Bulk alloyed magnets place HRE atoms uniformly — most of those atoms are wasted in grain interiors where they do nothing for coercivity but do depress remanence. GBD places HRE only at the boundaries where the magnetic reversal nucleates.

Yes, and it is the central design constraint. Terbium diffusion typically penetrates 3–6 mm per face under standard processing conditions. For magnets thicker than roughly 10 mm in any dimension, the core retains the original (lower) coercivity of the sintered substrate while only the surface layer achieves the elevated coercivity. For automotive traction pole pieces, robotics actuator magnets, and most compact-motor designs this is not a problem because parts are typically 5–8 mm thick. For wind turbine pole blocks, large scientific instrument magnets, and thick rotor segments, bulk alloying is still required.

Yes, favorably. Bulk alloyed NdFeB carries dysprosium or terbium throughout every grain, substituting for neodymium and depressing remanence by roughly 1 mT per percent HRE added. GBD avoids this penalty because the HRE atoms concentrate at grain boundaries, leaving grain interiors as nearly pure Nd2Fe14B. Comparison data on production-scale material typically shows GBD-processed N48SH running 1–3% higher Br than bulk alloyed N48SH at the same HcJ. For motor designers, that difference translates directly into higher torque density at the same temperature class.

The GBD process adds a post-sinter coating step and a diffusion anneal of 8–24 hours, which typically increases processing cost by 5–15%. On paper that looks like a price increase. But terbium metal traded near USD 4,030/kg in late May 2026 (roughly +103% year-to-date), and GBD needs only a thin surface application — Mainrich's factory-verified case reached an N45H to N45UH coercivity upgrade with a 0.35% Tb surface coating and 75% less Dy than conventional alloying. At current pricing, the material savings exceed the processing adder by a wide margin. GBD magnets are normally the lower total-landed-cost option when high-temp grades are required.

GBD reduces the per-magnet heavy rare earth footprint, which matters when MP Materials, Lynas, and other Western producers cannot yet supply commercial quantities of separated terbium or dysprosium. Switching a fleet of SH-grade magnets from bulk alloyed to GBD-processed cuts the program's HRE intake by roughly half without changing the application. It does not eliminate exposure — GBD still consumes HRE, and the majority of GBD capacity sits in China — but the volume reduction is meaningful. For OEMs building CRMA disclosure programs or supply-chain stress tests, the difference shows up directly in the numbers.