Single-grain grain-boundary engineering with Nd-Cu for sustainable recycling and reprocessing of Nd-Fe-B magnets

The sustainable recycling and reprocessing of Nd-Fe-B magnets are essential for reducing critical-material dependency
in next-generation permanent-magnet manufacturing. This study introduces a single-grain, grainboundary-
engineering strategy that revitalizes RE2Fe14B grains recovered and extracted via chemical leaching
from end-of-life (EoL) wind-turbine magnets. The study showed that the selective leaching disrupted the matrix
grains, producing Nd-, Dy-, and Fe-based oxides that shaped the subsequent microstructural evolution. A detailed
transmission-electron-microscopy (TEM) study was employed to track the chemical and structural evolution of
the modified grain boundaries and to correlate these changes with microstructural and magnetic performance.
Nd70Cu30 additions proved decisive in dissolving RE2Fe14B-based surface oxides into the liquid phase and reprecipitating
them at the triple pockets as oxygen-rich secondary phases, enabling surface reconstruction and
liquid-phase sintering. The magnetic properties of bulk Nd-Fe-B magnet samples improved markedly with Nd-Cu
additions (0–30 wt%): the remanence increased to ≈ 1.05 T, saturating above 5 wt% Nd-Cu due to enhanced
grain alignment. The coercivity rose continuously from 50 to 825 kA/m with increasing Nd-Cu, governed primarily
by grain-boundary characteristics rather than grain size. The maximum energy product also increased
from 10 to 195 kJ/m3 under the same additions. Simulations showed that the effect of in-plane anisotropy due to
the presence of RE-oxides at the surface of the Nd2Fe14B grains reduces the coercivity. On the other hand, the
better grain alignment markedly enhances the coercivity. The low-Fe-concentration grain boundaries, considered
to be paramagnetic, act as an effective magnetic decoupling phase. Conversely, excessive non-magnetic secondary
phases in the triple pockets generate local demagnetizing fields that lower the coercivity. Thus, optimizing
the oxygen pathways with Nd-Cu additions enables the sustainable recycling and reprocessing of Nd-Fe-B
magnets by replacing the Nd-rich phases with resource-efficient Nd70Cu30. The study demonstrates the potential
of microstructural single-grain, grain-boundary re-engineering to enhance the properties of recycled magnets.