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

Rebernik Gracej, Mihaela (author), Šturm, Sašo (author)

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, grain-boundary-engineering strategy that revitalizes RE$_2$Fe$_{14}$B 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. Nd$_{70}$Cu$_{30}$ additions proved decisive in dissolving RE$_2$Fe$_{14}$B-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/m$^3$ 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 Nd$_2$Fe$_{14}$B 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 Nd$_{70}$Cu$_{30}$. The study demonstrates the potential of microstructural single-grain, grain-boundary re-engineering to enhance the properties of recycled magnets.

• Language: English
• Keywords: Nd-Fe-B magnets, recycling, Nd-Cu eutectic, spark plasma sintering, single-grain grain-boundary engineering
• Work type: Article
• Typology: 1.01 - Original Scientific Article
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Publication status: Published
• Publication version: Version of Record
• Year: 2026
• Number of pages: 15 str.
• Numbering: Vol. 1050, art. 185605
• PID: 20.500.12556/RUL-182842
• UDC: 620.1/.2
• ISSN on article: 1873-4669
• DOI: 10.1016/j.jallcom.2025.185605
• COBISS.SI-ID: 262745091
• Publication date in RUL: 25.05.2026

Record is a part of a journal

• Title: Journal of alloys and compounds
• Publisher: Elsevier
• ISSN: 1873-4669
• COBISS.SI-ID: 23089925

Licences

• License: CC BY-NC 4.0, Creative Commons Attribution-NonCommercial 4.0 International
• Link: http://creativecommons.org/licenses/by-nc/4.0/
• Description: A creative commons license that bans commercial use, but the users don’t have to license their derivative works on the same terms.

Secondary language

• Language: Slovenian
• Keywords: fizika, kemija, materiali, Nd-Fe-B magneti, recikliranje, Nd-Cu evtektik, iskrovno plazemsko sintranje, inženirstvo mej zrn na ravni posameznega zrna