Correlation of coercivity, microstructure, and surface defects in HDDR-processed Nd-Fe-B powders for bonded magnet applications

Liu, Xubo B.; Kesler, Michael S.; Tener, Zachary P.; Kramer, Matthew J.; McGuire, Michael A.; Nlebedim, Ikenna C.

doi:10.1063/9.0001013

Correlation of coercivity, microstructure, and surface defects in HDDR-processed Nd-Fe-B powders for bonded magnet applications

Journal Article · Tue Feb 10 00:00:00 EST 2026 · AIP Advances

DOI:https://doi.org/10.1063/9.0001013· OSTI ID:3019934

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Ames Laboratory (AMES), Ames, IA (United States)
Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
Ames Laboratory (AMES), Ames, IA (United States); Iowa State Univ., Ames, IA (United States)

Hydrogenation-Disproportionation-Desorption-Recombination (HDDR) is an effective method for producing high coercivity, anisotropic powder from either fresh Nd-Fe-B alloy or recycled Nd-Fe-B magnets for use in bonded magnets. We investigated the impact of surface defects on the coercivity of HDDR-processed Nd-Fe-B using scanning electron microscopy (SEM), magnetic measurements, and micromagnetic simulations. We observed that coercivity decreases as particle size reduces, with SEM revealing surface defects and the detachment of Nd₂Fe₁₄B grains and Nd-rich phases from the particle surface. Micromagnetic simulations indicate that demagnetization initiates at the particle surface, where these defects are most concentrated, leading to reduced coercivity. The reduction in squareness of demagnetization curve, knee point field and coercivity for smaller HDDR particles is attributed to an increased specific surface area, which exhibits reduced nucleation field and weak domain wall pinning field during magnetization reversal. By addressing the role of surface defects in coercivity degradation, this study provides insights for improving both new powder production and recycling strategies, ultimately leading to enhanced performance of bonded magnets and contributing to more sustainable practices in the rare earth supply chain. One potential strategy to enhance the performance of HDDR Nd-Fe-B materials involves reducing the fraction of fine particles ($<$35 µm) and promoting the formation of grain boundary phases on particle surfaces.

View Accepted Manuscript (DOE)

Research Organization:: Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)

Sponsoring Organization:: USDOE Office of Energy Efficiency and Renewable Energy (EERE), Energy Efficiency Office. Advanced Materials & Manufacturing Technologies Office (AMMTO); USDOE Office of Science (SC)

Grant/Contract Number:: AC02-07CH11358; AC05-00OR22725

OSTI ID:: 3019934

Journal Information:: AIP Advances, Journal Name: AIP Advances Journal Issue: 2 Vol. 16; ISSN 2158-3226

Publisher:: American Institute of Physics (AIP)Copyright Statement

Country of Publication:: United States

Language:: English

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