Microstructure refinement of hot deformed Dy-free NdFeB magnets through a novel cold sintering approach by pressure-assisted spark plasma sintering

Wu, Ziyi; Zhao, Dong; Cui, Baozhi; Lian, Jie

doi:10.1016/j.jallcom.2025.183663

Microstructure refinement of hot deformed Dy-free NdFeB magnets through a novel cold sintering approach by pressure-assisted spark plasma sintering

Journal Article · Tue Sep 09 00:00:00 EDT 2025 · Journal of Alloys and Compounds

DOI:https://doi.org/10.1016/j.jallcom.2025.183663· OSTI ID:3012370

Wu, Ziyi ^[1]; Zhao, Dong ^[1]; Cui, Baozhi ^[2]; Lian, Jie ^[1]

Rensselaer Polytechnic Inst., Troy, NY (United States)
Ames Laboratory (AMES), Ames, IA (United States)

Anisotropic fine-grained Dy-free NdFeB high-performance magnets were produced using a novel cold sintering approach by pressure-assisted spark plasma sintering (SPS). The NdFeB nanocrystalline ribbon powders were subjected to cold sintering below 450 °C using pressure-assisted SPS, followed by hot deformation at 710 °C and 810 °C via SPS. Results indicate that cold sintering below 450 °C effectively restricts grain growth in the NdFeB magnets, yielding sub-100 nm scale grains across both fine-grained regions and most coarse-grained regions. The refined grain structure achieved through cold sintering allows for a lower hot deformation temperature as compared with the magnet by conventional sintering, as finer grains allow easier deformation and grain sliding/rotation. This process produces a highly fine-grained and anisotropic microstructure to achieve high magnetic performance (i.e., high intrinsic coercivity, remanence, and maximum energy product) in the magnets. The cold sintering (at 350 °C) and lower hot deformation temperatures required (e.g., 710 °C) help control grain coarsening and limit grain size that is commensurate with the size of a single magnetic domain of the Nd₂Fe₁₄B magnetic matrix phase, resulting in high intrinsic coercivity (H_ci =11.4 kOe). Furthermore, the reduced hot deformation temperature mitigates the formation of coarse grain bands, leading to improved magnetic properties, specifically an increased remanence (B_r = 14.3 kGs) and a high maximum energy product ((BH)_max = 48 MGOe). In conclusion, cold sintered hot deformed NdFeB magnet also shows good thermal stability with reversible temperature coefficient α(B_r)= -0.097 %/K and α(H_ci)= -0.616 %/K at elevated temperatures up to 400 K.

View Accepted Manuscript (DOE)

Research Organization:: Ames Laboratory (AMES), Ames, IA (United States)

Sponsoring Organization:: USDOE

Grant/Contract Number:: AC02-07CH11358

OSTI ID:: 3012370

Journal Information:: Journal of Alloys and Compounds, Journal Name: Journal of Alloys and Compounds Vol. 1040; ISSN 0925-8388

Publisher:: ElsevierCopyright Statement

Country of Publication:: United States

Language:: English

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