Effect of Nb doping on microstructure and magnetic properties of hot-deformed Nd-Fe-B magnets with Nd-Cu eutectic diffusion

doi:10.1016/j.jmst.2021.12.073

Abstract

Abstract:

To restrict grain growth in coarse grain regions caused by the diffusion of Nd-Cu eutectic alloys, the Nb element was introduced into the precursor alloy to regulate the microstructure of melt-spun powder and die-upset magnets. The magnetic properties and thermal stability of die-upset magnets were appreciably improved through the addition of Nb. For the Nb-doped diffusion die-upset magnet, the grains inside the ribbons were refined and the coarse non-oriented surface crystallite got suppressed on the interface of ribbons during the hot-deformation process to form the anisotropic magnet. Moreover, Nd gathers at the intergranular phases, which is considered to enforce domain wall pinning force. The Nb-modified microstructure is advantageous to thermal stability and coercivity enhancement. High-resolution transmission electron microscopy images revealed that the Nb element gathered on the grain boundary and triple grain boundary to form c-Nb and h-NbFeB to hinder the grain growth during the hot-deformation process, which led to direct enhancement in the coercivity. Furthermore, the c-Nb and h-NbFeB are nonmagnetic phases that strengthened the magnetic isolation. However, the h-NbFeB precipitated from the hard magnetic phase and formed crystal defects which led to remanence deterioration.

Key words: Nd-Fe-B, Hot deformation, Diffusion, Nb-doping, Coercivity

Tingting Song, Xin Li, Xu Tang, Wenzong Yin, Yang Luo, Dunbo Yu, Wenlong Yan, Jinyun Ju, Renjie Chen, Aru Yan. Effect of Nb doping on microstructure and magnetic properties of hot-deformed Nd-Fe-B magnets with Nd-Cu eutectic diffusion[J]. J. Mater. Sci. Technol., 2022, 122: 121-127.

Figures/Tables 8

Fig. 1. (a) Room-temperature demagnetization curves. (b) Temperature dependence of remanence. (c) Temperature dependence of coercivity for HD-Nb-free, DPHD- Nb-free, and DPHD- Nb-doped samples.

Fig. 2. X-ray diffraction spectra and the corresponding (006) polar figures of HD-Nb-free, DPHD-Nb-free, and DPHD-Nb-doped magnets, respectively.

Fig. 3. Low-magnification BSE-SEM image and enlarged image of (a), (b) DPHD-Nb-free magnet and (d), (e) DPHD-Nb-doped magnet; (c), (f) EDS element line scanning of the yellow line in (b) and (e). The insets of Fig. 3(b) and (e) show the statistics of the coarse region width for Nb-free and Nb-doped die-upset magnets, respectively.

Fig. 4. High magnification cross-sectional BSE-SEM images taken from the interface of ribbons of (a) Nb-free and (b) Nb-doped die-upset magnets. High magnification cross-sectional BSE-SEM images obtained from the inside of ribbons of (c) Nb-free and (d) Nb-doped die-upset magnets (c-axis is in-plane as the arrow indicates). (e) and (f) The statistics of transverse, lateral grain size, and shape ratio for Nb-free and Nb-doped die-upset magnets, respectively.

Fig. 5. (a) HADDF-STEM image of DPHD-Nb-doped sample; (b, c) STEM-EDS mappings for Fe, Nd, Nb, Pr, and Cu obtained from regions 1 and 2 marked with blue and red boxes (c-axis is indicated by the white arrows).

Fig. 6. HRTEM and the extracted FFT images of DPHD-Nb-doped sample.

Fig. 7. M-T and dM/dT-T curves of Nb-doped and Nb-free hot deformed magnets.

Fig. 8. Cross-sectional view Lorentz TEM images of (a, b) DPHD-Nb-free and (c, d) DPHD-Nb-doped die-upset magnets at 0 applied magnetic fields.

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