Concurrent improvements of corrosion resistance and coercivity in Nd-Ce-Fe-B sintered magnets through engineering the intergranular phase

doi:10.1016/j.jmst.2021.09.008

Abstract

Abstract:

Usually the improved coercivity of rare earth (RE) based 2:14:1-type permanent magnets via RE-rich intergranular additives is achieved at the cost of more corrosion channels and deteriorated corrosion resistance, which remains a challenging hurdle in the RE-Fe-B community. Distinctly, here we report the concurrent improvements of corrosion resistance and coercivity in 40 wt.% Ce-substituted Nd-Ce-Fe-B sintered magnets through engineering the intergranular phase using simple (Nd, Pr)H_x additive. The dehydrogenated Nd/Pr changes the RE concentration gradients between 2:14:1 matrix and intergranular phases during sintering and enlarges the fraction of corrosion-resistant REFe₂ phase, rather than the conventionally assumed Nd/Pr-rich intergranular phase with high chemical vulnerability. The spontaneous formation of REFe₂ intergranular phase after (Nd, Pr)H_x addition generates the uniquely enhanced corrosion resistance against the hot/humid and acidic environments, and counts as one peculiar feature of Nd-Ce-Fe-B magnets at high Ce substitution level, being distinct from previously reported Ce-free/lean RE-Fe-B. Simultaneously, the formation of continuous grain boundaries enhances the coercivity from 8.7 to 12.5 kOe with trace addition of (Nd, Pr)H_x. Above findings may spur progress towards developing a high-performance Nd-Ce-Fe-B permanent magnet.

Key words: Abundant rare earth Ce, Corrosion resistance, REFe₂, Coercivity, Microstructure

Jiaying Jin, Yongming Tao, Xinhua Wang, Zeyu Qian, Wang Chen, Chen Wu, Mi Yan. Concurrent improvements of corrosion resistance and coercivity in Nd-Ce-Fe-B sintered magnets through engineering the intergranular phase[J]. J. Mater. Sci. Technol., 2022, 110: 239-245.

Figures/Tables 6

Fig. 1. Magnetic and anti-corrosive performance of the Ce-40 magnets with different (Nd, Pr)Hx contents. (a) Second-quadrant demagnetization curves, (b) normalized dM/dT-T curves within the temperature range of 150-350 °C, (c) mass loss in hot/humid atmosphere as a function of exposure time, (d) released hydrogen volume in 0.1 mol/L H2SO4 solution as a function of immersion time.

Fig. 2. Low-magnification back-scattered SEM images and corresponding colored maps for Ce-40 magnets added with (a, e) 0 wt.%, (b, f) 1 wt.%, (c, g) 2 wt.% and (d, h) 3 wt.% (Nd, Pr)Hx. High-magnification images for Ce-40 magnets added with (i) 0 wt.% and (j) 3 wt.% (Nd, Pr)Hx, (k, l) corresponding Ce/Fe/Nd/Pr distributions along the blue line (across RE-rich phase) and the red line (across REFe2 phase).

Fig. 3. TEM characterizations for Ce-40 magnet with 3 wt.% (Nd, Pr)Hx addition. (a) Bright field image (BFI). Electron diffraction patterns (EDPs) for (b) grain 1 (G1), (c) grain 2 (G2), and (d, e) triple junction (TJ) along two different zone axes (ZAs). Fe/Ce/Nd/Pr mappings under scanning-TEM (STEM) mode for (f) TJ region and (g) continuous grain boundary (GB) isolating adjacent grains.

Fig. 4. (a) Rietveld refinement of step-scanned XRD profiles for 0 wt.% and 3 wt.% (Nd, Pr)Hx-added Ce-40 magnets. Back-scattered SEM images and the corresponding Fe/Nd/Ce mappings for (b) 0 wt.% and (c) 3 wt.% (Nd, Pr)Hx-added Ce-40 magnets.

Fig. 5. Open circuit potential curves for (Nd, Pr)11.76Fe82.36B5.88 (2:14:1), Ce33.33Fe66.67 (1:2) and (Nd, Pr)80Fe20 alloys in (a) 0.6 mol/L NaCl aqueous solution, (b) 0.75 mol/L NaOH alkali solution, and (c) 0.005 mol/L H2SO4 acidic solution. (d) Tafel curves and (e) Nyquist plots for 0 wt.% and 3 wt.% (Nd, Pr)Hx-added Ce-40 magnets in representative NaCl solution.

Fig. 6. Schematic illustrations for the microstructural evolutions during sintering and corrosion process of the Ce-40 magnets with (a-d) 0 wt.% and (e-h) 3 wt.% (Nd, Pr)Hx.

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