Formation of anti-shell/core structure of heavy rare earth elements (Tb, Dy) in sintered Nd-Fe-B magnet after grain boundary diffusion process

doi:10.1016/j.scriptamat.2018.12.034

Scripta Materialia

Volume 163, 1 April 2019, Pages 40-43

https://doi.org/10.1016/j.scriptamat.2018.12.034 Get rights and content

Abstract

The evolution of microstructure close to the surface of sintered Nd-Fe-B magnet after grain boundary diffusion process (GBDP) of heavy rare earth elements (HREE, Tb, Dy) was investigated. It was found that two new types of microstructure, anti-shell/core and no-shell structures, were formed in the magnet with the diffusion time longer than 0.5 h. The anti-shell/core structure was formed due to an abnormal diffusion of HREE from grain to grain boundary based on the dual-trend diffusion model, it contributes a negative effect on the coercivity of diffused magnets.

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Conclusions

An anti-shell/core structure was formed at the outer layer of the Nd-Fe-B magnets after grain boundary diffusion of HREE when the diffusion time is longer than 0.5 h, the depth range of the anti-shell/core structure increases with increasing diffusion time. This structure was predicted by the dual-trend diffusion model. For the HREE diffusion in GBP is faster than that in GP, the concentration of HREE in GBP close to the magnet surface drops faster than that in GP when the HREE coating was

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In this paper, the variations of composition, core-shell structure, and magnetic properties of diffused Nd-Fe-B magnet with the diffusion depth have been intensively investigated. It was confirmed that the Tb concentration, core-shell structure and intrinsic coercivity (H_ci) along the diffusion depth of magnets indicated obvious gradient characteristics. The H_ci of diffused magnet was increased by 824.4 kA/m and reached 1864.7 kA/m. The H_ci of the residual sample gradually decreased with cutting and removal of surface. However, it was emphasized that the H_ci of 1702.6 kA/m with the increment of 662.3 kA/m was obtained in the central sample, in which the concentration of Tb was 0.13 wt%. The electron probe microanalyzer (EPMA) analyses showed that Tb-rich shells were continuously visible in the depth of 100 μm, and became discontinuous in the depth of 500 μm, and were nearly invisible in the center of diffused magnet with the depth of 2500 μm. In order to further characterize the atomic-scale morphology and elements, a high-angle annular dark field (HAADF) detector and an energy dispersive X-ray spectroscopy (EDS) on the double aberration-corrected scanning transmission electron microscopy (STEM) were employed. The results indicated that the thick Tb-rich shells were clearly visible near the surface of the diffused magnet, and the lattice constant c of 2:14:1 phase was reduced by 0.05 Å due to the partial substitution of Tb element for Pr/Nd elements. However, Tb-rich shells in the central part of diffused magnet was discontinuous and inhomogeneous, in which the Tb-rich and Tb-poor regions were occurred. It was mainly attributed to the low Tb concentration at grain boundaries in the center of the magnet, which was difficult to form complete Tb-rich shells.
Understanding the coercivity enhancement mechanism of grain boundary diffused Nd-Fe-B magnets by comparing with commercial equivalent coercivity magnets
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The grain boundary diffusion (GBD) technology effectively utilizes heavy rare earth (HRE) elements to improve the coercivity of sintered Nd-Fe-B magnet. Significantly, clarifying its coercivity improvement mechanism is essential for further optimizing the process and performance. Through comparative analysis with commercial conventional magnets (CSH) processing equivalent coercivity, this study investigated the specific mechanism by which the GBD method enhances the coercivity of HRE elements in an efficient manner. To achieve the same coercivity (∼21.4 kOe), GBD magnets contained only 0.25 wt% Tb, whereas CSH magnets had 1.17 wt% Dy and 1.18 wt% Tb. The microstructure analysis revealed that the Tb element in the GBD magnet was concentrated in the outer layer of the grain, constituting 17.37 wt% of the total rare earth content. While, in CSH magnets, the HRE element in the main phase grains accounted for only 9.58 wt% of the total rare earth elements, including Dy (3.83 wt%) and Tb (5.75 wt%). The characteristic core-shell structural grains in GBD magnet also endowed it with an uneven magnetic domain structure and a more concentrated “abrupt” magnetization reversal. Based on the characterization results the nucleation positions of the two types of magnets were analyzed by fitting H_ci/M_s and H_a/M_s. The results can serve as a guide for enhancing coercivity in grain boundary diffusion sintered Nd-Fe-B magnets, aiding in process and property optimization, and advancing the understanding of nucleation type mechanisms.
Coercivity enhancement of NdCeFeB magnet by two-step grain boundary diffusion with Pr<inf>70</inf>Cu<inf>10</inf>Al<inf>15</inf>Ga<inf>5</inf> and DyH<inf>x</inf>
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Nd-Ce-Fe-B sintered magnets with Ce-substitution amounts of 30 wt% were treated by one- and two-step grain boundary diffusion processes, in which DyH_x and low-melting-point Pr₇₀Cu₁₀Al₁₅Ga₅ (at%) alloys were designed as the diffusion sources and diffused into the magnets according to the different steps and orders. Results showed that the increase of coercivity was only 2.7 kOe when introducing heavy rare-earth Dy by the one-step diffusion of DyH_x, reflecting the difficulty of conventional diffusion modification on high Ce content magnets. Note that by the two-step grain boundary diffusion (no matter which diffusion source was diffused preferentially in the first step), the coercivities were significantly enhanced over 18.5 kOe from the original 13.64 kOe, with the obvious improvements of thermal stability. Microstructures and compositions analyses revealed that the grain boundaries were greatly optimized after the diffusion of Pr₇₀Cu₁₀Al₁₅Ga₅, supporting favorable diffusion channels for the introduction of Dy in the subsequent second step. By contrast when introducing Dy first and then diffusing the Pr₇₀Cu₁₀Al₁₅Ga₅ second, the elements in the low-melting-point alloys could drive the superficial Dy to migrate into the deep interior of the magnets. Thus, both the two types of two-step grain boundary diffusions were conductive to improving the utilization of heavy rare-earth Dy in high Ce content magnets. Besides, dynamic domain evolution analyses demonstrated the positive effect of diffusion on anti-demagnetization. This work provided a foundation for the development of high Ce content magnets with high performances.
The magnetic hardening and exchange-decoupling strengthening of combined diffusion Nd-Fe-B magnet
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Microstructure and magnetic properties of single-sided diffused magnet by TbF₃ following Pr-Cu-Al alloy were investigated. It was confirmed that the core–shell structure of TbF₃ diffused magnets was developed at the depth of about two hundred micrometers from coated-surface and the coercivity enhancement was mainly resulted from magnetic hardening of Tb-rich shells. However, microstructure and magnetic properties indicated obvious gradient characteristics along diffusion depth and Tb element was partially blocked at near surface of magnet. It was also verified that the following diffusion treatment by Pr₈₀Cu₁₀Al₁₀ alloy for TbF₃-diffused magnets can obviously promote grain boundary diffusion of Tb element into deeper regions and optimize uniformity of Tb-rich shells. Meanwhile, additional Pr-Cu-Al phase was introduced and continuously distributed at intergranluar regions. Therefore, magnetic hardening of Tb-rich shells and exchange-decoupling strengthening of adjacent 2:14:1 grains isolated by intergranluar phases contributed to further coercivity increment ΔH_ci of magnet simultaneously. Furthermore, it was demonstrated that the H_ci of diffused magnets was enhanced with a reduction of magnet thickness and an increase in Pr₈₀Cu₁₀Al₁₀ source proportion and then close to saturation due to the limitation of diffusion depth.
Clarifying the dominated coercivity enhancement mechanism of grain boundary diffused Nd-Fe-B magnets by Pr-based alloys
2024, Journal of Magnetism and Magnetic Materials
Low-melting point Pr-based alloys have been confirmed to be cost-effective grain boundary diffusion sources for enhancing the coercivity of Nd-Fe-B magnets. Various coercivity enhancement mechanisms have been reported for these alloys so far, but the dominated one has not been clarified. Here, the sintered Nd-Fe-B magnet was treated by Pr₇₅Al₂₅ alloy grain boundary diffusion at different temperatures. A detailed investigation on the diffusion kinetics of the elements and the microstructure evaluation have been carried out. After 800 °C diffusion, the intrinsic coercivity of the magnet increased from 1070 kA/m to 1348 kA/m without significant reduction of remanence. The increased rare earth-rich phase continuously distributed along the grain boundaries has great contribution to the magnetic decoupling, leading to high coercivity. In comparison, after 900 °C diffusion, although Pr and Al diffused more sufficiently in the magnet, a drastic lattice diffusion occurred and less grain boundary layer was formed, resulting in insufficient coercivity enhancement. The results thus indicate that, for Pr-Al diffusion, the formation of RE-rich layer is more important than that of (Nd,Pr)₂Fe₁₄B shell in order to enhance the coercivity. Therefore, different from that for heavy rare earth diffusion, the diffusion treatment for Pr-based sources should be carefully optimized to form continuous grain boundary phase and suppress the excessive lattice diffusion.
On dysprosium utilisation in multi-main-phase Nd–Dy–Fe–B magnets with core–shell microstructures
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The development of high-performance Nd–Dy–Fe–B magnets that minimise the consumption of the scarce rare earth (RE) element Dy remains a major global scientific and technological quest. Here, we designed an alloy microstructure comprising of a uniform Dy-lean core–Dy-rich shell in a series of multi-main-phase (MMP) Nd–Dy–Fe–B magnets. The resulting MMP Dy1 and Dy3 magnets with an overall Dy level of 1 and 3 wt.% possessed values of 0.48 and 0.29 T/wt.% of coercivity increment per unit weight percentage of the Dy addition, respectively. Most importantly, the resulting MMP Dy3 magnet exhibited a high coercivity (2.38 T), an excellent thermal stability of the coercivity (|β| = 0.531%/°C), a high squareness factor (> 95%), all with little diminishment in the remanent magnetisation (1.35 T) and maximum energy product (43.6 MGOe). These properties are superior to the currently available sintered Nd–Dy–Fe–B magnets which utilise higher levels of Dy of 5 wt.%. Via magnetic and multi-scale microstructural characterisation experiments and micromagnetic simulations, the formation of the Dy-lean core–Dy-rich shell microstructure is rationalised via solid-state-diffusion and solution reprecipitation during liquid-phase sintering. The Dy-lean core–Dy-rich shell microstructure and the non-ferromagnetic low-Fe RE-rich grain boundary phase led to the synergistic magnetic performance. This is significant in the context of the MMP Nd–Dy–Fe–B magnets being applied to large-scale production. The present work establishes a pathway for the more sustainable utilisation of Dy in permanent magnets via formation of a uniform core–shell microstructure.

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Formation of anti-shell/core structure of heavy rare earth elements (Tb, Dy) in sintered Nd-Fe-B magnet after grain boundary diffusion process

Abstract

Graphical abstract

Section snippets

Conclusions

J. Alloys Compd.

Acta Mater.

Acta Mater.

J. Magn. Magn. Mater.

J. Magn. Magn. Mater.

Acta Mater.

Scr. Mater.

Acta Mater.

Acta Mater.

Acta Mater.

Scr. Mater.

J. Alloys Compd.

Acta Mater.