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• Anisotropic diffusion of heavy rare earth can be avoided for sintered Nd-Fe-B magnets. • The alloying of diffusion source distinctly enhance diffusion efficiency of Tb along a -axis. • The alloying of diffusion source inhibits the excessive lattice diffusion of Tb. Grain boundary diffusion (GBD) efficiency of heavy rare earths (HREs) for sintered Nd-Fe-B magnets along the easy magnetization direction (c -axis) was reported to be higher than that along the difficult one (a -axis). However, this conclusion is challenged by the present results, which indicate that the anisotropic diffusion does not exist, at least for some low-melting HRE-based diffusion sources. The alloying of Tb by Pr, Al and Cu can distinctly enhance the diffusion efficiency of Tb along the a -axis, resulting in quite similar coercivity enhancement to that diffused along the c -axis. The diffusion kinetic analysis and microstructure characterizations demonstrate that the alloying lowers the melting point of diffusion source and wets the intergranular phase as effective diffusion paths for Tb. This suppresses the excessive lattice diffusion of Tb and the formation of over-thick Tb-rich shells, which is beneficial to enhancing the Tb diffusion efficiency along the a -axis and reducing the total Tb consumption. [ABSTRACT FROM AUTHOR]

Nd-Fe-B magnets containing no heavy rare earth (HRE) elements exhibit insufficient coercivity to withstand the demagnetization field at elevated temperatures. The grain boundary diffusion (GBD) process provides the best route to fabricate high-coercive Nd-Fe-B magnets with low consumption of expensive HRE resources. Here we give a special review on the grain boundary diffusion sources and their coating methods. Up to now, various types of grain boundary sources have been developed, starting from the earliest Tb or Dy metal. The HRE-M eutectic alloys were firstly proposed for reducing the cost of the diffusion source. After that, the diffusion sources based on light rare earth and even non rare earth elements have also been proposed, leading to new understanding of GBD. Now, the diffusion sources including inorganic compounds, metals, and alloys have been employed in the industry. At the same time, to coat the diffusion source on the magnets before diffusion treatment, various methods have been developed. Different from the previous review articles for GBD, this review gives an introduction of typical types of diffusion sources and their fabrication approaches. The effects of diffusion source on the microstructure and magnetic properties are summarized briefly. In particular, the principles and applicability of different coating approaches were discussed in detail. It is believed that this review can provide a technical guidance for the industry for designing the diffusion process and products meeting specific requirements. [ABSTRACT FROM AUTHOR]

Grain boundary diffusion (GBD) is an effective process to enhance the thermal stability and coercivity of sintered Nd-Fe-B magnets, but the low diffusion efficiency and high consumption of heavy rare earth (HRE) elements prevent further performance improvement and cost reduction of the magnets. Here, a Ni-containing dual-alloy Pr-Ni-Al/Dy-Ni-Al diffusion source is used to enhance both the diffusion efficiency of the Dy element and the anti-corrosion properties of Nd-Fe-B magnets. Pr-Ni-Al and Dy-Ni-Al alloys with different melting points exhibit different diffusion behaviors. The diffused low-melting Pr-Ni-Al alloy can construct continuous grain boundary, acting as an effective diffusion channel for the GBD of Dy-Ni-Al. As a result, the coercivity was enhanced from 1057 to 1662 kA/m by dual-alloy diffusion. For comparison, the coercivity increased only to 1548 kA/m by the diffusion of Pr-Dy-Ni-Al single-alloy with the same element contents. It was confirmed by composition and microstructure characterizations that Pr-Ni-Al/Dy-Ni-Al diffusion leads to a deeper diffusion of Dy. The constitution of continuous grain boundary phase and core-shell structure attributed to the enhancement of coercivity. Different from many HRE-based diffusion sources, the present Ni-containing sources can also greatly enhance the anti-corrosion properties of Nd-Fe-B magnets after GBD. The dual-alloy diffusion with low consumption and high diffusion efficiency of HREs, thus, shows good prospect for practical applications. • A Pr-Ni-Al/Dy-Ni-Al dual-alloy is proposed as grain boundary diffusion source. • Low-melting Pr-Ni-Al alloy provides diffusion channels for deep diffusion of Dy. • Deep diffusion and core-shell structure lead to enhanced coercivity. • Increased electric potential of GB phase results in improved corrosion resistance. [ABSTRACT FROM AUTHOR]

Grain boundary diffusion of heavy rare earth-based alloys can effectively enhance the coercivity of Nd-Fe-B magnets. Although the light rare earth elements can further improve the diffusion efficiency of Dy and Tb, they generally lead to reduced anti-corrosion properties of the magnets. Here, we successfully enhanced the coercivity and corrosion resistance of Nd-Fe-B magnets by GBD of Ni alloyed Pr-Tb-Cu diffusion source. The ternary Pr 35 Tb 35 Cu 30 alloy diffusion increased the coercivity of Nd-Fe-B magnet from 1330 to 1923 kA/m, but the coercivity was enhanced to 1990 kA/m by Pr 35 Tb 35 Ni 15 Cu 15 alloy diffusion. The formation of high-anisotropy (Nd,Tb) 2 Fe 14 B phase is the main reason for the coercivity improvement. Partially substitution of Cu by Ni can reduce the melting point of Pr 35 Tb 35 Cu 30 alloy, which is effective in promoting the infiltration of Tb. Cu is also a necessary element for improving the diffusion process, and the diffusion of Pr 35 Tb 35 Ni 30 alloy only increased the coercivity to 1747 kA/m. More interestingly, the diffusion of Pr 35 Tb 35 Ni 15 Cu 15 also enhanced the chemical stability of the magnet in NaCl solution. Scanning Kelvin probe force microscopy characterized that the modified GB phase in Pr 35 Tb 35 Ni 15 Cu 15 treated magnet exhibits a higher Volta potential than the 2:14:1 main phase, which is the main reason for the improved corrosion resistance. The present results suggest that the non-rare earth elements in the alloy diffusion sources play important roles in enhancing both coercivity and corrosion resistance of Nd-Fe-B magnets. • Both coercivity and corrosion resistance of Nd-Fe-B magnet are distinctly enhanced. • Ni alloying lowers melting point of Pr-Tb-Cu and deepens the infiltration of Tb. • Ni alloying results in higher potential of grain boundary than that of 2:14:1 phase. • Scanning Kelvin probe force microscopy was used to measure local potential. [ABSTRACT FROM AUTHOR]