Dy-based dual-alloy grain boundary diffusion for sintered Nd-Fe-B magnets with improved magnetic performance and corrosion resistance

Abstract

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.