Presentation Information
[O2-4]A comparative study on shell formation and coercivity improvement of Pr-free and Pr-alloyed Nd-Fe-B sintered magnets during grain boundary diffusion process with low-melting Pr-Cu-Al-Ga alloy
*Sujin Lee1, Ganghwi Kim2, Ki-Suk Lee2, Sumin Kim1, Tae-Hoon Kim1, Kyoung-Hoon Bae3, Dong-Hwan Kim3, Jung-Goo Lee1 (1. Korea Institute of Materials Science (Korea), 2. Ulsan National Institute of Science and Technology (Korea), 3. Star Group Ind. CO., Ltd. (Korea))
Keywords:
Nd-Fe-B sintered magnets,Grain boundary diffusion process,Chemically induced liquid film migration,Heavy rare-earth free,Pr-rich shell
The grain boundary diffusion process (GBDP) using heavy rare-earth elements (HRE = Dy, Tb) has been widely employed to resolve the thermal demagnetization problem by forming the high anisotropy field HRE-rich shells around Nd2Fe14B grains1). However, due to the limited availability and high-cost of HRE have led to strict restrictions on its usage2). As an alternative, the GBDP using Pr, a light rare-earth element, has demonstrated its effectiveness in enhancing the coercivity of Nd-Fe-B magnets without relying on the HRE. The origins of coercivity enhancement of the Pr-GBDP are the formation of high-anisotropy Pr-rich shells, which suppresses reverse domain nucleation at the grain boundary region, and the dissolution of Pr into the Nd-rich grain boundary phases, which weaken the exchange coupling between Nd2Fe14B3). The coercivity improvement of Pr-GBDP magnets is strongly influenced by the microstructure of Pr-rich shell, including its concentration, thickness, and distribution4). Therefore, in order to develop highly efficient Pr-GBDP, a deep understanding on the mechanism of Pr-rich shell formation within the Nd-Fe-B magnets is essential. Despite the fact that commercial Nd-Fe-B magnets typically contain 5~8 wt.% Pr, the impact of pre-alloyed Pr on the microstructure of Pr-rich shell remains unclear. Thus, this work examines magnetic and microstructural changes of Pr-free and Pr-alloyed Nd-Fe-B magnets during the GBDP using low-melting Pr-Cu-Al-Ga (PCAG) alloy as the diffusion source.
The Pr-free [Nd26.8Dy3Al0.35B0.9Co1.0Cu0.2Ga0.3Zr0.1Febal. (wt.%)] and Pr-alloyed [Nd20 Pr6.8Dy3Al0.35B0.9Co1.0Cu0.2Ga0.3Zr0.1Febal. (wt.%)] base magnets were prepared. The coating amount of PCAG source for the base magnets were 0 (base), 4 (PCAG-4), 8 (PCAG-8), 12 (PCAG-12) wt.%. The coated magnets were then heat treated at 970℃ for 15 h for the GBD of PCAG, followed by post-diffusion annealing at 550℃ for 2 h. Fig. 1(a) shows the variations in magnetic properties of Pr-free and Pr-alloyed magnets as a function of the PCAG diffusion amount. The Pr-free magnets exhibited a more substantial increase in coercivity compared to the Pr-alloyed magnets, despite a comparable reduction in remanence. Specifically, after the PCAG-12 GBDP, the coercivity of Pr-free magnet increased from 2.15 to 2.58 T (coercivity gain : 0.43 T), whereas that of Pr-alloyed magnet improved from 2.25 T to 2.46 T (coercivity gain : 0.21 T). These results demonstrate that the PCAG-GBDP enhances the coercivity of Pr-free magnets more effectively than that of Pr-alloyed one. Fig. 1(b) shows the back scattered electron (BSE) images and electron probe micro-analyzer (EPMA) maps for Pr taken from the surface region (50 μm depth) of the PCAG-12 Pr-free and Pr-alloyed magnets. Compared to the Pr-alloyed magnet, the formation of shallower Pr-rich shell is observed at the surface region of the Pr-free magnets after the PCAG-GBDP. This indicates that fewer Pr atoms were consumed for shell formation at the surface region of Pr-free magnets, allowing for deeper diffusion of Pr into the center of the magnets. Consequently, the Pr-free magnets exhibited a greater GBD depth than the Pr-alloyed magnets. The micromagnetic simulation demonstrates that the increased diffusion depth of Pr is a dominant factor contributing to the superior coercivity gain of Pr-free magnets by the PCAG-GBDP. In this presentation, the influence of pre-alloyed Pr on the thickness and distribution of Pr-rich shell will be discussed in detail in terms of the chemically induced liquid film migration (CILFM)5). Based on our findings, we will propose a guide to develop highly efficient Pr-based GBDP.
References
1) H. Sepehri-Amin et al., Acta Mater. 61 (2013) 1982.
2) H. X. Zeng et al., J. Mater. Sci. Technol. 36 (2020) 50.
3) M. Tang, et al., Scr. Mater. 117 (2016) 60.
4) Z. Wei et al., J. Magn. Magn. Mater. 589 (2024) 171593.
5) T. H. Kim et al., Scr. Mater. 178 (2020) 433.
The Pr-free [Nd26.8Dy3Al0.35B0.9Co1.0Cu0.2Ga0.3Zr0.1Febal. (wt.%)] and Pr-alloyed [Nd20 Pr6.8Dy3Al0.35B0.9Co1.0Cu0.2Ga0.3Zr0.1Febal. (wt.%)] base magnets were prepared. The coating amount of PCAG source for the base magnets were 0 (base), 4 (PCAG-4), 8 (PCAG-8), 12 (PCAG-12) wt.%. The coated magnets were then heat treated at 970℃ for 15 h for the GBD of PCAG, followed by post-diffusion annealing at 550℃ for 2 h. Fig. 1(a) shows the variations in magnetic properties of Pr-free and Pr-alloyed magnets as a function of the PCAG diffusion amount. The Pr-free magnets exhibited a more substantial increase in coercivity compared to the Pr-alloyed magnets, despite a comparable reduction in remanence. Specifically, after the PCAG-12 GBDP, the coercivity of Pr-free magnet increased from 2.15 to 2.58 T (coercivity gain : 0.43 T), whereas that of Pr-alloyed magnet improved from 2.25 T to 2.46 T (coercivity gain : 0.21 T). These results demonstrate that the PCAG-GBDP enhances the coercivity of Pr-free magnets more effectively than that of Pr-alloyed one. Fig. 1(b) shows the back scattered electron (BSE) images and electron probe micro-analyzer (EPMA) maps for Pr taken from the surface region (50 μm depth) of the PCAG-12 Pr-free and Pr-alloyed magnets. Compared to the Pr-alloyed magnet, the formation of shallower Pr-rich shell is observed at the surface region of the Pr-free magnets after the PCAG-GBDP. This indicates that fewer Pr atoms were consumed for shell formation at the surface region of Pr-free magnets, allowing for deeper diffusion of Pr into the center of the magnets. Consequently, the Pr-free magnets exhibited a greater GBD depth than the Pr-alloyed magnets. The micromagnetic simulation demonstrates that the increased diffusion depth of Pr is a dominant factor contributing to the superior coercivity gain of Pr-free magnets by the PCAG-GBDP. In this presentation, the influence of pre-alloyed Pr on the thickness and distribution of Pr-rich shell will be discussed in detail in terms of the chemically induced liquid film migration (CILFM)5). Based on our findings, we will propose a guide to develop highly efficient Pr-based GBDP.
References
1) H. Sepehri-Amin et al., Acta Mater. 61 (2013) 1982.
2) H. X. Zeng et al., J. Mater. Sci. Technol. 36 (2020) 50.
3) M. Tang, et al., Scr. Mater. 117 (2016) 60.
4) Z. Wei et al., J. Magn. Magn. Mater. 589 (2024) 171593.
5) T. H. Kim et al., Scr. Mater. 178 (2020) 433.
