[O5] RE-Fe-B Magnets III

9:40 AM - 9:55 AM JST(12:40 AM - 12:55 AM UTC)

[O5-3] High Coercivity of 2.8 T in HRE-Free Anisotropic Magnets by Microstructure Engineering

*Xin Tang¹, H. Sepehri-Amin¹, T. Kajiwara², H. Miyawaki², T. Ohkubo¹, K. Hono¹ (1. National Institute for Materials Science (Japan), 2. Daido Corporate Research & Development Center, Daido Steel Co., Ltd., Nagoya 457-8545, Japan (Japan))

Keywords:

Nd-Fe-B,coercivity,heavy rare earth free

Currently, (Nd,Dy)-Fe-B-based anisotropic magnets used in major applications exhibit coercivity above 2.8 T, typically containing more than 8 wt.% Dy [1]. However, the scarcity of Dy in the earth's crust has driven efforts to develop Dy-free Nd-Fe-B-based magnets with properties comparable to conventional (Nd,Dy)-Fe-B sintered magnets. One approach to enhancing coercivity without reliance on Dy is by reducing grain size [1]. Recent studies on hot-deformed magnets have shown that, despite achieving ultra-fine grain sizes, coercivity remains limited to 2 T due to strong exchange coupling between grains [2]. This suggests that further coercivity enhancement could be achieved by a well-established grain boundary diffusion process [3]. In this study, we applied a heavy rare-earth-free grain boundary diffusion process to ultrafine-grained hot-deformed magnets and demonstrated that an ultra-high coercivity of 2.8 T can be achieved without Dy through microstructure engineering.The starting magnet, with a nominal composition of Nd_13.5Fe_bal.Co_3.5Ga_0.57B_5.64 (at.%), was processed via hot extrusion. Its microstructure, shown in Figure 1(a), reveals a grain size of approximately 80 nm (as seen in the inset). In the as-deformed state, the grain boundary phase exhibits weak contrast, indicative of an Fe-rich intergranular phase, which limits coercivity to 2 T. After the diffusion process, an Fe-lean intergranular phase with bright contrast is formed, significantly enhancing coercivity to 2.8 T as shown in Figure 1(b). Further analysis, benchmarked against 8-9 wt.% Dy-containing sintered magnets, confirms that the Dy-free magnets developed in this study exhibit comparable performance at elevated temperatures. This work demonstrates the potential of ultrafine-grained Dy-free magnets as viable alternatives to commercial 8 wt.% Dy-containing sintered magnets.
References
[1] K. Hono et al, Scr. Mater. 67 (2012) 530.
[2] N. Kulesh et al, Acta Mater. 276 (2024) 120159.
[3] H. Sepehri-Amin et al Scr. Mater. 63, 1124-1127 (2010).

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