[O2] RE-Fe-B Magnets II

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[O2-5] Two-Step Grain Boundary Diffusion of Dy/Tb-Nd-Cu for Enhanced Coercivity and Thermal Stability in Nd-Fe-B Hot-Deformed Magnets with Reduced Tb Content

*Zulfa Hilmi Kautsar¹, Xin Tang¹, Keiko Hioki², Hossein Sepehri-Amin^1,3, Tadakatsu Ohkubo¹, Kazuhiro Hono^1,3 (1. Research Center for Magnetic and Spintronic Materials, National Institute for Materials Science (Japan), 2. Daido Corporate Research & Development Center, Daido Steel Co., Ltd. (Japan), 3. Graduate School of Science and Technology, University of Tsukuba (Japan))

Keywords:

Nd-Fe-B,Hot-deformed magnets,GBDP,Two-step diffusion process,Microstructure

Nd-Fe-B-based permanent magnets are widely used in electric traction motors for electric and hybrid electric vehicles due to their high maximum energy product. For these applications, the magnet must maintain coercivity above 0.8 T at temperatures exceeding 150 °C. Conventionally, this is achieved by either increasing room-temperature coercivity or reducing the temperature coefficient of coercivity (β), typically through the partial substitution of Nd with heavy rare earth elements (HREs) such as Dy and Tb. However, the limited availability and high cost of HREs, especially Tb, have driven efforts to minimize their use. In recent years, the grain boundary diffusion process (GBDP) has emerged as a promising technique for reducing HRE consumption by enabling more efficient utilization of these elements. Due to the higher anisotropy field of the Tb₂Fe₁₄B phase compared to Dy₂Fe₁₄B, Tb-GBDP generally yields higher coercivity than Dy-GBDP [1–3]. However, given that Tb is significantly scarcer and more expensive than Dy, developing an HRE-GBDP approach that achieves comparable properties to Tb-GBDP while using less Tb is highly desirable. In this study, a two-step eutectic GBDP [4] was applied to a 5 mm thick Nd-Fe-B hot-deformed magnet, with an initial diffusion using Nd₅₀Dy₃₀Cu₂₀, followed by a second diffusion step with Nd₅₀Tb₃₀Cu₂₀. This treatment led to a significant increase in coercivity from 1.1 T to 2.54 T, with only a slight decrease in remanent magnetization from 1.50 T to 1.33 T. The β improved from -0.47%/°C to -0.36%/°C, resulting in a high coercivity of 1.29 T at 150 °C (Fig.1). This substantial enhancement in coercivity is attributed to the partial formation of Dy-rich and Tb-rich double shells on the surface of platelet-shaped Nd₂Fe₁₄B grains while maintaining their ultra-fine grain size. Notably, this two-step process achieves coercivity and remanent magnetization comparable to those obtained using 9 wt.% Nd₅₀Tb₃₀Cu₂₀ GBDP, while offering an even more favorable β value, indicating superior thermal stability. These findings demonstrate an alternative approach to develop high coercivity and thermal stability Nd-Fe-B permanent magnets with reduced reliance on Tb.

References
[1] S. Hirosawa, Y. Matsuura, H. Yamamoto, S. Fujimura, M. Sagawa, H. Yamauchi, J. Appl. Phys. 59 (1986) 873–879.
[2] J. Li, L. Liu, H. Sepehri-Amin, X. Tang, T. Ohkubo, N. Sakuma, T. Shoji, A. Kato, T. Schrefl, K. Hono, Acta Mater. 161 (2018) 171–181.
[3] Z. Liu, J. He, R. V. Ramanujan, Mater. Des. 209 (2021) 110004.
[4] X. Tang, J. Li, H. Sepehri-Amin, T. Ohkubo, K. Hioki, A. Hattori, K. Hono, Acta Mater. 203 (2021) 116479.

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