Research Center for Magnetic and Spintronic Materials

(ESICMM-G8 Symposium on Next Generation Permanent Magnets, Tsukuba, 2015)

Structure and chemistry of the grain boundary phase in Nd-rich Ga-doped Nd-Fe-B sintered magnet

T. T. Sasaki¹⁾, T. Ohkubo¹⁾, Y. Takada²⁾, T, Sato²⁾,
Y. Kaneko²⁾, A. Kato³⁾, K. Hono¹⁾

1) ESICMM, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, 305-0047, Japan
2) Toyota Central Research Institute, 41-1, Nagakute, 480-1192, Japan
3) Toyota Motor Corporation, Susono, 410-1193, Japan

Abstract:

The coercivity of Nd-Fe-B permanent magnets varies depending on grain size and the magnetism of thin Nd-rich grain boundary (GB) phase. Recent analytical studies on the chemical composition and the magnetism of the Nd-rich GB phase in commercial magnets have convincingly shown that the GB phase is ferromagnetic containing Fe about 65at.% [1,2]. This suggests that further coercivity increase is anticipated by the formation of non-ferromagnetic GB phase with high Nd content. Recently, Showa Denko has developed a Nd-rich Ga-doped Nd-Fe-B strip cast alloy, from which sintered magnets with the coercivity of μ₀H_c=1.8 T can be processed with the standard powder metallurgy process [3]. It is intriguing to know how such a high coercivity can be achieved without grain size refinement. In this study, the authors investigated detailed microstructure of the Nd-rich Ga-doped sintered magnets using aberration corrected STEM/EDS, SEM, and three-dimensional atom probe. The GB phase that is formed uniformly by the addition of Ga contains no ferromagnetic elements, indicating that the Nd₂Fe₁₄B grains are fully exchange decoupled. The process-dependent microstructure feature of this alloy will also be reported correlating with the coercivity. This work will give new insights on how to enhance the coercivity of Nd-Fe-B without using heavy rare earth elements and costly grain size refinement process.

[1] H. Sepehri-Amin et al., Acta Mater., 60 (2012) 819-830.
[2] Murakami et al., Acta Mater., 71 (2014) 370-379.
[3] T. Yamazaki et al., Annual Congress of the Japan Society of Powder and Powder Metallurgy (2013) 79