Presentation Information
[O14-2]Micromagnetic Simulations of Hot-deformed Nd-Fe-B Magnets Subjected to Eutectic Grain Boundary Diffusion Process
*Anton Bolyachkin1, Xin Tang1, Nikita Kulesh1, Hossein Sepehri-Amin1 (1. National Institute for Materials Science (Japan))
Keywords:
Micromagnetic simulations,Nd-Fe-B magnets,grain boundary diffusion
The grain boundary diffusion process (GBDP) using a low melting point alloy is recognized as an efficient technique for engineering the microstructure of hot-deformed Nd-Fe-B magnets aiming for high coercivity [1,2]. GBDP directly addresses the most critical microstructural feature affecting the coercivity that is a thin intergranular phase (IGP). This treatment is usually performed by coating a magnet with the eutectic alloy, e.g., Nd70Cu30, and then annealing at temperature 500-600 °C. During this process, the Nd-rich liquid phase penetrates from the surface to the interior of the magnet through a network of triple junctions. At the same time, the liquid phase infiltrates between the grains, modifying the chemical composition of the IGP. Figure 1(a) shows a typical microstructure of a hot-deformed Nd-Fe-B magnet with an improved magnetic isolation of the Nd2Fe14B grains from each other after GBDP. This isolation is at the expense of the magnet’s remanence. Interestingly, while some grains are well separated by the thick Nd-rich phase, which is supposed to be nonmagnetic (Fig. 1b), other grains are still in contact through the thin magnetic IGP with Fe content similar to that in the IGP before GBDP (Fig. 1c). Thus, the grains are expected to be still exchange coupled, although the contact area for coupling tend to be decreased by GBDP. The question arises how the coercivity is influenced by the remaining exchange coupling and observed change of grain packing density, that apparently affects the magnetostatic interaction between grains. In this work, micromagnetic simulations were employed to address this question.
To imitate the microstructural transformations in hot-deformed Nd-Fe-B magnets after the Nd-based GBDP, a series of micromagnetic models was developed with a gradually increasing volume fraction of the nonmagnetic region, representing the infiltrated Nd-rich phase. One of these models is shown in Fig. 1d, demonstrating that there were also the regions of thin magnetic IGP which number and area decreased upon the GBDP. The mean grain size and aspect ratio were maintained constant. Micromagnetic simulations were used to analyze the tradeoff between coercivity and remanence as the magnetic properties of IGP (magnetization and exchange stiffness) and the volume fraction of Nd-rich phase were varied systematically (Fig. 1e) [3]. These results allow to define realistic limits for coercivity achievable in hot-deformed Nd-Fe-B magnets via the Nd-based GBDP. The coercivity limits strongly depend on the IGP magnetization, which was estimated to be 0.9 ± 0.1 T by reproducing experimental Mr vs. Hc data from the literature. In this report, further extension of this study will be presented, accounting for the core-shell grain structure and covering other aspects of coercivity such as its angular and temperature dependencies.
The support by the MEXT (JPMXP1122715503) and JSPS (JP23H01674) is acknowledged.
References
[1] Hioki, Sci. Technol. Adv. Mater. 22 (2021) 72.
[2] Sepehri-Amin et al., Acta Mater. 61 (2013) 6622.
[3] Bolyachkin et al., Scripta Mater. 247 (2024) 116095.
To imitate the microstructural transformations in hot-deformed Nd-Fe-B magnets after the Nd-based GBDP, a series of micromagnetic models was developed with a gradually increasing volume fraction of the nonmagnetic region, representing the infiltrated Nd-rich phase. One of these models is shown in Fig. 1d, demonstrating that there were also the regions of thin magnetic IGP which number and area decreased upon the GBDP. The mean grain size and aspect ratio were maintained constant. Micromagnetic simulations were used to analyze the tradeoff between coercivity and remanence as the magnetic properties of IGP (magnetization and exchange stiffness) and the volume fraction of Nd-rich phase were varied systematically (Fig. 1e) [3]. These results allow to define realistic limits for coercivity achievable in hot-deformed Nd-Fe-B magnets via the Nd-based GBDP. The coercivity limits strongly depend on the IGP magnetization, which was estimated to be 0.9 ± 0.1 T by reproducing experimental Mr vs. Hc data from the literature. In this report, further extension of this study will be presented, accounting for the core-shell grain structure and covering other aspects of coercivity such as its angular and temperature dependencies.
The support by the MEXT (JPMXP1122715503) and JSPS (JP23H01674) is acknowledged.
References
[1] Hioki, Sci. Technol. Adv. Mater. 22 (2021) 72.
[2] Sepehri-Amin et al., Acta Mater. 61 (2013) 6622.
[3] Bolyachkin et al., Scripta Mater. 247 (2024) 116095.
