(ESICMM-G8 Symposium on Next Generation Permanent Magnets, Tsukuba, 2015)
Computational approaches to elucidate the roles of grain boundary phase on magnetic property of NdFeB hard magnet


Toshiyuki Koyama*1), Taichi Abe2), Yuhki Tsukada1) and Yoshinao Kobayashi3)

1) Department of Materials, Physics and Energy Engineering, Nagoya University
2) Environment and Energy Materials Division, National Institute for Materials Science
3) Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology
 　Corresponding author*: koyama@numse.nagoya-u.ac.jp

Abstract:

  It is widely known that Nd-rich amorphous phase forms at the grain boundary (GB) region in Nd2Fe14B polycrystalline microstructure during optimization annealing, and the coercive force depends crucially on the Nd concentration in the amorphous GB phase. In this study, we attempted for understanding the thermodynamic nature of Nd-rich amorphous phase based on the Calphad method and phase-field simulation, and calculated the effect of Nd-rich amorphous phase on the coercive force by micromagnetics simulation. Results obtained are as follows: (1) The metastable Nd-Fe-B ternary phase diagram is calculated based on the conventional Calphad method, where liquid phase and two Nd-Fe-B ternary compounds including Nd2Fe14B phase are considered. The metastable phase diagram reveals that Nd concentration in the amorphous GB phase, which has been measured in experiment, is reasonably explained if we assume the Gibbs energy of amorphous phase to be that of liquid phase. (2) The Nd concentration in amorphous GB phase was analyzed based on the modified grain-boundary-phase (GBP) model. The phenomenological segregation behavior of micro-alloying elements at grain boundary region has been theoretically explained based on the GBP model proposed by M.Hillert [1]. We modified the GBP model by considering the composition gradient energy through the phase-field methodology, then the extended GBP model was applied to the Nd segregation behavior in Nd2Fe14B hard magnet. Figure 1 is the typical example of the calculation results, which shows the Nd segregation at grain boundary region (center part of each figure) dependent on the average composition of Nd in alloy. Note that the Nd segregation is drastically affected by the tiny change of average composition of Nd. (3) The magnetic property of Nd2Fe14B polycrystalline microstructure including GB phase was calculated by the image-based micromagnetics simulation based on the Landau–Lifshitz–Gilbert equation, where the maximum magnetization of GB phase was systematically changed. The behavior of magnetic domain boundary migration during magnetization and the value of coercive force depend on the magnetic property of GB phase, i.e. the coercive force increases with decreasing maximum magnetization of GB phase.
Fig.1 Nd segregation at grain boundary region (center part of each figure). The segregation tendency is drastically affected by the average composition of Nd in alloy.
[1] M.Hillert：Chapter 1 in Lectures on the theory of phase transformations, TMS, (1999).