Grain boundary diffusion behaviors of hot-deformed magnets in different directions with additional pressure

Abstract

The diffusion behaviors of hot-deformed magnets were investigated in two different directions with additional pressure. For magnets diffusion along the direction parallel to the C-axis (∥C magnets) and magnets diffused perpendicular to the C-axis (⊥C magnets), the coercivity increased by 3.66 kOe and 4.94 kOe, while the remanence decreased by 1.39 kGs and 1.04 kGs, respectively. The coercivity and remanence are both higher in ⊥C magnets as compared to ∥C magnets. SEM indicates that the orientation in ⊥C magnets is better than in ∥C magnets, which contributes to the more excellent remanence. The SEM also suggests the distribution of the rare-earth-rich phase was more uniform in ⊥C magnets. EPMA shows that the diffusion depth of ⊥C magnets reaches around 1500 μm, while ∥C magnets is less than 1250 μm. Both the homogeneous distribution of the rare-earth-rich phase and deeper diffusion depth are responsible for the improved coercivity of ⊥C magnets. EDS shows that the Nd70Cu30 preferentially entered the interior of the magnets through the interface formed by the fine grains of melt-spun ribbon edges for ∥C magnet during grain boundary diffusion process. But the ⊥C magnet did not show this preference during diffusion. The grain morphology and diffusion resistance of magnets in different directions is the fundamental cause for the difference of Cu distribution in ∥C magnet and ⊥C magnet. The initial magnetization curves suggests that the coercivity mechanism of the initial magnets combines nucleation and domain wall pinning, while the domain wall pinning is enhanced for the diffused magnets. The optimum performances with Br = 13.21 kGs, Hcj = 18.23 kOe, and (BH)max = 43.01 MGOe were achieved in ⊥C magnets.