36 research outputs found
Coercivity enhancement of sintered Nd-Fe-B magnets by chemical bath deposition of TbCl3
No full textThe chemical bath deposition (CBD) and the grain boundary diffusion method were combined to diffuse the heavy rare earth for obtain the thick magnets with high coercivity and low heavy rare earth. The jet mill powders were soaked into the alcohol solution of 0.2 wt. % TbCl3. A thin layer of TbCl3 was wrapped to the surface of (PrNd)(2)Fe14B powder particles. The coercivity of magnet is increased from 11.89 kOe to 14.72 kOe without significant reduction of remanence after grain boundary diffusion in the sintering and the annealing processes. The temperature coefficients of the remanence and the coercivity are improved by the substitution of PrNd by Tb in the surface of grains. The highly accelerated temperature/humidity stress test (HAST) results indicate that the CBD magnet has poor corrosion resistance, attributing to the present of Cl atoms in the grain boundaries. (C) 2014 AIP Publishing LLC
Highly efficient Tb-utilization in sintered Nd-Fe-B magnets by Al aided TbH2 grain boundary diffusion
No full textAl aided TbH2 grain boundary diffusion (GBD) was applied to sintered Nd-Fe-B magnet in comparison with the TbH2 GBD magnet. The coercivity was enhanced from 13.73 kOe to 23.29 kOe, higher than the 22.26 kOe of TbH2 GBD magnet. The Tb content in the bulk magnet was reduced to 0.47 wt%, much lower than the 1.85 wt% of the TbH2 GBD magnet. Microstructure analysis showed that continuous grain boundary phases were formed and completely enveloped the grains with thin Tb-rich shells, which can be attributed to the reduced Tb content in diffusion source and the promoted GBD aided by Al. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved
Coercivity enhancement in Dy-free sintered Nd-Fe-B magnets by effective structure optimization of grain boundaries
No full textIn this study, we presented an effective approach for the substantial coercivity enhancement of Dy-free sintered Nd-Fe-B magnets with Ga-doping. The coercivity was substantially enhanced from 10.1 kOe to 19.8 kOe by effective microstructure optimization and phase constitution regulation in antecedent alloys. A good isolation of 2:14:1 grains by Nd-rich intergranular phase is obtained in final magnets accompanying with the formation of intermetallic Nd6Fe13Ga phase. However, when dehydrogenating above 500 degrees C, masses of preformed Nd6Fe13Ga phases are detected in the initial strip casting alloys and the homologous magnet suffers an unanticipated degradation of coercivity. Magnetic domain observation and thermal magnetic analysis imply a pronounced difference in the magnetism of grain boundary (GB) phases, which closely associated with the composition homogeneity of Nd-rich phases in antecedent alloys. The mechanism of the coercivity variation in the sintered magnets with different GBs magnetism is discussed based on magnetic behavior analysis. (C) 2017 Elsevier B.V. All rights reserved
Whole process metallurgical behavior of the high-abundance rare-earth elements LRE (La, Ce and Y) and the magnetic performance of Nd0.75LRE0.25-Fe-B sintered magnets
No full textBalanced usages of the high-abundance rare-earth elements, such as La, Ce and Y, have attracted tremendous interests of the industrial and scientific societies because of the economic and environmental merits. One of the key issues is of understanding the whole process metallurgical behavior of the high-abundance rare-earth elements in order to optimize the microstructures for a high magnetic performance of the RE-Fe-B sintered magnets, which contain a high concentration of the high-abundance rare-earth elements. In this work, we systematically characterized the metallurgical behaviors during the whole process of strip casting, sintering and annealing for the magnets (Nd0.25LRE0.25)(30.5)FebalAl0.1Cu0.1B1 (LRE = La, Ce and Y, wt. %). During the strip casting (SC), La and Ce mainly distribute in the grain boundary phase while Y distributes uniformly in the 2:14:1 matrix phase grains. Under the sintering and annealing, La is squeezed out from the 2:14:1 matrix phase to segregate in the grain boundaries, while Ce and Y migrate from the grain boundary to the 2:14:1 matrix phase. The crystalline structures of matrix phase and grain boundary phase of Nd-La/Ce/Y-Fe-B are tetragonal phase (space group P42/mnm) and hcp-Re2O3. The investigations of the grain boundary of Nd-La/Ce/Y-Fe-B magnets demonstrate that the grain boundary of the magnet with Ce is thicker than that of magnets with La and Y. The different metallurgical behaviors of elements La, Ce and Y are attributed to the different solidification temperature and substitution energy of Re2Fe14B phases. Core-shell grain structure was formed in the case of Y-containing magnets. Elemental analysis demonstrates that Y prefers to locate in the core of the grain boundary and Nd distributes in the shell of the grain boundary. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved
Coercivity enhancement of Ce-Fe-B sintered magnets by low-melting point intergranular additive
No full textCe-Fe-B sintered magnets with enhanced coercivity were prepared by the powder metallurgy method. The mechanism of the coercivity enhancement in Ce-Fe-B sintered magnets with the low-melting point intergranular additive was discussed in details. It was speculated that the low coercivity of Ce-Fe-B sintered magnet was related to the irregular sharps and relatively low magneto-anisotropy field of the matrix phase. After introducing a 20 wt.% Nd-based intergranular additive, the coercivity markedly increased from 108 Oe to 2560 Oe due to the formation of thin and continuous grain boundary layers and the surface modification of the matrix phase grains. Additionally, the formation of the high anisotropy field (Nd,Ce)(2)Fe14B shell was beneficial to the increase of the coercivity as well. This work suggested that adding low-melting point intergranular additives was effective to fabricate the practical Ce-Fe-B sintered magnets
Effect of diffusing TbF3 powder on magnetic properties and microstructure transformation of sintered Nd-Fe-Cu-B magnets
No full textThe coercivity of sintered Nd-Fe-Cu-B magnets is markedly enhanced from 12.57 to 21.70 kOe while the remanence decreases from 13.80 to 13.49 kGs by grain boundary diffusion of TbF3 powder for 2 h. Microstructure analysis suggests that, during the diffusion process, F diffuses into the magnets easily and forms a new F-rich phase. The enrichment of F in grain boundary near the surface leads to the Cu movement into the interior and the Cu reduction in the surface of magnets. Diffusion of Tb leads to an increase of local total rare earth elements content. Under the combined effect of Cu reduction and increase of local total rare earth elements content, grain growth area is formed and further diffusion is suppressed. That excessive Tb diffuses into matrix phase leads to a decrease in remanence. When the grain growth area is removed, the deterioration of remanence recovers to 13.80 kGs without any reduction of coercivity. (C) 2017 Elsevier B.V. All rights reserved
Improved corrosion resistance and thermal stability of sintered Nd-Fe-B magnets with holmium substitution
No full textThe effects of Ho substitution for Nd on the microstructure, corrosion resistance and thermal stability of the Nd-Fe-B magnets were investigated. The (Nd,Ho)-O phase was formed with increasing Ho substitution. The results of potentiodynamic polarization and highly accelerated stress test show improved corrosion resistance with increasing Ho substitution. The optimum mass loss 0.29 mg/cm(2) is achieved. Moreover, the average temperature coefficients for remanence and coercivity in the range of 25-150 degrees C are both closer to zero, indicating improved thermal stability. The mechanisms for the improved corrosion resistance and thermal stability are discussed in relation to the microstructure featuring the (Nd,Ho)-O phase. (C) 2018 Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths
Study on Ultrafine-Grained Sintered Nd-Fe-B Magnets Produced From Jet-Milled HDDR Powders
No full textUltrafine-grained sintered Nd-Fe-B magnets were produced from jet-milled hydrogenation-disproportionation-desorption-recombination powders in a proper sintering condition. The coercivity of similar to 15 kOe was obtained by sintering below 950 degrees C, and the average grain size was <1 mu m. For the newly fine-grained magnets, a higher magnetic field is needed to magnetize them, giving rise to a two-step initial magnetization curve involving domain wall pinning at the grain boundary phase. Microstructure observation indicated that the Nd-rich phases did not encapsulate the 2:14:1 phases effectively, resulting in defects and interactions between adjacent 2:14:1 grains, which are responsible for the relatively low coercivity of the fine-grained magnets
Coercivity Enhancement of Dy-Free Sintered Nd-Fe-B Magnets by Grain Refinement and Induction Heat Treatment
No full textUsing conventional N-2 gas jet milling, a coercivity of 19.01 kOe was obtained in Dy-free sintered Nd-Fe-B magnets with the grain size refined to 2.6 mu m. Through the high-frequency induction heat treatment (HIHT) at 600 degrees, the coercivity was enhanced to 20.56 kOe, while the remanence remained unchanged. The transmission electron microscope observation showed that the grain boundaries were broadened through the HIHT process. The increasement of coercivity may be related to the strong stirring effect of induction vortex, which facilitates the continuous distribution of the Nd-rich grain boundary phase and thus enhances the magnetic insulation between the matrix grains
