The formation of continuous thin layers of a Nd-rich amorphous phase surrounding Nd2Fe14B grains is the key microstructural feature of high-coercivity Nd–Fe–B magnets. Contrary to popular belief, a ...quantitative 3D atom probe investigation suggests that the thin grain boundary phase is ferromagnetic in both sintered and hydrogen disproportionation desorption recombination magnets. Model experiments using Nd–Fe–B thin films indicate that the intrinsic coercivity of μ0Hc=3T with improved temperature dependence is achievable by magnetically isolating Nd2Fe14B grains of ∼100nm.
The coercivity of the sintered Nd-Fe-B permanent magnets prepared by the dual-main-phased (DMP) process can be significantly enhanced. This paper provides a comprehensive review of this method. The ...methodology and process of the DMP method are outlined, along with its mechanisms and advantages in improving magnet performance. Furthermore, this study highlights the outstanding performance of DMP magnets in terms of mechanical properties and corrosion resistance. Lastly, the future development trends and research directions for the DMP method are explored, such as optimizing process parameters, exploring novel alloy designs, and developing higher-performance magnet materials.
Many new applications of Nd–Fe–B magnets have emerged in the past two decades. Recently, motor application has taken over VCM position which was the dominant application in Nd–Fe–B sintered magnet ...previously. Motor market is expected to expand in future because of strong energy saving requirements from the automobile and electric appliances markets.
Magnetic property improvement in residual magnetization (Br) and coercive force (
H
cJ) helped the rapid growth of the motor market. Magnetic properties are still rapidly improving and many technologies for powder alignment are also being invented. A new world record of magnetic properties with
B
r=1.555
T, (BH)
max=474
kJ/m
3
H
cJ=653
kA/m was established by NEOMAX Co. Ltd.
The dependence of the coercivity of hot-deformed anisotropic Nd–Fe–B magnets on grain size has been studied by processing the magnets at different temperatures. Higher coercivity was obtained in ...fine-grained magnets processed at lower temperature (700°C), in which intergranular phases formed uniformly along the grain boundaries. On the other hand, large Nd-rich triple-junction phases were frequently observed in the larger grain-sized magnets processed at higher temperature (900°C). Three-dimensional atom probe analyses showed that the Nd content in the intergranular phase decreased as the processing temperature increased. The origin of the coercivity difference in these hot-deformed magnets processed at different temperatures is discussed based on micromagnetic analysis of the observed microstructures complemented with finite-element micromagnetic simulations.
The grain boundary diffusion process using an Nd70Cu30 eutectic alloy has been applied to hot-deformed anisotropic Nd–Fe–B magnets, resulting in a substantial enhancement of coercivity, from 1.5 T to ...2.3 T, at the expense of remanence. Scanning electron microscopy showed that the areal fraction of an Nd-rich intergranular phase increased from 10% to 37%. The intergranular phase of the hot-deformed magnet initially contained ∼55 at.% ferromagnetic element, while it diminished to an undetectable level after the process. Microscale eutectic solidification of Nd/NdCu as well as a fine lamellae structure of Nd70(Co,Cu)30/Nd were observed in the intergranular phase. Micromagnetic simulations indicated that the reduction of the magnetization in the intergranular phases leads to the enhancement of coercivity in agreement with the experimental observation.
Nd–Fe–B permanent magnets have been coated with 0.6wt.% dysprosium and annealed at various temperatures to study the impact of the temperature-dependent Dy diffusion processes on both the magnetic ...properties and the microstructure. When optimum annealing conditions are applied the Dy processed magnets with initial coercivity of ∼1100kAm−1 yield coercivity increases which can exceed 400kAm−1 without a significant reduction of the remanent magnetic polarization. The improved stability against opposing magnetic fields can be observed up to a depth of ∼3mm along the diffusion direction, restricting the application of the Dy diffusion process to either thin magnets or magnets with tailored coercivity gradients. While in the proximity of the Dy-coated surface, each grain has a Dy-enriched shell with a Dy content of ∼6at.%; the Dy concentration decreases exponentially to ∼1.8at.% after a diffusion depth of 400μm and to ∼1at.% after a diffusion depth of 1500μm, as was found with wavelength dispersive X-ray spectroscopy and scanning transmission electron microscopy–energy dispersive X-ray spectroscopy, respectively. In the vicinity of the Dy-coated surface, the mechanism of the Dy-shell formation is attributed to the melting/solidification of a heavy-rare-earth-rich intermediate phase during high-temperature annealing. This is based on the observation that a constant Dy concentration over the width of the shells was found. Also an epitaxial relation between the Dy-poor core and the Dy-rich shell was observed by electron backscattered diffraction, which is supported by results obtained with Kerr microscopy.
We have characterized the microstructures of as-sintered and optimally post-sinter annealed Nd-rich Ga-doped Nd–Fe–B magnets by scanning electron microscopy (SEM) and aberration-corrected scanning ...transmission electron microscopy (STEM). While the Nd2Fe14B grains in the as-sintered sample with a coercivity of 0.99T are in direct contact with each other, those in the optimally annealed sample with a coercivity of 1.8T are completely enveloped by typically 10-nm-thick Nd-rich phase that contains little Fe. This strongly suggests that the Nd2Fe14B grains in the optimally annealed Nd-rich Ga-doped Nd–Fe–B magnets are exchange decoupled in contrast to those in the commercial sintered magnets.
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The microstructure of hot-deformed Nd–Fe–B permanent magnets with different Nd contents was investigated in order to correlate them with the hard magnetic properties. A thick distinct Nd-rich grain ...boundary (GB) layer was observed in a high Nd content sample by scanning electron microscopy and transmission electron microscopy. Three-dimensional atom probe results showed a significant increase in the Nd content in the GB as the overall Nd content in the alloy increased. We found a clear correlation between the Nd concentration in the GB layer and the coercivity. The mechanism of the coercivity increase is discussed based on the microstructure characterization and micromagnetic simulation results.
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The coercivity of hot-deformed Nd–Fe–B magnet was substantially enhanced from 1.0T to 2.6T by the grain boundary diffusion process using Pr–Cu alloy. However, the temperature ...dependence of coercivity is larger compared to the sample diffusion processed with Nd–Cu. Microstructure studies showed a good isolation of platelet shaped Nd2Fe14B grains by Pr-rich intergranular phase, which explains pronounced coercivity at room temperature. Small portions of Nd2Fe14B become (Nd,Pr)2Fe14B phase, which has a higher anisotropy field compared to that of the Nd2Fe14B phase at room temperature, while it becomes lower than that of the Nd2Fe14B phase above ∼110°C. Co is depleted from the (Nd,Pr)2Fe14B phase, which is considered to cause a slight decrease in Curie temperature. Micromagnetic simulations with the magnetically isolated grains including (Nd,Pr)2Fe14B regions showed that the degradation of thermal stability of coercivity in the Pr–Cu diffusion processed sample is due to the large temperature dependence of anisotropy field in the (Nd,Pr)2Fe14B regions.
The RE6Fe13Ga phase (RE, Rare Earth) exhibits tremendous promise for achieving high-performance magnets. However, the challenge of controlling its formation and distribution hinders improvements in ...coercivity and squareness of Ga-doped magnets. In this study, we present a strategy to regulate the chemical composition of the RE-rich phase (0–9 at.% Cu, without Ga) and subsequently modify the phase transformation pathways of the RE6Fe13Ga phase in Ga-doped magnets by incorporating a Pr-rich Ga-Cu-containing aiding alloy. Remarkable magnetic performance was achieved, with values of Br=13.36 kGs, μ0Hcj=20.82 kOe, (BH)max=43.75 MGOe, and μ0Hk90/μ0Hcj=0.99. Through quantitative analysis of chemical and structural properties and phase fractions at grain boundary (GB) triple junctions, we elucidate the phase transformation pathways in the magnets. The reaction between amorphous RE-Ga-rich phases and the matrix phases, along with partial consumption of the RE-rich phases with the Mn2O3-type structure (space group Ia3¯), led to the formation of the RE6Fe13Ga phase, accompanied by the presence of an amorphous Fe-rich transitional phase sharing a similar chemical composition. The gradual and partial involvement of the RE-rich phase improved the melt's wettability and delayed the formation of the RE6Fe13Ga phase. Consequently, a uniform microstructure was achieved, establishing the conditions for high magnetic performance. Thin metallic GBs and thick RE-rich, amorphous Fe-rich, and RE6Fe13Ga GBs were formed in the magnet. The former ensured squareness, while the sufficient coverage of the latter contributed to exceptional coercivity. The Pr-rich aiding alloy induces chemical heterogeneity, reinforcing the magnetic properties. This study advances our understanding of phase transformation pathways in Ga-doped magnets and introduces an effective approach for achieving high-performance magnets.
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