Coercivity Of Hot-deformed Magnets Research Articles

Significant Progress for Hot-Deformed Nd-Fe-B Magnets: A Review.

High-performance Nd-Fe-B-based rare-earth permanent magnets play a crucial role in the application of traction motors equipped in new energy automobiles. In particular, the anisotropic hot-deformed (HD) Nd-Fe-B magnets prepared by the hot-press and hot-deformation process show great potential in achieving high coercivity due to their fine grain sizes of 200-400 nm, which are smaller by more than an order of magnitude compared to the traditional sintered Nd-Fe-B magnets. However, the current available coercivity of HD magnets is not as high as expected according to an empirical correlation between coercivity and grain size, only occupying about 25% of its full potential of the anisotropy field of the Nd2Fe14B phase. For the sake of achieving high-coercivity HD magnets, two major routes have been developed, namely the grain boundary diffusion process (GBDP) and the dual alloy diffusion process (DADP). In this review, the fundamentals and development of the HD Nd-Fe-B magnets are comprehensively summarized and discussed based on worldwide scientific research. The advances in the GBDP and DADP are investigated and summarized based on the latest progress and results. Additionally, the mechanisms of coercivity enhancement are discussed based on the numerous results of micromagnetic simulations to understand the structure-property relationships of the HD Nd-Fe-B magnets. Lastly, the magnetization reversal behaviors, based on the observation of magneto-optic Kerr effect microscopy, are analyzed to pinpoint the weak regions in the microstructure of the HD Nd-Fe-B magnets.

Texture development and grain boundary phase formation in Ce- and Ce-La-substituted Nd-Fe-B magnets during hot-deformation process

In this study, the influence of Ce- and Ce-La-substitution for Nd in Nd-Fe-B magnets on their magnetic and microstructural changes in the course of the hot-deformation process was investigated to gain insight into further reducing the Nd content in fine-grained Nd-Fe-B hot-deformed magnets. We found that the [001] texture and microstructure of the RE-rich grain boundary phases, which are the factors that determine the remanence and coercivity of hot-deformed magnets, were deteriorated by the Ce- and Ce-La-substitution. This is because the volume fraction of the RE-rich liquid formed in the grain boundaries during the hot-press and hot-deformation decreased due to the Ce- and Ce-La-substitution, thereby increasing the friction between the 2:14:1 grains while the c-axes of grains were aligned by grain rotation and decreasing the RE concentration of grain boundary phases. In particular, the Ce-La-co-substitution further prevented the liquid formation within the grain boundaries of the magnets during hot-press and hot-deformation because the wettability of the RE-rich liquid on the 2:14:1 grains became poor when La was substituted for Nd in the 2:14:1 grains. These results indicate that there is room for further improvement in both the remanence and coercivity of the Ce- and Ce-La-substituted magnets by introducing additional liquid into the grain boundaries prior to hot-press and hot-deformation. By applying the Nd-Cu infiltration process to the melt-spun ribbons (intermediate infiltration, I-infiltration), both the remanence and coercivity of the Nd-saving (Nd0.7Ce0.3)-Fe-B and (Nd0.7Ce0.225La0.075)-Fe-B hot-deformed magnets were successfully improved.

High thermal stability of hot-deformed Nd–Fe–B magnets by spraying Pr70Cu30 powders on melt-spun powders

The coercivity of hot-deformed magnets is enhanced with slight reduction of remanence by spraying Pr70Cu30 powders on magnets’ precursor melt-spun Nd–Fe–B powders. The distribution of rare earth rich phases in diffused sample is superior to original magnet. The thermal stability of the diffused magnets is comparable to the sintered sample with 1 wt% Dy content and much better than the sintered grade 42SH HRE-free sample. The reversed magnetic domains of the pre-saturated samples appear when the temperature reaches 125 °C in coarse grains near ribbon surface and propagated like avalanche by further increasing the temperature. The thermal stability of hot-deformed magnet can be largely optimized from the magnetic domain variation dependent on temperature enhancement.

The effect of Nd-Cu diffusion during hot pressing and hot deformation on the coercivity and the deformation ability of Nd-Fe-B HDDR magnets

HDDR processed Nd-Fe-B powders blended with Nd-Cu powders were hot-pressed and then hot-deformed to obtain fully dense, anisotropic magnets. The microstructure evolution in hot-pressed and hot-deformed HDDR magnets was studied to illustrate the impact of Nd-Cu diffusion on coercivity and deformation ability. It was found that the coercivity and the deformation ability were improved significantly by Nd-Cu addition. The coercivity of hot-deformed magnets was increased from 3.81kOe to 13.01kOe after 2wt% Nd-Cu additions and the annealing at 900°C for 2h. Scanning electron microscopy and transmission electron microscopy observations revealed that the variation in coercivity was attributed to the diffusion of Nd-Cu along the grain boundaries during hot pressing and hot deformation.