Ultrahigh coercivity of hot-deformed Nd-Fe-B magnets was obtained by low melting point PrNd-Cu alloys diffusion process. The coercivity was largely increased from 15.35 to 27.30 kOe. Magnetic isolation by nonmagnetic phases should be the primary explanation for the enhancement of coercivity. High susceptibility of the recoil loops at low field indicated that magnetic domain wall motion within grains was easy, while a near-zero reversible susceptibility of recoil loops implied that domain wall strong pinning at grain boundaries was dominant at higher field in initial magnetization process. Open recoil loops were also obtained in the infiltrated magnets and disappeared when the external field exceeded about 16 kOe lightly higher than the coercivity of the magnet without diffusion process. The inhomogeneous distribution of infiltrated phases that led to the grains aggregation and single grains with softer and harder magnetic characteristics, respectively, may be the primary reason for the phenomenon. The stray field produced by isolated single grains plays a critical role in the magnetization behavior. A phenomenological model for the magnetization reversal was built to interpret the open recoil loops in a recoil loop process.

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.

Ultrafine-grained die-upset Nd-Fe-B magnets are of importance because they provide a wide researching space to redesign the textured structures. Here is presented a route to obtain a new die-upset magnet with substantially improved magnetic properties. After experiencing the optimized heat treatment, both the coercivity and remanent magnetization of the Dy-Cu press injected magnets increased substantially in comparison with those of the annealed reference magnets, which is distinct from the reported experimental results on heavy rare-earth diffusion. To study the mechanism, we analyzed the texture evolution in high-temperature annealed die-upset magnets, which had significant impact on the improvement of remanent magnetization. On basis of the results, we find that the new structures are strongly interlinked with the initial structures. With injecting Dy-Cu eutectic alloy, an optimized initial microstructure was achieved in the near-surface diffused regions, which made preparations for the subsequent texture improvement. Besides, the Dy gradient distribution of near-surface regions of the Dy-Cu press injected magnets was also investigated. By controlling the initial microstructure and subsequent diffusion process, a higher performance magnet is expected to be obtained.

Hot-deformed Nd-Fe-B magnets with a larger maximum energy product at higher operating temperature are desirable in a wide range of applications but are very challenging to realize in a common “single phase” structure. Here, we show the macroscopic structural design in hot-deformed magnets by using two kinds of melt-spun powders with/without heavy rare earth. Higher coercivity with a remarkably improved maximum energy product is obtained in the separated multilayer magnet. We find that the multilayer structure can improve the c-axis alignment of platelet-shaped grains in each layer and propose the possible interlayer's long-range magnetic interaction explaining the recoil loop open of designed magnets, coupled with visible field-induced domain evolution. This experimental approach reveals exciting applications of structural design in ultrafine-grained hot-deformed magnets.