Microstructure and magnetic properties evolution of isotropic nanocrystalline NdFeB hot-pressed magnets with CeCu addition

Bulk dense anisotropic rare earth (RE)-Fe-B magnets synthesized by hot pressing and subsequent hot deforming melt-spun powders have attracted much attention because of their good all-around properties. It is well known that the formation of the desired crystallographic texture is critical to obtain good magnetic properties in the preparation process of anisotropic RE-Fe-B magnets. Melt-spun ribbons with nominal compositions of Nd13Fe78Co3B6 and Ce7Cu3 were consolidated into isotropic nanocrystalline NdFeB hot-pressed magnets by the spark plasma sintering technique, and the microstructure and magnetic properties of the magnets were investigated. Results demonstrate that the coercivity of the magnets is enhanced dramatically by the grain boundary diffusion of the low-melting-point CeCu alloy. With 10 wt% CeCu addition, the coercivity increases from 7.5 to 12.44 kOe. The excessive CeCu, however, reduces the remanence of the NdFeB magnets from 7.76 to 5.66 kGs. Moreover, an appropriate holding time effectively improves the magnetic properties of the magnets. The microstructure indicates that the main phase NdFeB with a grain thickness of 20 $\mu \text{m}$ , and the rest are Nd-rich phase and CeCu grain boundary phase. The intergranular phase of CeCu acts as a diffusion channel, which improves the diffusion efficiency and the uniform distribution of elements. The magnets sintered by spark plasma form a special two-zone structure with a coarse grain region and a fine grain region, and the CeCu alloy effectively inhibits the growth of the grains.