Synergistic regulation and optimization of microstructure, coercivity, and thermal stability in sintered NdFeB magnets through grain boundary diffusion of Dy and Al elements

The grain boundary diffusion process (GBDP) is a well-established method for producing high-performance NdFeB magnets. However, the limited diffusion ability of heavy rare earth (HRE) elements, which are only attached to the surface of the magnet, has been a major concern in the industry. In this study, we utilize the GBDP method to diffuse low-melting-point DyAl alloy into magnets, resulting in the formation of continuous Dy-, Al-, and Nd-rich thin grain boundary phases and (Nd,Dy) 2 Fe 14 B shells with diffusion depths over 1500 μm. These results suggest that the presence of continuous Dy-, Al-, and Nd-rich thin grain boundary phases is crucial in promoting the diffusion of Dy elements in magnets. Furthermore, these thin and continuous grain boundary phases effectively isolate the Nd 2 Fe 14 B grains, preventing the simultaneous reversal of adjacent grains. As a result, the coercivity of the N42-DyAl magnet increases from 12.07 kOe to 19.95 kOe at room temperature and from 5.17 kOe to 10.76 kOe at high temperature (120 °C). The N42-DyAl magnets also exhibit a large coercivity temperature coefficient of −0.485%/°C and excellent thermal stability. This optimized effect of synergistic multi-element grain boundary diffusion creates an extensive core-shell structure, enhancing the anisotropy field and increasing the coercivity.