Temperature-dependent microstructure and magnetic properties evolution in the sintered Nd-Fe-B magnets with Pr-Al-Cu diffusion

The grain boundary diffusion process of Pr-Al-Cu at various temperatures has been applied in the sintered Nd-Fe-B magnets. Detailed evolution of the grain boundary phases and its influence on the magnetic properties were investigated. Microstructure analysis showed that the shell morphology rich in Pr and Al was significantly formed in the diffusion process at 700 °C, which was determined as (Pr,Nd)2(Fe,Al)14B phase, while Al is homogeneous in the 900 °C diffused magnets. After the annealing treatment, continuous grain boundary phases were generated in the 700 °C diffused magnet to isolate the neighbouring grains even at the depth of 1000 µm, whereas that in the 900 °C diffused magnet were scarce. Thus a pronounced enhancement of coercivity from 16.21 kOe to 21.36 kOe was yielded in the annealed 700 °C diffused magnet. Transmission electron microscopy showed that the thickness of the Pr and Al rich shell in the matrix phase grains was dramatically decreased during the annealing process. The absent shell layers facilitate the formation of (Pr,Nd)6(Fe,Al,Cu)14 phase in the intergranular regions. Also the residual diffusion source on the surface of the diffused magnets was supposed to promote the development of continuous grain boundary phases during the annealed process. Based on these results, the temperature-dependent microstructure and magnetic properties evolution was detailedly discussed.