Improving Tb diffusion and magnetic properties in fine-grained NdFeB magnet by Zr addition

In this work, efficient grain boundary diffusion (GBD) of Tb can be achieved in fine-grained sintered Nd-Fe-B magnet through the addition of Zr. The magnets were prepared using Nd-Fe-B powder with an average particle size of 2.6 μm. Compared with the magnet without Zr addition, the intrinsic coercivity of the Zr-added magnet increases by 3.55 kOe after GBD with the increment of 15.0 %, and the squareness of the demagnetization curve is improved by 5.7 %. The Zr element can combine with the C element in the magnet to form block-like ZrC precipitates, which inhibits the formation of harmful rare-earth carbon phase and reduces the ineffective depletion of rare earth elements. In addition, the Zr element can also combine with the B element to form needle-like ZrB₂ precipitates. The consumption of B element increases the volume fraction of RE-rich phase, thus providing more liquid-phase channels for the diffusion of Tb. The ZrC and ZrB₂ precipitates distributed at the grain boundary (GB) can broaden the width of the GB phase, which not only provide broader channels for Tb diffusion, but also improve the magnetic isolation of the main phase grains. By the addition of Zr, the diffusion depth of Tb in the magnet is increased. Meanwhile, the accumulation of Tb on the diffusion surface of the magnet reduces significantly, and the thin and uniform Tb-rich shell is formed on the exterior of the main phase grains in the magnet.