Microstructure and magnetic properties of grain boundary modified recycled Nd-Fe-B sintered magnets

Full-density recycled Nd-Fe-B sintered magnets were produced from collected waste Nd-Fe-B magnets with the coercivity, μ₀H_c, of 1.30 T and remanence, μ₀M_r, of 1.37 T. A small fraction of Nd₆Dy₂₁Co₁₉Cu_2.5Fe powder (Grain Boundary Modified (GBM™) powder) was blended to the powders obtained from the Nd-Fe-B waste magnets. The addition of 5.0 wt% GBM™ powder resulted in μ₀H_c= 2.36 T with μ₀M_r= 1.29 T in recycled magnets. The temperature coefficient of intrinsic coercivity, β, in the recycled magnet was measured to be −0.47%/^oC. Microstructure studies showed that the addition of the GBM™ powder increases the volume fraction of a metallic Nd-rich phase in the recycled magnets. 3D tomography revealed a good interconnection of Nd-rich phases through the grain boundaries of the recycled magnet. Microstructure studies suggested that obtained high coercivity in the recycled magnet with 5.0 wt% addition of GBM™ is due to the formation of distinct grain boundary phase combined with the formation of Dy-rich shell in the Nd₂Fe₁₄B grains with ∼0.8 at. % Dy.