Grain boundaries are thought to be the primary demagnetization sites of precipitate-hardening 2:17-type Sm-Co-Fe-Cu-Zr permanent magnets with a unique cellular nanostructure, leading to a poor squareness factor as well as a much lower than ideal energy product. In this work, we investigated the grain boundary microstructure evolution of a model magnet Sm₂₅Co_46.9Fe_19.5Cu_5.6Zr_3.0 (wt. %) during the aging process. The transmission electron microscopy (TEM) investigations showed that the grain boundary region contains undecomposed 2:17H, partially ordered 2:17R, 1:5H nano-precipitates, and a Sm_n+1Co_5n−1 (n = 2, 1:3R; n = 3, 2:7R; n = 4, 5:19R) phase mixture at the solution-treated state. After short-term aging, further decomposition of 2:17H occurs, characterized by the gradual ordering of 2:17R, the precipitation of the 1:5H phase, and the gradual growth of Sm_n+1Co_5n−1 compounds. Due to the lack of a defect-aggregated cell boundary near the grain boundary, the 1:5H precipitates are constrained between the 2:17R and the Sm_n+1Co_5n−1 nano-sheets. When further aging the magnet, the grain boundary 1:5H precipitates transform into Sm_n+1Co_5n−1 compounds. As the Sm_n+1Co_5n−1 compounds are magnetically softer than the 1:5H precipitates, the grain boundaries then act as the primary demagnetization sites. Our work adds important insights toward the understanding of the grain boundary effect of 2:17-type Sm-Co-Fe-Cu-Zr magnets.

Transmission electron microscopy (TEM) is indispensable to reveal the cellular nanostructure of the 2:17-type Sm-Co based magnets which act as the first choice for high-temperature magnet-associated devices. However, structural deficiencies could be introduced into the TEM specimen during the ion milling process, which would provide misleading information to understand the microstructure–property relationship of such magnets. In this work, we performed a comparative investigation of the microstructure and microchemistry between two TEM specimens prepared under different ion milling conditions in a model commercial magnet Sm₁₃Gd₁₂Co₅₀Cu_8.5Fe₁₃Zr_3.5 (wt.%). It is found that additional low-energy ion milling will preferably damage the 1:5H cell boundaries, while having no influence on the 2:17R cell phase. The structure of cell boundary transforms from hexagonal into face-centered-cubic. In addition, the elemental distribution within the damaged cell boundaries becomes discontinuous, segregating into Sm/Gd-rich and Fe/Co/Cu-rich portions. Our study suggested that in order to reveal the true microstructure of the Sm-Co based magnets, the TEM specimen should be carefully prepared to avoid structural damage and artificial deficiencies.