Local Phase Segregation Induced by Ion Milling in 2:17-Type Sm-Co Based 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.