# Local Phase Segregation Induced by Ion Milling in 2:17-Type Sm-Co Based Magnets
###### Abstract  
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 Sm\({}_{13}\)Gd\({}_{12}\)Co\({}_{96}\)Cu\({}_{8.5}\)Fe\({}_{13}\)Zr\({}_{3.5}\) (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. Sm-Co magnets; ion milling; phase segregation; TEM +
Footnote †: journal: materials  
0000-0002-319-2223-2  
X-ray diffraction technologies. Thus, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) are indispensable to reveal the true structure of the 2:17-type Sm-Co based magnets [3; 4; 5; 7; 8; 9; 10; 11; 12; 13]. However, the TEM (or HR-TEM) is not risk-free, in terms of introducing microstructural artifacts, especially during the TEM specimen preparation process. Generally, the thin film specimens for TEM examinations of the 2:17-type Sm-Co based magnets are fabricated by ion milling or focused ion beam, since these magnets are too brittle to suffer electropolishing [4; 5; 7; 8; 10; 11; 12; 13]. Although the energy used for ion milling or focused ion beam is relatively low (2-6 keV), the possible structure damage can be easily introduced into metallic materials since the temperature rising of the specimen surface is unavoidable during the ion milling process. In addition, many studies in ion implantation/ion beam modification of materials point out that the structure of the specimens might become modified by energetic ions, and artifacts can be formed in this way [16; 17; 18; 19; 20; 21]. Besides the widely known amorphous layer near the hole, there are other artifacts including defects clusters [17], elemental segregation [18], and phase transformation [19] that can be observed during the particle irradiation process. Note that the high-performance magnets will suffer damaging environmental effects, especially in applications such as space probes, high energy particle accelerators, spectrometers, and new synchrotron light sources; thus, the potential structure damage induced by ion irradiation must be considered. Previous investigations about the Sm-Co based magnets all focus on the high energy heavy ion irradiation damage, indicating that the magnetic properties, especially the magnetization, drop drastically when these magnets suffer from high energy irradiation [3; 22; 23]. However, the possible structure damage induced by low-energy ion irradiation in the 2:17-type Sm-Co based magnets are rarely considered. Herein, we performed a comparative investigation between two TEM specimens fabricated by low-energy ion milling under different conditions of a commercial magnet Sm\({}_{13}\)Gd\({}_{12}\)Co\({}_{50}\)Cu\({}_{8.5}\)Fe\({}_{13}\)Zr\({}_{3.5}\) (wt.%). The detailed structure and composition of these specimens are unveiled using TEM equipped with an energy dispersive X-ray spectroscopy (EDS) detector.