# Local Phase Segregation Induced by Ion Milling in 2:17-Type Sm-Co Based Magnets
## 3 Results
### Microstructure and Microchemistry for Both TEM Specimens  
Figure 2 shows the bright-field images (BFIs) and the selected area electron diffraction (SAED) patterns of the two TEM specimens taken along [001]-2:17R zone axis. To perform the real structure to the most extent, two SAED patterns were taken using the largest aperture of our TEM to include diffraction information of an area with a diameter of \(\sim\)4 \(\upmu\)m. In the SAED patterns [Figure 2b,d], the 1:5H phase produces only fundamental reflections ([001]- and \(\{\overline{1}20\}\)-1:5H) which are overlapped by the fundamental reflections of the 2:17R phase ([003]- and \(\{\overline{1}20\}\)-1:5H), which is consistent with the XRD results in Figure 1. The superlattice reflections at [011] *, [012] *, \(\{\)021 *, and \(\{\)022 * positions are attributed to the 2:17R nano twins. The 1:3R phase produces extra superlattice reflections [003]-1:3R at  
Figure 1: Powder XRD patterns of the final magnet. the [003/2]*-2:17R positions. The orientation relationship of these three phases is [100]-2:17R // [210]-1:5H/1:3R and (006)-1:3R // (003)-2:17R // (001)-1:5H. The additional weak reflections at the positions of {001} *-, [002] *-, [010] *-, [020] *-, [031] *-, [032] *-, [013] *- and {023} *-2:17R are attributed to the 2:17R' phase, which is an intermediate during the transformation from the supersaturated solid solutions to the 2:17R equilibrium phase [10; 11]. The BFIs (Figure 2a,c) reveal that both specimens have a diamond-shaped cellular nanostructure, consisting of the 2:17R cell interiors, the cell boundaries along the {011}-2:17R pyramidal plane, and the 1:3R platelets along the {001}-2:17R basal planes. The cell boundaries in S1 (Figure 2a) all exhibit dark contrast which can be deemed as the 1:5H phase. However, the cell boundaries near the hole in S2 (right region in Figure 2c) exhibit alternately bright and dark contrasts, which are rarely observed in the 2:17-type Sm-Co based magnets, while the cell boundaries far away from the hole exhibit a dark contrast. The abnormal microstructure in specimen S2 indicates that the structure and chemistry of the cell boundaries near the hole may be altered during the relatively lower energy pips process. The bright-field images shown in Figure 2 were selected in an area with a size of ~500 \(\times\) 500 nm\({}^{2}\) to show the cellular structure clearly; however, this region is too large to perform the microchemistry near the cell boundaries region. Thus, higher-resolution scanning TEM high-angle annular dark field (STEM-HAADF) images and the corresponding elemental distribution for both specimens were performed in Figures 3 and 4, respectively. The cell boundaries are shown in in Figure 3a; the bright contrasts along the {011} pyramidal planes can be deemed as 1:5H phases. The corresponding elemental mapping images in  
Figure 2: Bright field images, selected area electron diffraction patterns for specimen 1 (**a**,**b**) and specimen 2 (**c**,**d**). Figure 3b-g further reveal that Cu is distinctly concentrated at the cell boundary phase with relatively depleted Fe and Co at the cell boundaries in S1, while the cell interior is Fe/Co-rich and Co-depleted. The Zr-rich 1:3R platelets along the {001} basal planes may act as the elemental distribution paths to form the chemistry heterogeneous between the cell interiors and cell boundaries [4; 27; 28]. The TEM-EDS line scanning along the black arrow in Figure 3h shows the elemental distribution difference more clearly. The marked region (gray rectangle region in Figure 3h) refers to the cell boundary, in which the Cu concentration is higher and Fe/Co concentration is lower than those in the cell interior. Figure 4a shows the STEM-HAADF image of specimen S2. It can be seen that the cell boundaries in S2 are distinctly different from those in S1. The cell boundaries are segregated into two types of lamellae (with a thickness of \(\sim\)2-5 nm and a length of \(\sim\)10 nm) with bright and dark contrasts.