# Local Phase Segregation Induced by Ion Milling in 2:17-Type Sm-Co Based Magnets
## 4 Discussion  
The above comparative investigations between two TEM specimens with different ion milling conditions but the same composition and same heat treatment history reveal that the primary 1:5H cell boundaries near the hole in the TEM specimen with additional low-energy ion milling will transform 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. The possible phase segregation mechanism is discussed below. The formation of the cellular nanostructure in the 2:17-type Sm-Co magnets was deemed as a concurrent recrystallization and precipitation process, which involved the growth of the 2:17R cell interior phase (sub-grain interior) and the precipitate of the 1:5H phase along the cell boundaries (sub-grain boundaries) [5; 8; 9]. The whole process is controlled by a coupled diffusive and displacive phase transformation. The motion of the partial dislocations transforms the 2:17H solution-treated precursor into the 2:17R phase, leaving a number of interstitials and vacancies in the {011} pyramidal planes, which act as the nucleation site for the 1:5H precipitates. In addition, the elemental partition occurs, including the rare earth atoms (Sm or Gd) gathering into the pyramidal planes, the Fe and Co atom enrichment into the cell interiors, and the Zr atoms gathering into the {001} basal planes to form the 1:3R platelets. During the ion milling process, the temperature-rising effect contained with the bombardment of the argon ion may offer extra driving force to the atom's diffusion, which can change the structure and microchemistry of the 1:5H cell boundaries phase. Although the energy for the TEM specimen's preparation is too low to induce structural artifacts into the 2:17-type Sm-Co based magnets since the irradiation resistance of these magnets is good enough at room temperature, the temperature-rising effect during the ion milling cannot be ignored. Earlier investigation [34] implied that elevated temperature (200-400 \({}^{\circ}\)C) can be easily generated at the specimen surface under a normal ion milling process, even for the metallic specimens with good thermal conductors. In addition, the thermal effect is induced by the incoming ion bombardments to the specimen surface [35]. Consequently, the possible ion damage at high temperatures must be considered for the Sm-Co based magnets during the low-energy ion milling process. According to previous investigation, the formation of the 1:5H phase is highly controlled by the diffusion of the interstitial atoms [8]. On the one hand, the rising temperature during the ion milling (especially after the hole perforation is achieved for specimen 2) may  
Figure 6: HR-TEM image near the cell boundary region for the specimen 2 (**a**), enlarged views (**b**,**c**) of the white dashed square regions in (**a**), and FFT patterns (**e**,**f**) taken from dashed square regions in (**a**). [011]-FCC structure projection (**d**) and simulated electron diffraction pattern (**g**) for FCC structure. provide extra freedom to alter the atom diffusion process; e.g., the extra temperature field enlarges the thermodynamic driving force. On the other hand, the mechanical interaction between the Ar\({}^{+}\) and Sm/Gd/Cu/Fe/Co atoms in the hole-edge region with an extremely small thickness also may change the diffusion behavior. It should be noted here that ion milling of specimens for TEM examination is not a static process, since the outer surfaces of the specimen are continually being eroded by the ion beam. Thus, these combined effects in the hole-edge region in specimen 1 induce further atom diffusion, resulting in cell boundaries segregation from the 1:5H phase to the Sm/Gd-rich and Co/Fe/Cu-rich FCC structures. Since the ion milling energy used in this work was determined empirically, the systematic effect of the ion milling energy on the structure and microchemistry of the TEM specimen needs to be further investigated.