## 2 Materials and Methods  
The sample is from the EAST upper divertor's G3 outer target (Figure 1), which employs five strings of W/Cu monoblocks with the aim to withstand high projected heat loads up to 10 MW/m\({}^{2}\) on the top surface [23]. The tungsten part was prepared via the powder metallurgy process. It was sintered at 2000 \({}^{\circ}\)C, and then warm-rolled at 1100 \({}^{\circ}\)C with a thickness reduction of 50%. After rolling, tungsten plates were partitioned by using electrical discharge machining (EDM) wire cutting. The CuCrZr/Cu tube was welded with hot isostatic pressure (HIP) technology and then installed and commissioned in EAST [23]. After a melting failure during the plasma campaign in 2019, parts of the damaged outer targets were replaced. As shown in Figure 1a, melting and resolidification occurred at the G2 outer target. Figure 1b shows the most severe melting string in the G2 outer target from Figure 1a. Besides the severe leading-edge-induced melting of W armor, a Cu layer is visible on the surface of the failed W/Cu monoblocks, and such a Cu layer is also found on the edge sides of the toroidal gap between the W/Cu monoblocks [30]. The plasma strike point in EAST is concentrated around monoblocks #7 to #14. Therefore, in this paper, the monoblocks were located at the same place, where the monoblocks numbered #W511-7, #W511-11, and #W511-14 in string No. W511 were located in the G3 outer target, which is the adjacent string to G2 in Figure 1a. The string was cut into a single monoblock via EDM. The surface of the monoblock was observed by using an optical microscope. The surfaces of the samples were prepared for optical microscope observation via mechanical grinding with a silicon carbide paper and polishing with a 1 \(\upmu\)m diamond. The corrosion solution was a boiled H\({}_{2}\)O\({}_{2}\) solution with a 3% volume fraction, and the corrosion time was about 40-60 s depending on the sample condition. Samples for EBSD measurements were first prepared via mechanical  
Figure 1: (**a**) View of the damaged G2 CM in the vacuum chamber of EAST (number represents the monoblock number, and the arrow points to the monoblock #W511); (**b**) the leftmost string of W/Cu monoblocks with severe leading-edge-induced melting of the W and Cu layer on the surface after disassembly (number represents the monoblock number, and the arrows point to the cooper layer). polishing and then via electropolishing; the polishing solution was a 2% NaOH solution, and the current density was 0.02 A/mm\({}^{2}\). The electropolishing time was about 200 s. EBSD measurements were performed with an Oxford C-Swift detector in a Zeiss Gemini 500 SEM using a voltage of 20 kV. A step size of 1 \(\upmu\)m was chosen to study the details of the deformed microstructure and the recrystallization grains. The open-source software MTEX [32] was used to analyze the EBSD data. Monoblocks were cut from the rolled plate, and the height (H), width (W), and thickness (T) direction of the monoblocks corresponded to the RD (rolling direction), TD (transverse direction), and ND (normal direction) of the rolled plate (Figure 2b). Both the length and width of each W/Cu monoblock were designed to be 26 mm, whereas the thickness of the monoblocks varied from 7-12 mm according to the requirements. The distance from the top surface (W-T plane) to the tube wall was 6 mm. The inner diameter of the CuCrZr coolant tube was 12 mm, and the thickness was 1.5 mm. The thickness of the OFHC-Cu interlayer between the CuCrZr coolant tube and the W armor was 1 mm [23]. Generally, to mitigate the leading-edge-induced high thermal effects, both of the leading edges of each W/Cu monoblock were chamfered with dual charfering shaping (1 mm \(\times\) 1 mm) [33] since EAST operates in both the normal and reverse magnetic field directions. However, the H-T plane on the outer side (blue arrow) in string No.