## 3 Results and Discussion  
Considering the melting phenomenon in Figure 1b, the melting site of G2 was exposed to a very high heat flux, and string No. W511 in the adjacent region withstood high temperatures due to thermal radiation. From the direction of the view shown in Figure 3a, it can be seen that the heat load of the monoblock caused cracking and recrystallization on the leading edge. As shown in Figure 3b, the recrystallization area is concentrated on the upper part of the monoblock, corresponding to the heat load on the top surface. The recrystallization depth on the left side is greater than that on the right side. On the left side of monoblock #W511-7 is monoblock #W511-8; thus, it can be seen that #W511-8 will have a great depth of recrystallization, corresponding to the higher heat flux. On the contrary, #W511-6 on the other side may have a shallow recrystallization depth, or no recrystallization may have occurred. The red circle in Figure 3e-g has a higher magnification than in Figure 3b-d. The recrystallized grains near the heat load surface can be seen in these pictures. Cracks start from the leading edge and extend from the recrystallized area to the deformed area. #W511-7 has the least number of cracks, but also has a deep crack propagation, in which the maximum crack length is 2376 \(\upmu\)m and the maximum crack width  
Figure 2: Schematic diagram of the outer target: (**a**) composition diagram of the outer target (number represents the monoblock number); (**b**) schematic diagram of monoblock (the arrow represents the direction of heat flux). is 57.5 \(\upmu\)m. In Figure 3c, recrystallization also appears in the leading edge of #W511-11. The maximum crack length is 2394 \(\upmu\)m, which is the deepest crack in these monoblocks, but the number of cracks increases significantly and the maximum crack width is 44.5 \(\upmu\)m. In Figure 3d, only a few recrystallized grains are found in the upper right corner near the heat load surface. The maximum crack length of #W511-14 is 1410 \(\upmu\)m, with a maximum crack width of 38.6 \(\upmu\)m, and the number of cracks is similar to that in #W511-11. However, since the monoblock #W511-14 only has a few recrystallization grains in the upper right corner, all of the cracks occurred in the deformed state. The propagation mode of all of the cracks is an intergranular fracture in these monoblocks. In most cases, when recrystallization occurs, cracks develop as a microcrack network [18], but in Figure 3b-d, only macrocracks appeared and no crack networks were found. Commonly, recrystallization will reduce the mechanical strength of materials [34], leading to a decrease in the cracking threshold [35], which is then more likely to generate cracks with high thermal stress. However, #W511-14 had cracks without recrystallization, where the cracks may have been formed before recrystallization. Even though #W511-7 withstands the highest heat flux and has the largest recrystallization area, a fewer number of cracks are found in it, whereas the widest crack releases most of the energy. For mono-blocks with smaller crack widths, more cracks were generated during the process of releasing thermal stress. From the current results, it was found that the primary cracks were not propagated deep enough to reach the cooling tube due to such self-custellation effects [23]. The metallographic microstructure of the monoblocks' H-W plane is shown in Figure 4. There is a large recrystallization area in the upper right corner of #W511-7, and the difference in the microstructure is obvious, as shown in Figure 4b. A similar recrystallization process happened in #W511-11, whereas the area where the microstructure changed is smaller, and there is no recrystallized grain found at the same area in #W511-14, as shown in Figure 4c,d, respectively. From the results mentioned before, #W511-7 has the largest high-temperature distribution area and the highest temperature, which is higher than the recrystallization temperature of pure tungsten.