## 3 Results and Discussion
### Microstructural Characterization of the Bonded Samples
[19] also pointed out that the formation of Ni-rich boride was due to the eutectic reaction of residual liquid phase during the TLP bonding of the IN738 LC superalloy. Similarly, the existence of Cr-rich boride was found in zone C. Further increasing chromium concentration during the cooling in the liquid phase allowed the composition of the liquid phase to reach a new eutectic composition. This part of the liquid solidified to a binary eutectic consisting of Cr-rich boride and \(\gamma\)-Ni solid solution. This can be verified by [PERSON] et al. 's [17] work on the TLP bonding of IN718 and IN625. Figure 2b shows a magnified image of zone D. There are many fine particles in this region. EDS analysis showed that the Si in zone D was as high as 5.42 wt.%. According to Ni-Si-B phase diagrams [15; 18], the solubility of Si in the \(\gamma\)-Ni solid solution was about 4 wt.% at room temperature. It can be preliminarily concluded that these fine particles were products of precipitation rather than a eutectic reaction. No nickel silicide was found in the ISZ, which can also be explained by the fact that Si in ISZ (\(\sim\)3.39 wt.%) was lower than the solubility of Si in the \(\gamma\)-Ni solid solution. It can be inferred that these fine particles were Ni\({}_{3}\)Si precipitates from the \(\gamma\)-Ni solid solution according to the Ni-Si phase diagram. These results were in agreement with previous work on the TLP bonding of duplex stainless steels [15; 16]. It can be concluded that athermal solidification occurred during the cooling process. The formation of ASZ due to the bonding temperature was insufficient to complete isothermal solidification. The SEM image of the DAZ of the GH4169 joint bonded at 1040 \({}^{\circ}\)C is shown in Figure 3. It can be seen that extensive blocky and needle-like compounds were formed in the DAZ. The EDS results of these compounds are shown in Table 3. The Cr, Nb, and Mo contents in zone E and zone F were significantly higher than those in the base metal, which confirms that the gray blocky and needle-like compounds were Cr-Nb-Mo-rich boride compounds. Similarly, The EDS results of zone G indicated that the black blocky compounds were Ni-Cr rich boride compounds. Although both B and Si are MPD elements, the silicon content in the DAZ was very low. Silicide is not clearly seen in Figure 3 due to the smaller atomic radius of B. During diffusion, B diffused into the interstitial site and Si would take the substitutional site so the diffusion rate of B was much higher than that of Si. It can also be seen that compounds were formed both in the grains and grain boundaries near the joint. However, compounds were formed only at the grain boundaries in the region far from the joint. This is because the free energy of the grain boundary is lower than that inside of the grain, and moreover, the concentration of B is relatively low in the farther region, according to diffusion theory. In conclusion, the formation of DAZ can be explained by the diffusion of B from the BNi-2 paste into the base metal, which reacted with the strong boride formers (e.g., Cr, Nb, and Mo) in the base metal.