## 3 Results and Discussion  
### Microstructural Characterization of the Bonded Samples  
A typical microstructure of TLP-bonded GH4169 using BNi-2 solder paste at 1040 \({}^{\circ}\)C for 30 min is shown in Figure 2. It can be seen that the TLP joint was divided into three zones: the athermal solidification zone (ASZ), the isothermal solidification zone (ISZ) and the diffusion-affected zone (DAZ). The EDS results of different phases in ASZ are listed in Table 2. Boron is a light element, which can only be qualitatively analyzed but not quantitatively analyzed by the EDS, so it was not included in Table 2. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Material** & **Ni** & **Cr** & **Fe** & **Nb** & **Mo** & **Ti** & **Al** & **Si** & **B** & **Mn** & **Co** & **Cu** & **C** \\ \hline GH4169 & Bal & 19 & 17 & 4.8 & 3 & 0.8 & 0.5 & 0.35 & - & 0.35 & 1 & 0.3 & 0.08 \\ BNi-2 solder paste & Bal & 7 & 3 & - & - & - & - & 4.5 & 3.2 & & & 0.06 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition (wt.%) of the base metals and solder paste. Figure 1: (**a**) Samples for microstructure observation and (**b**) a schematic illustration of fixture for the shear test. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Zone** & **Ni** & **Cr** & **Fe** & **Nb** & **Mo** & **Ti** & **Si** & **Suggested Phase** \\ \hline A & 71.49 & 18.44 & 4.29 & 0.31 & 0.91 & 1.18 & 3.39 & \(\gamma\)-Ni solid solution \\ B & 83.67 & 3.31 & 3.50 & 5.61 & 1.65 & 1.86 & 0.41 & Ni-rich boride \\ C & 1.10 & 84.18 & 0.84 & 0.94 & 10.83 & 1.63 & 0.49 & Cr-rich boride \\ D & 81.69 & 6.12 & 5.77 & 0.83 & 0.20 & 0.47 & 5.42 & Ni\({}_{3}\)Si + \(\gamma\)-Ni solid solution \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chemical composition (wt.%) of different zones marked in Figure 2.  
Figure 2: SEM (**a**) micrographs of the transient liquid phase (TLP) bonded GH4169/BNi-2/GH4169 at 1040 \({}^{\circ}\)C for 30 min; and (**b**) A magnified image of the white square in (a). It can be seen that the ISZ had a monophasic microstructure. The EDS results of zone A indicate that the ISZ was composed of a \(\gamma\)-Ni solid solution. Similar results have been reported by other researchers [15; 18]. During the process of TLP bonding, the bonding temperature was higher than the melting point of the BNi-2 interlayer. The MPD (melting point depressant) elements (e.g., B and Si in the molten BNi-2) diffused into the base metal and the melting point of BNi-2 increased. When the melting point rose to the bonding temperature, the grains of the \(\gamma\)-Ni solid solution nucleated on the side of the base metal and an epitaxial growth grew to the center of the joint. It should also be noted that elements such as Nb, Mo, and Ti were found in the ISZ. These elements were not included in the original composition of the BNi-2 paste (Ni-Cr-Fe-Si-B). It can be inferred that these elements were dissolved from the base metal during the bonding process. In conclusion, the formation of the ISZ can be attributed to the composition and solidifying point of the melting point change in BNi-2, which was caused by the interdiffusion between the base metal and BNi-2 interlayer. Figure 2a demonstrates that the ASZ of the joint was composed of three different phases (marked as B, C, and D). According to the EDS results listed in Table 2, Ni-rich boride was found in zone B. During isothermal solidification, the formation and growth of the \(\gamma\)-Ni solid solution phase rejected excess Cr and B in the liquid phase. With the increase of Cr and B concentration the composition of the liquid phase moved to the eutectic composition. During the subsequent cooling process, a eutectic reaction occurred in the liquid phase, which resulted in the formation of Ni-rich boride and \(\gamma\)-Ni solid solution as a binary eutectic. A similar phase composition appeared during the TLP bonding of the IN718 superalloy conducted by [PERSON] et al. [15]. [PERSON] et al.