## 3 Results and Discussion
### Shear Strength and Fractography of the Joint  
Shear tests were conducted to investigate the effect of bonding temperature on the mechanical behavior of TLP-bonded GH4169 joints, and the results are illustrated in Figure 5. It can be seen that the highest shear strength (728.03 MPa) was obtained at 1080 \({}^{\circ}\)C. The average shear strength of the GH4169 joints at different brazing temperatures were 397.52 MPa at 1040 \({}^{\circ}\)C, 551.15 MPa at 1060 \({}^{\circ}\)C, and 572.45 MPa at 1100 \({}^{\circ}\)C. The maximum average shear strength is relatively high in the industrial applications of nickel-base alloys. The shear strength increased with increasing bonding temperature from 1040 \({}^{\circ}\)C to 1080 \({}^{\circ}\)C. This result is probably the consequence of increasing the width size of the ISZ and decreasing the intermetallic compounds in the joint area. The intermetallic compounds in the ASZ and the DAZ can deteriorate their mechanical properties due to their brittle nature [23]. As mentioned in the microstructural analysis, higher bonding temperatures promote the diffusion of alloying elements (e.g., Cr, Nb, Mo) from the base metal into the bonding region, enhancing the solid solution (e.g., \(\gamma\)-Ni solid solution) strengthening mechanism. As the bonding temperature increased from 1080 to 1100 \({}^{\circ}\)C, the effect of temperature on the concentration gradient exceeded that on the diffusion coefficient. Increasing the temperature decreased the rate of isothermal solidification, leading to the increase of harmful intermetallic compounds. It can be concluded that the maximum shear strength was achieved at 1080 \({}^{\circ}\)C because the ASZ disappeared and the intermetallic compounds in the centerline of the joints were eliminated completely.  
Figure 4: SEM images of TLP-bonded GH4169/BNi-2/GH4169 at different temperatures: (**a**) 1040 \({}^{\circ}\)C, (**b**) 1060 \({}^{\circ}\)C, (**c**) 1080 \({}^{\circ}\)C, and (**d**) 1100 \({}^{\circ}\)C.  
In order to study the fracture modes of joints with different structures and different shear properties, the fractographs were also investigated by SEM after the shear tests. The cross section and fracture surface of the TLP-bonded samples at 1040 and 1080 \({}^{\circ}\)C are shown in Figure 6. According to the cross-section image of the sample bonded at 1040 \({}^{\circ}\)C (Figure 6a), fracture occurred along the central area of the joint where the ASZ existed. The morphology of the fracture surface in Figure 6c showed that the fracture mode of this sample was brittle fracture. Therefore, it can be deduced that the boride compounds in the ASZ were the chief culprit in the failure of joints due to their hard and brittle nature. Crack initiation and propagation induced by stress concentration around these boride compounds ultimately lead to the failure of joints. This can be verified by [PERSON] et al. (2019) in the case of TLP bonding between IN718 and IN600. Other joints bonded at 1060 and 1000 \({}^{\circ}\)C, which contained the boride compounds in the joint center, showed similar fractography.  
Figure 5: Effect of bonding temperature on the shear strength of TLP-bonded GH4169/BNi-2/GH4169 joints.  
Figure 6: _Cont._  
However, without the ASZ in the central area of the joint, the sample bonded at 1080 \({}^{\circ}\)C showed a different fracture behavior. According to the cross-section image in Figure 6b, a fracture occurred along the DAZ of the joint where the intermetallic compounds existed. The intensive dimples in the fracture surface (Figure 6d) indicated that the fracture mode of the sample bonded at 1080 \({}^{\circ}\)C was ductile fracture. This can be attributed to the completion of isothermal solidification. The intermetallic compounds in the DAZ were the source of the cracks and the main cause of joint fracture.