Influence of transient liquid phase bonding followed by homogenization on the fatigue lifetimes of inconel 738 at elevated temperature

An ideal transient liquid phase (TLP) joint is achievable with the complete isothermal solidification and homogenization of the Diffusion Affected Zone (DAZ), in which case, a joint with a similar microstructure to the Base Metal (BM) is obtained. In this study, Inconel 738 (IN-738) was joined by TLP bonding using a BNi-2 filler alloy at 1120 degrees C for different hold times followed by a Standard Heat Treatment (SHT) for the homogenization of the joint. After a microstructural study of the joints, the BM and the homogenized TLP joint were fatigue tested at 850 degrees C under similar conditions. A comparison of the fatigue test results (S-N curve) was indicative of the reduced fatigue lifetime of IN-738 as a result of TLP bonding which can be attributed to the shorter elongation of the joint, gamma grain growth during the bonding, and the larger number of fatigue crack initiation sites at the homogenized joint compared with the BM. In addition, several grain boundary cracks were observed across the fracture of the homogenized joint that may have been due to the reduced cohesive strength of the material at the boundaries or the increased density of carbides at grain boundaries. Eventually, Basquin's equation was presented to estimate the fatigue life of the joint and the fatigue strength coefficient and fatigue strength constant of the joint were calculated -0.049 and 325.65, respectively.