Transient Liquid Phase Bonding of IN738LC/MBF-15/IN738LC: Solidification Behavior and Mechanical Properties

Transient liquid phase (TLP) bonding of IN738LC superalloy was carried out using a rapidly solidified MBF-15 Ni-based foil. The effects of bonding temperature (1130–1170 °C) and time (5–120 min) as well as foil thickness (35–140 µm) were studied on the microstructure of joint region and its mechanical properties. The solidification sequence in the joint region was found to be (i) formation of γ solid solution in the isothermally solidified zone, followed by (ii) ternary eutectic of γ + Ni3B + CrB, and finally (iii) binary eutectic of γ + Ni3Si in the athermally solidified zone. Fine Ni3Si particles were also formed via a solid state transformation within the γ matrix in the vicinity of eutectic products. A deviation of isothermal solidification kinetics from the standard parabolic TLP model was observed by increasing the bonding temperature to 1170 °C, which resulted in the formation of eutectic constituents at the joint centerline. The analysis of mechanical and fractographic test results revealed that the samples with complete isothermal solidification exhibit the highest shear strength, whereas the hard eutectic constituents act as preferential failure sites and lead to a significant reduction in the joint shear strength in samples with incomplete isothermal solidification.