The effects of temperature (1050, 1100 and 1150 °C) and time (1–20 min) on the microstructure of the transient liquid phase (TLP) joints of IN-738/MBF-20/IN-718 system were investigated using OM, SEM and SEM/EDS analysis. During bonding, no pressure was applied and just the weight of the upper specimen was effective. A continuous eutectic structure was observed at the joint centerline of the samples bonded at 1050 °C for 30 min. The same was seen at the joints made at 1100 °C for 1 min which were detected as Cr- and Ni-borides. The morphology and composition of phases at diffusion affected zone (DAZ) were somewhat different in IN-718 and IN-738 halves. Increasing the bonding time at 1050 and 1100 °C bonding temperatures increased the degree of isothermal solidification. In case of the samples, TLP-bonded at 1150 °C, the diffusion of the liquid phase into the grain boundaries of the base metals occurred and a wavy bonding interface and no DAZ phases were observed. A critical bonding temperature (between 1100 and 1150 °C) was seen above which the rate of isothermal solidification was reduced. Larson-Miller equation (P = TB [C + ln(tB)]) was used to estimate the required time for completion of the isothermal solidification. The calculated value of 33 for system-dependent constant C, resulted in the best estimate at different bonding temperatures.
