MICROSTRUCTURAL EVOLUTION DURING THE TRANSIENT LIQUID-PHASE BONDING OF DISSIMILAR NICKEL-BASED SUPERALLOYS OF IN738LC AND NIMONIC 75

The joining of dissimilar nickel-based superalloys, i.e., IN738LC to Nimonic 75, using transient liquid-phase bonding with a Ni-15Cr-3.5B interlayer (MBF-80) was carried out at temperatures of 1080 degrees C to 1180 degrees C for different bonding times of 30-150 min. A joint cross-section was surveyed using optical and scanning electron microscopy. Microstructural examinations showed that after short bonding times, the joint microstructure consists of continuous eutectic intermetallic phases and longer bonding times lead to a eutectic-free microstructure. It was clear that for all the bonding times and temperatures, boride phases were precipitated at the interface of the base metal and the interlayer due to boron diffusion in to the base metals. The results also showed that the morphology of the precipitates in the diffusion-affected zone (DAZ) varies from globular to acicular by increasing the bonding time. Completion of the isothermal solidification, which prevents the formation of continuous intermetallic phases at the joint centerline, was studied at different temperatures.