Microstructure Evolution and Phase Identification in Ni-Based Superalloy Bonded by Transient Liquid Phase Bonding

This study investigates the development of the microstructure in the interconnection zone of an IN718/Al/IN718 couple obtained by the Transient Liquid Phase Bonding (TLPB) process at 1000 °C. The crystal structure, composition and orientation of the phases formed during joining were examined by SEM-EDS EBSD. The results showed that the interconnection zone is a multilayered region, constituted mainly by sigma (σ), Laves and α-Cr phases and the AlNi intermetallic. The evolution of the microstructure over time shows that the topologically close-packed (TCP) phases grow as columnar grains, while the AlNi phase forms exiaquied grains. The AlNi phase is split into two layers, Al-rich and Ni-rich, both having the same crystal structure but different chemical composition. EBSD mapping revealed that there is no grain boundary along the split line, suggesting that splitting results from a solid-state transformation requiring no nucleation. At longer bonding time, the AlNi phase is enriched in Ni and presents nanoprecipitates homogeneosly dispersed. These results provide experimental data that contribute to the understanding of TLPB of superalloys.