Transient liquid phase bonding of dissimilar nickel base superalloys - Wettability, microfracture and mechanical properties

The present paper investigates the wettability-microstructure-mechanical property relationships in wide gap transient liquid phase (TLP) bonds between the single crystal nickel base superalloy CMSX-4 and two polycrystalline superalloys, IN 738 and IN 939, using wide gap style composite interlayers Niflex-110 and Niflex-115 in comparison with conventional BNi-3 brazing foils. Microstructural studies using scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM/EDS) showed the preferential flow of the liquid along the grain boundaries of the Niflex core, rather than the wetting of the faying surfaces, leaving non-bonded regions at the latter. This problem was overcome by increasing the boron content of the Niflex foils to form additional liquid. Suppression of bondline boride formation was achieved with the wide gap composite interlayers. Two factors were of potential importance in the room temperature mechanical properties of the wide gap bonds. The first was the extent of γ' formation at the bondline and the second was the presence of second phases in the diffusion zone of the polycrystalline substrate. This second phase formation resulted in brittle secondary cracks in the polycrystalline substrate. However, the extent of γ' formation was the dominant factor in the room temperature shear strength of the bonds.