Transient liquid phase bonding of a Hf bearing single crystal nickel aluminide to MM247 superalloy

An investigation of transient liquid phase bonding of single crystal NiAl to a conventional polycrystalline nickel base superalloy MM247 is presented here. The NiAl substrate material employed was cast single crystal NiAl (B2 compound, β phase) with 1·5 at.-% hafnium (Hf) as an alloying element. The interlayers employed in the bonds were 50 μm thick pure copper foils. Based on investigations employing transmission electron microscopy and scanning electron microscopy with energy dispersive X-ray spectrometry, the microstructural changes in the joint region and the adjacent substrates were examined as a function of bonding time at a temperature of 1150°C. The results indicated that hafnium as an alloying element in single crystal NiAl had a strong effect on the microstructure of joints. In particular, attention is focused on the formation of Hf bearing phases and the suppression of σ phase precipitation during bonding.