Ni3Al based composite interlayers for wide gap transient liquid phase bonding of NiAl-Hfsingle crystals to a nickel base superalloy

The formation of undesirable second phases, for example sigma and/or carbid es, represents a major problem during transient liquid phase (TLP) bonding of NiAl alloys to nickel base superalloys, Unlike many other TLP bonding sy stems, the formation of these second phases does not appear to be associate d directly with the presence of the interlayer, Instead, the formation of s econd phases in the bonds results from a combination of extensive interdiff usion between the two substrates and the low solubility, in NiAl, of common alloying additions to nickel base alloys (such as Cr, Mo, and W), The use of a composite interlayer consisting of NiAl las a non-melting constituent) and copper las a liquid former) allows very short bonding times and this h as been found previously to suppress the formation of rr and carbides, duri ng bonding. In contrast, the use of NiAl-Cu composite interlayers had littl e effect on the formation of second phases during post-bond heat treatment and this represents a considerable problem. Hence, the present paper invest igates the use of an alternate, Ni3Al-Cu, interlayer, which is able to prev ent the entry of Cr, Mo, and W into the NiAl alloy and hence suppress the f ormation of undesirable second phases (such as sigma) during post-bond ther mal exposure.