Ni 3 Al based composite interlayers for wide gap transient liquid phase bonding of NiAl–Hf single crystals to a nickel base superalloy

Abstract

The formation of undesirable second phases, for example σ and/or carbides, represents a major problem during transient liquid phase (TLP) bonding of NiAl alloys to nickel base superalloys. Unlike many other TLP bonding systems, the formation of these second phases does not appear to be associated directly with the presence of the interlayer. Instead, the formation of second phases in the bonds results from a combination of extensive interdiffusion 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 (as a non-melting constituent) and copper (as a liquid former) allows very short bonding times and this has been found previously to suppress the formation of σ and carbides, during bonding. In contrast, the use of NiAl–Cu composite interlayers had little effect on the formation of second phases during post-bond heat treatment and this represents a considerable problem. Hence, the present paper investigates the use of an alternate, Ni₃Al–Cu, interlayer, which is able to prevent the entry of Cr, Mo, and W into the NiAl alloy and hence suppress the formation of undesirable second phases (such as σ) during post-bond thermal exposure.