Transient liquid phase bonding of structural intermetallic compounds

The intermetallic compound NiAl is a candidate material for high–temperature structural applications, for example in the hot–zone of aero gas turbine engines. However, if this material is to find significant structural applications, methodologies for primary fabrication of complex components and post–service repair will be required. In particular, joining technologies suitable for bonding NiAl to nickel–base alloys are required. This paper examines transient liquid phase (TLP) bonding as a method of joining NiAl to nickel–base alloys. The focus of the paper is microstructural development at the bond–line and in the adjacent (nickel–base and NiAl) substrates. Two different interlayer materials are considered in this paper, namely: Ni – 4.5 wt.% Si – 3.2 wt.%B (designated by the American Welding Society as 'BNi–3') and commercially pure copper. Bonds between binary, nominally–stoichiometric NiAl and both commercially pure nickel and Martin Marietta 247 are examined using edge–on transmission electron microscopy. The paper considers the character of the isothermal solidification process, the formation of second–phases at the bond–line and in the adjacent substrates and the extent to which NiAl–Ni–base alloy TLP bonds obey the assumptions implicit in standard models of the TLP process.