Liquid-assisted diffusion bonding of NiAl

A new isothermal joining method is investigated which could allow full utilization of the high temperature properties of advanced intermetallic alloys, such as NiAl, by producing joint compositions which match the parent material after bonding. In this method, interlayers of the parent material constituents are pre-placed in stoichiometric proportions on the surfaces to be joined. Bonding occurs without the application of pressure by raising the temperature above the liquidus of the lower melting constituent. The constituents then interdiffuse to produce the parent material phase and homogenize to the proper stoichiometry. This method differs from conventional liquid-assisted diffusion bonding in that no foreign melting-point depressants are added. Interrupted bonding treatments were used to characterize the kinetics of the isothermal solidification and homogenization processes in NiAl couples, and the compositional profiles of the bond region were characterized by electron probe microanalysis. Homogenization, as defined by the Ni and Al concentration maxima, follows a logarithmic time dependence once the bond region becomes single-phase NiAl.