A microstructural investigation of NiAl/Ni-Si-B/NiAl transient liquid phase bonds

Microstructural development during transient liquid-phase bonding of cast polycrystalline near-stoichiometric NiAl (B2 compound, beta-phase) was investigated using cross-sectional transmission electron microscopy. Bonds made using Ni-4.5 wt % Si-3.2 wt % B (AWS designation BNi-3) interlayers in the form of melt-spun foils were studied. Microstructural changes in the joint region and surrounding substrates were examined as a function of holding time at temperatures of 1065 and 1150 degrees C. The transformation of the substrate beta-phase to L1(0)-type martensite due to aluminium transfer to the joint has been discussed, and the formation of L1(2) type (gamma' structure) layers at the joint-substrate interfaces considered. The nature of the isothermal solidification process has also been discussed and the changes, with holding time at the bonding temperature, in the character of the residual eutectic examined. The formation of M(23)X(6)-type borides within both the joint and surrounding substrates was also considered.