Microstructural characterisation of transient liquid phase zone of activated diffusion brazed nickel base superalloy

Microstructural investigations of activated diffusion brazed (ADB) joints i n IN738 plate were conducted. Joints of this type, which had wide gaps, wer e formed using the brazing alloys Nicrobraz 150, BRB, and DF4B. Tests on th e wettability of the brazing alloys when associated with an IN738 surface s howed that a slight degree of wetting was associated with a rapid onset of isothermal solidification and conversely a heavy degree of wetting was asso ciated with a relatively low rate of isothermal solidification. The microst ructure of the joint area was characterised using a scanning electron micro scope and an electron probe X-ray microanalyser (EPMA). These showed that b orides with a blocky morphology were present in joints associated with all three brazing alloys examined. A major difference in matrix phase chemistry was observed however for th three brazing alloys, e.g. an Ni-Ni3B eutectic phase was observed in Nicrobraz 150, but DF4B and BRB exhibited a coarsene d gamma prime (gamma') phase and an absence of a Ni3B matrix phase.