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

Abstract

Microstructural investigations of activated diffusion brazed (ADB) joints in IN738 plate were conducted. Joints of this type, which had wide gaps, were formed using the brazing alloys Nicrobraz 150, BRB, and DF4B. Tests on the wettability of the brazing alloys when associated with an IN738 surface showed that a slight degree of wetting was associated with a rapid onset of isothermal solidification and conversely a heavy degree of wetting was associated with a relatively low rate of isothermal solidification. The microstructure of the joint area was characterised using a scanning electron microscope and an electron probe X-ray microanalyser (EPMA). These showed that borides 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 the three brazing alloys, e.g. an Ni–Ni₃ B eutectic phase was observed in Nicrobraz 150, but DF4B and BRB exhibited a coarsened gamma prime (γ′) phase and an absence of a Ni₃ B matrix phase.