Phase transformations during high temperature brazing of dissimilar bonding of IN738 to TiAl intermetallic compound using Ni-Si-B ternary filler alloy

In this research microstructural evolution during high temperature brazing of dissimilar bonding of IN738 Ni-base superalloy to TiAl intermetallic compound using an amorphous Ni-Si-B ternary alloy was investigated. Phase transformations via solidification and solid state reactions are discussed. Observations indicated that the microstructure of IN738/MBF-30/TiAl joint consist of four different zones; isothermal solidification zone in both sides, athermally solidified zone in the bond center, diffusion affected zone in the IN738 side and reaction layer in the TiAl side. γ-Ni solid solution phase in ISZ of the IN738 side and binary isostructural solid solutions in ISZ of the TiAl side were formed during holding time at bonding temperature. Ni-Cr borides have been formed due to binary eutectics associated with γ-Ni solid solution in the ASZ during cooling. Cr-Mo borides and Ni-rich boride with different morphologies were precipitated in the DAZ. Ni element from MBF-30 molten interlayer reacted with γ-TiAl base, leading to the formation of the reaction layer containing single phase δ-Ti2Ni and triple phase τ2-Al2TiNi+τ4-AlNi2Ti+β1-NiAl layers adjacent to TiAl substrate. Microhardness evaluation of different zones indicated that some high hardness phases have formed in the bond region and presence of the γ-Ni solid solution in the ASZ cause to decrease the detrimental effects of them.