Interface microstructure and mechanical response of similar and dissimilar joints of (γ + α2) titanium aluminide and α-titanium alloy produced by diffusion bonding

Abstract

Titanium aluminides are one of the potential materials for aerospace applications because of their high specific strength and good resistance to creep and oxidation at elevated temperatures. However, it is quite challenging to obtain similar and dissimilar joints with these materials by fusion welding due to their high crack sensitivity, limited ductility, and low thermal conductivity. In this study, joints of (γ + α2)-titanium aluminide (TiAl) to itself and with α-Ti alloy were produced by solid-state diffusion bonding performed under vacuum in the temperature range of 750–950 °C and pressure of 10–20 MPa for 10–30 min. The resultant interface microchemistry and microstructures were examined in-detail by scanning electron microscope in conjunction with energy dispersive spectroscopy and electron backscattered diffraction. Instrumented indentation and shear testing were employed to examine hardness variation across the interface and to evaluate the bond strength of the joints, respectively. The TiAl/TiAl joint possessed a single thin interface layer of α2 (Ti3Al), whereas two interphase layers comprised, viz. α + α2 (layer 1) and α2 (layer 2) were formed at interface of the α-Ti/TiAl joint. The minimum bonding temperatures of 900 °C and 800 °C were identified for producing metallurgically sound joints of TiAl/TiAl and α-Ti/TiAl, respectively, under an applied pressure of 10 MPa for 30 min. The bond strength of ∼400 MPa, which is ∼90% shear strength of TiAl base alloy, was achieved for TiAl/TiAl joints produced at 900 °C and above. The α-Ti/TiAl joint produced at 950 °C showed a maximum bond strength of ∼504 MPa. The intimate contact of surfaces through the complete collapse of their asperities and formation of phases, similar to that present in TiAl base alloy, at the interface led to enhanced joint strengths. The fracture surface of the shear tested specimens displayed a transition in failure mode from brittle to transgranular failure of the interface along with TiAl base alloy with an increase in bonding temperature.