Strengthening mechanisms of TiAl/Ti₂AlNb diffusion bonding joint after post-bonded heat treatment and hot deformation

Brittle compounds and large residual stresses are easily formed at the diffusion bonding (DB) joints of dissimilar intermetallic compounds, which will deteriorate the performance of the joints. In this study, in order to conduct an in-depth study of the strengthening mechanisms, the post-bonded heat treatment (PBHT) and post-bonded hot deformation (PBHD) were applied to the TiAl/Ti₂AlNb DB joint. Comparative studies were implemented aiming to reveal the strengthening mechanisms and their influence on interfacial microstructure and mechanical properties. The experimental results indicated that the harmful intermetallic compound AlNb₂ phase in the DB joint was eliminated by PBHT and PBHD. Meanwhile, the diffusion layers were changed from O + (AlNb₂+α₂) + (α₂+β/B2+γ) three layers of DB joints to (α₂+β/B2+O + Ti₃NbAl₂) + (α₂+β/B2+γ) two layers of PBHD joint. Interestingly, the Ti₃NbAl₂ phase was first found in the DB joint after PBHD. After testing, the tensile strength and the relative tensile strength (defined as the tensile strength ratio of the joint and the TiAl alloy) of samples increased by 272.54 MPa and 39.2% after PBHD, respectively. In comparison, the tensile strength and the relative tensile strength of samples increased by 75.96 MPa and 10.96% after PBHT. This demonstrated that PBHD could induce the formation and uniform dispersion of the Ti₃NbAl₂ phase, which is indispensable to improve the tensile strength of the TiAl/Ti₂AlNb DB joint.