Diffusion bonding of TA2 titanium and 20# steel with vanadium/chromium bimetal interlayers: Microstructure, unexpected carbides, and mechanical properties

V and Cr bimetals are utilized as interlayers in the vacuum diffusion bonding of TA2 Ti and 20# steel to prevent the formation of Ti–Fe intermetallic compounds and improve the interface compatibility. The microstructure and mechanical properties of the diffusion-bonded joints are investigated using optical microscopy, scanning electron microscopy combined with energy dispersive spectroscopy, transmission electron microscopy, X-ray diffraction, nanoindentation, and tensile testing. The results indicate that the V/Cr bimetallic interlayer effectively inhibits the formation of Ti–Fe intermetallic compounds, with solid solutions without these compounds present at the Ti/V and steel/Cr interfaces. However, a 1–3-μm-thick continuous, hard, brittle vanadium carbide layer with hardness and elastic modulus value of 6.66 GPa and 201.55 GPa is unexpectedly discovered at the interface between V and Cr. The fracture morphology analysis reveals that the continuous carbide layer at the V/Cr interface is the primary cause of joint failure. The tensile strength of the joint remains relatively stable with an increase in the bonding temperature, whereas the elongation gradually decreases. A maximum tensile strength of 252 MPa is achieved with an elongation of 4.3%.