Effect of Carbon Content on Titanium–Carbon Steel Heterostructure Interfacial Elemental Diffusion and Chemical Reaction

Differences in the physicochemical properties of titanium (Ti) and steel lead to complexity during solid-state bonding. In particular, the effect of carbon (C) in steel on elemental diffusion and interfacial reactions is still insufficiently understood. This study analyzed the interfacial morphology and the diffusion and reaction behavior of Ti, iron (Fe), and C diffusion-bonded couples, using commercial pure titanium (TA2) Ti and steels with different C contents (20#, 45#, and 85#) by diffusion bonding at 710, 870, and 960 °C, respectively. A continuous C-enriched layer formed on the Ti side of the interfaces, comprising nano- to sub-micrometer-scale face-centered cubic TiC grains, with the grain size increasing with the increasing distance from the interface. No Ti-Fe intermetallic was detected. The thickness of the C-enriched (TiC) layer increased with the increasing temperature and C content of steel, exceeding 2.5 μm in the TA2-85# joint bonded at 960 °C. During bonding, C atoms exhibited a faster diffusion rate and a stronger tendency to react with Ti, which led to preferential TiC formation at the Ti-side interface. Although Fe atoms also diffused toward the Ti side, the strong Ti-C reaction reduced the probability of Ti-Fe, and Fe was therefore mainly distributed along TiC grain boundaries. Ti atoms diffused more slowly and preferentially reacted with C, which prevented the formation of Ti-rich phases near the steel side. The activation energy of the Ti + C → TiC interfacial reaction decreased with increasing C content of steel. These findings address the research gap related to C diffusion in Ti-steel heterostructures and provide a scientific basis for controlling the detrimental diffusion of C during the fabrication.