Sub-stoichiometry-facilitated oxidation kinetics in a δ-TixC-doped Ti-based alloy

Titanium carbide has been widely considered as a stabilizer to alleviate the serious chemical activity of Ti-based alloys at elevated temperatures. However, a reverse effect may also take place: we show the sub-stoichiometry characteristic of delta -TixC can facilitate the oxidation kinetics of a Ti-6Al-4V alloy to a significant extent. In particular, oxygen atoms tend to preferentially occupy the vacant carbon sites within the Ti2C phase, giving rise to a higher oxidation rate. The intrinsic mechanisms were understood with a combination of ab initio simulation and experimental validation. Titanium carbides: from point defects to oxidation mechanismInstead of stabilizing titanium alloys during oxidation, specific titanium carbide stoichiometries can expedite oxidation kinetics. A team led by Lujun Huang and Lin Geng at Harbin Institute of Technology in China combined ab initio molecular dynamics simulations and experiments to investigate the role of titanium carbides with different stoichoimetries (TiC, Ti2C) on the oxidation kinetics of a titanium lattice. Their simulations showed that, unlike interstices in Ti and TiC, the vacant carbon sites in Ti2C are preferentially occupied by oxygen, leading to the stronger oxidation propensity. Experiments showing an enhanced initial oxidation rate confirmed that Ti2C facilitated oxidation when doped into a conventional titanium alloy at elevated temperature but, once it was consumed, a local passivation layer formed, extending oxidation life. Understanding the detrimental effect of stoichiometry on oxidation may enable us to better mitigate it.