First-principles insights on phase stability of titanium interstitial alloys

Titanium can dissolve unusually high concentrations of interstitial elements such as carbon and nitrogen to form a rich variety of compounds with beneficial structural and functional properties. A first-principles statistical mechanics study was performed to predict phase stability in the Ti-C and Ti-N binaries. Density functional theory calculations were combined with the cluster-expansion approach to determine ground-state carbon-vacancy and nitrogen-vacancy orderings over the octahedral sites of hcp and fcc Ti. A large number of vacancy-ordered rocksalt phases were found to be stable at low temperature. Monte Carlo simulations showed that the ordered rocksalts transform to a disordered rocksalt that can tolerate high vacancy concentrations at intermediate to high temperatures. Clear trends in phase stability, rooted in electronic structure, are revealed upon a comparison of the calculated Ti-C and Ti-N phase diagrams with the Ti-O phase diagrams from a previous study [N. S. H. Gunda et al., Phys. Rev. Mater. 2, 033604 (2018)].