First-Principles Study of Point Defects in Ti–N Compounds Including Oxygen Insertion – Consequences on Oxidation of Ti Alloys

A detailed overview of oxygen insertion in titanium nitrides is presented. Our atomic-scale approach is based on DFT point defect energetics and thermodynamics based on the Independent-Point-Defect Approximation (IPDA). In the first part, the study of intrinsic defects (vacancies, anti-sites, interstitials, and dumbbells) is carried out in the δ-TiN, ϵ-Ti2N, and δ′-Ti2N binary compounds. It is shown that nitrogen vacancies are the main point defects in all of the Ti-rich nitrides. The N-rich phases show a more complicated behavior, with either interstitial N or Ti vacancies as the predominant defects. Noticeably, while the stability of δ′ is pointed out as controversial, the thermodynamic modeling of ϵ- and δ′-Ti2N shows that a proper identification of the relevant interstitial sites is crucial for understanding the physico-chemistry of these two nitrides. The thorough investigation of the Ti–N phase equilibria resulting from point defect thermodynamics confirms the validity of the chosen IPDA approach, which provides a solid basis for further studies, forming the second part of this work, on oxygen effects in Ti nitrides. The effect of O on the point defect structures of Ti nitrides is studied, and it is shown that this element has a strong tendency to substitute nitrogen, explaining the diffusion barrier effect of the nitrides. Finally, to emphasize the far-reaching practical implications of these atomic-scale results, we consider the oxygen behavior and related oxidation trends of Ti-based alloys under air conditions. We show how ab initio IPDA helps to rationalize the identification of the relevant Ti–N–O system at stake for oxidation, the roles of the phases, and the external conditions to which it is subjected as it relaxes toward equilibrium. We also show how the thermodynamic criteria derived from IPDA can help unravel the ambiguous experimental links between nitride formation and lower amounts of O dissolved in the metal of preoxidized Ti-based alloys.