Mechanisms of oxidation of pure and Si-segregated α-Ti surfaces

Using first-principles molecular dynamics, we identified the mechanisms of the oxidation of α-Ti surfaces. Si segregation was found to suppress α-case formation in Ti, which was also confirmed experimentally. Charge transfer from the metal atoms to the gas molecules drives the initial stages of oxidation on the pure and Si-segregated α-Ti (0001) surfaces, while during the later stages, oxidation proceeds via oxygen penetration into the slab. Growth of the oxide network was strongly dependent on the oxidation state of the surface Ti atoms. Oxide growth in the Si-segregated material was retarded with the formation of TiO _x (0.5 ≤ x ≤ 1) on the surface, which corresponds to the +1.5 oxidation state of the Ti atoms. The simulations and experiments clearly showed that Si reduces the ingress of oxygen into Ti, even at high temperatures. The primary and critical steps of identifying, understanding, and controlling the mechanisms of oxidation of Ti surfaces at high temperatures, as performed here, are expected to aid the design of new alloys with improved oxidation resistance.