First-principles study of oxygen incorporation and migration mechanisms in Ti₂AlC

We performed density-functional calculations of oxygen incorporation and diffusion in layered Ti₂AlC for a range of intrinsic- and impurity-element chemical potentials. In view of the thermal equilibrium coexistence between oxygen-dissolved Ti₂AlC and the oxide scale, a thermodynamic scheme is presented that allows the comparison of the relative stability of oxygen defects in different exterior environments. The calculations show that the oxygen atom favors substitution on carbon lattice sites (O_C) under oxygen-lean conditions and high temperatures, whereas the occurrence of an oxygen interstitial in the aluminum atomic layer (I_O-tri) becomes more preferential in an oxygen-rich atmosphere and low temperatures. Interstitial oxygen (I_O-tri) diffusion via a metastable interstitial site (I_O-oct) has a comparatively low migration energy. The substitutional oxygen defect (O_C) diffuses by exchanging with neighboring carbon vacancy, which needs a relatively high diffusion barrier.