Theoretical investigation of C, N and O insertion and diffusivity in the α 2 -Ti 3 Al-D0 19 system

The insertion and diffusivity of nitrogen and carbon atoms in the α 2 -Ti 3 Al D 0 19 system have been studied using first-principles calculations. The case of oxygen is also reexamined and compared with other interstitial species. The results show that the octahedral site ( 2a positions) composed only of Ti atoms is always found to be the most stable position. The other sites (tetrahedral, hexahedral or second octahedral sites) show significantly higher energies than the 2a sites regardless of the interstitial species. This high difference in energy also explains the slow diffusivity of the interstitial species in the α 2 system. Elastic effects and the chemical environment explain part of this remarkable stability: the 2a sites offer the lowest elastic deformation. The diffusivity is then discussed in terms of trajectories, diffusion coefficients and anisotropy. The results are compared with experimental values where available.