DFT studies on the nature of Y-Ti-O nanoclusters in bcc Fe

Nanostructured Ferritic Alloys (NFA) are promising candidates for structural materials of future fusion and fission reactors. They consist of a ferritic or ferritic/martensitic Fe-Cr matrix with a high dispersion of nanometer-size yttria-based oxide particles. In this research project the nature of the yttria-based oxide nanoclusters in a bcc Fe matrix is investigated by Density Functional Theory (DFT). The main goal of the studies is the better understanding of structure, energetics and composition of the clusters.
In the first part of the work three types of structures are considered: (i) clusters consisting of parts of the bixbyite (Y2O3) or pyrochlore (Y2Ti2O7) structure embedded in bcc Fe, (ii) clusters with Y, Ti, and O on bcc sites, and (iii) clusters with of Y, Ti, on bcc sites and O on octahedral interstitial sites of the bcc lattice. Simulation cells containing the three different structures but the same composition of atoms (Fe, Y, Ti, O) are considered, and relaxation calculations are performed using the DFT code VASP. It is found that in the three cases the energetics, i.e. the total binding energy of the clusters, is very similar. This contradicts the statement of Barnard et al. [1] that type (i) structures are most favorable. Further alternative cluster models with a core similar to the NaCl structure and an oxygen atom in the center are constructed and investigated in the second part of the work. For the compositions considered some of these clusters are more stable than those investigated before. Finally, the binding energy of O, Y, Ti atoms, and of the vacancy to selected cluster structures was studied. Oxygen and the vacancy are strongly attracted by the nanoclusters, while the interaction with metal atoms is weaker.
[1] L. Barnard et al. Acta Mater. 60, 935 (2012)