Investigation of structural models for O-Y and O-Y-Ti clusters in bcc Fe: a density functional theory study

Six different structural models for atomic clusters in bcc Fe are investigated by density functional theory (DFT) calculations. Results for clusters with identical numbers of constituents (O, Y, Ti, and vacancies) are compared. It is found that the data on the stability or energetics of the relaxed clusters are comparable although their atomic configurations are often different. This contradicts the prevailing opinion in the related theoretical literature that favors the so-called structure matching model. In all studied cases, the absolute value of the total binding energy per cluster constituent becomes lower if Y is partially replaced by Ti. Therefore the driving force for the growth of O-Y clusters should be higher than that of O-Y-Ti clusters. This may be correlated with the experimental observation that the presence of Ti leads to a reduction of the size of the oxide clusters in nanostructured ferritic alloys and to a higher dispersion. Not only cage-like clusters but also clusters with oxygen in the center (cage) are investigated. In the absence of Ti, clusters with oxygen in the center attain more stability that cage-like clusters and the opposite holds for clusters with Ti. It is also shown that adding O atoms to cage-like clusters leads to structures with O in the center. In the present comprehensive DFT study only clusters with dimensions below 1 nm could be treated. This is still below or close to the limit of the experimental resolution of methods allowing for a simultaneous determination of atomic structure and composition of the clusters. These small clusters may be considered as nuclei for further structural evolution and growth during which a selection of the most favored cluster structures could occur.