Atomistic study on structure and energetics of yttria-based oxide nanoclusters in bcc-iron

Oxide Dispersion Strengthened (ODS) steels consist of a ferritic matrix with dispersed nanometer-size oxide particles. Compared to conventional steels these materials exhibit two remarkable properties that are not fully understood yet: (i) Stability: Up to rather high temperatures the number and size of the oxygen-rich nanoparticles do not change significantly. (ii) Tolerance: The nanoclusters act as sinks for transmutation helium, vacancies and self-interstitials. The first property is the reason for the improved creep strength at high temperature, whereas the second property is related to the radiation resistance of these materials. Therefore, ODS steels are promising candidates for applications as structural materials in extreme environments, i.e. at high temperature and intense particle irradiation, such as in advanced nuclear fission and fusion reactors.
The detailed structure and composition of the nanoclusters containing Y, Ti, O, along with other minor alloying and impurity elements, is still under discussion. In this work simulated annealing (SA) based on the Metropolis Monte Carlo method on a rigid lattice is used in order to determine the structure of the oxide nanoclusters with the lowest formation energy. The ferritic matrix of ODS steels is modeled by bcc-iron. Since cluster sizes up to a few nm are of interest, first-principle methods cannot be used throughout since they are computationally too expensive. However, extensive first-principle calculations on the structure and energetics of point defects and small clusters must be performed in order to obtain the parameters describing the atomic interactions in the rigid lattice used for SA. In this work not only parameters for pair interactions but also for triple interactions are determined. SA is performed for various clusters and the results are compared with available experimental and theoretical data from literature. The binding energies of the oxide nanoclusters obtained in this work can be used as input parameters of coarse-grained method such as object kinetic Monte Carlo simulations and rate theory that are often used to consider the evolution of a system of nanoclusters.