Energetics, structure and composition of nanoclusters in oxide dispersion strengthened Fe–Cr alloys

Abstract

Extensive first principle calculations on embedded clusters containing few O, Y, Ti and Cr atoms, as well as vacancies, are performed to obtain the interaction parameters to be applied in Metropolis Monte Carlo simulations, within the framework of a rigid lattice model. A novel description using both pair and triple parameters is shown to be more precise than the commonly used pair parameterization. Simulated annealing provides comprehensive data on the energetics, structure and stoichiometry of nanometer-size clusters at T = 0. Additionally, Metropolis Monte Carlo simulations are carried out at high temperature in order to investigate the dependence of nanocluster composition on temperature. The absolute value of the binding energy per O atom unit increases with cluster size and approaches a constant for large clusters. The presence of Ti and/or vacancies increases the value of this quantity. In alloys without vacancies, clusters show a planar structure, whereas the presence of vacancies leads to three-dimensional configurations. Cr is not part of the nanoclusters, except in alloys without Ti but with vacancies. In the latter case, clusters consist of a core containing O, vacancies, as well as Y and a Cr shell, which have also been observed experimentally. A good agreement between the existing experimental data on the ratios (Y + Ti) : O, Y : Ti, (Y + Cr) : O, and Y : Cr and the simulation results is found. The comparison of experimental data with those obtained by simulations demonstrates that the assumption of nanoclusters consisting of nonstoichiometric oxides that are essentially coherent with the bcc lattice of the Fe–Cr matrix leads to reasonable results.