Lattice Monte Carlo simulations of nanocluster formation in nanostructured ferritic alloys

Atom probe, electron microscopy and small angle neutron scattering data all indicate that nm-scale Y–Ti–O clusters precipitate from solid solution during high temperature consolidation of mechanically alloyed Fe–Cr–Ti–Y2O3 powders. These apparently coherent transition phases are precursors to equilibrium Y2Ti2O7 pyrochlore oxides, and are distinguished by a low O/M ratio and lack of TEM diffraction contrast. In order to better understand the nature of these non-equilibrium features, Lattice Monte Carlo simulations of the nanocluster composition and structure were performed. The simulations used a rigid body-centered cubic lattice, with oxygen atoms placed on the octahedral interstitial sub-lattice, and considered variations in the cluster coherency strain, bulk composition and temperature. Pair bond energies were obtained from ab-initio calculations of the mixing enthalpies within a regular solution thermodynamics model. While the model is clearly oversimplified, the results provide helpful atomic-level insight into the Y–Ti–O nanoclusters and provide a basis for understanding the thermal and radiation stability. Future work will focus on including the effect of off-lattice relaxation and highly non-equilibrium vacancy concentrations on the precipitation kinetics.