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