First-principles prediction of thermodynamics and ordering in metallic alloys

Cluster expansions are increasingly used for multi-scale simulations combining first-principles electronic-structure calculations and Monte Carlo methods to predict thermodynamic properties of alloys. A cluster expansion is a basis set expansion in terms of lattice geometrical objects (clusters) and the effective cluster interactions; it can be exact and infinite, but is tractable only if truncated. Yet until now a truncation procedure was not well-defined and did not guarantee a reliable truncated cluster expansion. We present a methodology for the optimal truncation of a cluster expansion basis set that leads to reliable thermodynamics. Then we exemplify its use in calculating quantitative thermodynamic properties of the fcc Ni3V, and show good agreement to experiment.;Next we study the short- and long- range chemical ordering in the bulk hcp Ag2Al using the Monte Carlo method based on a canonical (fixed-composition) cluster expansion. Our results provide the structural and thermodynamic properties of Ag2Al, with good agreement to experimental short-range order data. We explain the discrepancy between the Ag2Al structures proposed from the X-ray scattering on bulk Ag2Al and from the high-resolution transmission electron microscopy (TEM) experiment on the hcp gamma precipitates. We also discuss the influence of the Al:Ag2Al interface, coherency strain, and off stoichiometric disorder on the structure of metastable Ag 2Al gamma' nano-precipitates in an fcc Al matrix. We show that gamma' precipitates are off-stoichiometric and predict a new metastable AgAl structure that reproduces the observed TEM image.;After that we construct a grand-canonical cluster expansion for the hcp Al-Ag and perform ground-state search and Monte Carlo simulations to determine the metastable hcp Ag-Al phase diagram. We predict a new equilibrium hcp AgAl ground state with zero-energy domain boundary defects. From thermodynamic results, we discuss the precipitate structure and composition in Al-rich Al-Ag alloys and explain the experimental TEM data.