Structure and stability of hcp bulk and nano-precipitated Ag2Al Original Research Article

We study the short- and long- range chemical ordering in hcp bulk Ag2Al using the Monte Carlo method based on a Hamiltonian constructed via structural formation energies from ab initio electronic-structure calculations. We find that the ground-state structure of bulk Ag2Al is that proposed from X-ray data, but there are several competing metastable hcp structures. Our results provide the structural and thermodynamic properties of Ag2Al, with good agreement to experimental short-range order data (after reprocessing our data according to experimental procedure). We also discuss the influence of the Al:Ag2Al interface, coherency strain, and off-stoichiometric disorder on the structure of metastable Ag2Al γ′ nano-precipitates in an fcc Al matrix. We show that γ′ precipitates are off-stoichiometric and we provide a new structure that reproduces the observed transmission electron microscopy image while allowing for a distribution of Ag concentrations amongst the precipitates.