(Meta-)stable phases and pre-Guinier-Preston zones in AlCu alloys constructed from ab initio relaxed atomic positions - Comparison to experimental methods

Abstract

Vacancies on different sublattices in the stable and meta-stable Al2Cu phases Θ and Θ′ have been studied by ab initio calculations. We calculate their formation energies and relaxation features. These phases exist in over-aged dilute AlCu alloys and have to grow under Al-rich conditions. Our calculations show that the preferred site for structural vacancies is on the Cu sublattice. Characteristic for early stages of aging dilute AlCu alloys are pre-Guinier–Preston (GP) zones. Here, we calculated the atomic arrangements of small clusters of Cu atoms in the aluminum matrix (Cu platelets on the {100}-planes of Al). Strong – energy lowering – relaxations of Al atoms toward the habit plane of Cu atoms possibly explain the stability of these planar configurations favored over three-dimensional agglomerations in real materials. The ab initio atomic positions for Θ- and Θ′-phases as well as for pre-GP zones have been finally used to calculate both X-ray absorption fine structure spectra and positron annihilation data – namely positron lifetimes and momentum distributions. Comparisons to the few existing experimental data are discussed and suggestions for further experiments are given.