(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 Al 2 Cu phases Q and Q 0 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 Q-and Q 0 -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.

Time-resolved X-ray absorption spectroscopy on Al–Cu alloys – from solute copper to stable precipitates

“…With increasing ageing time, a growth of the area of this dissolution peak as well as a shift to higher temperatures is observed. This indicates a growth in volume fraction and size of these Cu clusters/pre-GP-I zones, which become more stable with increasing size, as shown by ab initio calculations (Kohlbach et al, 2010;Staab et al, 2018).…”

“…convergence in total energy. This closely follows the first calculations of these systems by SIESTA, where the reliability of the code has been intensively tested and compared with experimental data on lattice constants, bulk moduli and vacancy formation energies (Kohlbach et al, 2010). The supercell sizes used are specified in Table 1.…”

Kinetics of natural aging in Al-Mg-Si alloys studied by positron annihilation lifetime spectroscopy

“…36 Another calculation finds that 4 Cu atoms attached to a vacancy even reduce the lifetime by 0.019 ns. 37 Cu decoration of vacancies therefore has a stronger effect on positron lifetime than Si decoration which is why the lifetimes in Al-Cu alloys are generally shorter after NA, 23 but even here, the theoretical calculations underestimate the observed positron lifetime change reported, e.g. in Ref.…”

“…36 Another calculation finds that 4 Cu atoms attached to a vacancy even reduce the lifetime by 0.019 ns. 37 Cu decoration of vacancies therefore has a stronger effect on positron lifetime than Si decoration which is why the lifetimes in Al-Cu alloys are generally shorter after NA, 23 38 The lifetime of positrons in such traps has been described by averages of the individual elements. 39 This simple approximation leads to a positron lifetime of 0.200 ns for a cluster containing, e.g., 60% of Mg and Si in equal parts and 40% Al, see TABLE 1.…”

The decomposition process in high-purity Al-1.7 at.% Cu alloys with trace elements: preservation of quenched-in vacancies by In, Sn and Pb influencing the $$\theta \, '$$ formation

“…This is most likely caused by the formation of semi-coherent precipitates such as the h 0 phase, which can also act as a positron trap with a characteristic positron lifetime (s 2 ). According to abinito calculations in [48] the characteristic positron lifetime (here named s 2 ) of h 0 bulk was calculated to be 178 ps, while a vacancy on the Cu-sublattice would result in 190 ps [48]. This view is supported by the data for the pure Al-Cu alloy showing for temperatures above 250 C a characteristic positron lifetime s 2 of 180-185 ps.…”