Analysis of electron-positron momentum spectra of metallic alloys as supported by first-principles calculations

Abstract

Electron-positron momentum distributions measured by the coincidence Doppler broadening method can be used in the chemical analysis of the annihilation environment, typically a vacancyimpurity complex in a solid. In the present work we study possibilities for a quantitative analysis, i.e., for distinguishing the average numbers of different atomic species around the defect. First-principles electronic structure calculations determining self-consistently electron and positron densities and ion positions are performed for vacancy-solute complexes in Al-Cu, Al-Mg-Cu, and Al-Mg-Cu-Ag alloys. The ensuing simulated coincidence Doppler broadening spectra are compared with measured ones for defect identification. A linear fitting procedure, which uses the spectra for positrons trapped at vacancies in pure constituent metals as components, has previously been employed to find the relative percentages of different atomic species around the vacancy [A. Somoza et al., Phys. Rev. B 65, 094107 (2002)]. We test the reliability of the procedure by the help of first-principles results for vacancy-solute complexes and vacancies in constituent metals.

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

“…In Al-Cu alloys, one Cu atom attached to a vacancy was calculated to lower the positron lifetime by 0.003 ns, while eight Cu atoms reduce the lifetime by 0.016 ns. 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.…”

Positron annihilation studies of HPT deformed aluminium alloys

“…It has to be pointed out that because of the high annealing temperatures at all annealing stages no Guinier Preston zones can be observed [7,8]. A more detailed analysis of the lifetime spectra, separating two lifetime components, gives for the undeformed sample a value for τ 1 of 160 ps and for the second lifetime component 225 ps, close to the value of Cu-vacancy complexes [10]. The lifetime values for the deformed sample are 180 and 240 ps, respectively [7,8].…”

Advances in positron studies of age hardening in light alloys

“…This refinement is achieved by subtracting from the CDB spectrum taken for a severely deformed sample the contribution of bulk annihilations, which is determined by a two-component analysis of a PALS spectrum taken with high statistics at the same temperature and in the same time of the CDB measurement. First principle calculations [16,18] indicate that the accuracy of this procedure is of 5-10% for Cu in Al. Less satisfactory seems the case of Mg as a ternary element in Al, but in this case the calculation is quite delicate for a balance of two opposite effects on the electron density probed by a trapped positron.…”