Vacancies and Precipitates in Al–1.9 at% Cu Studied by Positrons

Abstract

Experimental and theoretical investigations of positron trapping by disk shaped Guinier-Preston zones as appearing in AlCu alloys are performed. Using positron lifetime and angular correlation S-parameter measurements the decomposition phenomena are studied in an AI-1.9 at% Cu alloy. The energy and the wave function of a positron within a cylindrical shaped trap is calculated using Ritz's variational principle. Although a weak localization of the positrons by multilayer Guinier-Preston zones (GP zones) is to be expected from theory, the experimental spectra are dominated by positron annihilation in vacancies surrounded by Cu atoms. Above 200 "C positron trapping by semicoherent and incoherent precipitates is found.Es werden experimentelle und theoretische Untersuchungen des Positroneneinfangs durch scheibenformige Guinier-Preston-Zonen, wie sie in A1Cu-Legierungen auftreten, vorgenommen. Am Beispiel einer Al-1,9 at% Cu-Legierung werden die Entmischungsvorgange mittels Positronen-Lebensdauersowie Winkelkorrelations-S-Parameter-Messungen untersucht. Die Energie und die Wellenfunktion cines Positrons in einer zylinderformigen Haftstelle werden mit Hilfe des Ritzschen Variationsverfahrens berechnet. Obwohl ein Positroneneinfang durch die mehrschichtigen Guinier-Preston-Zonen theoretisch moglich ist, werden die Spektren durch Annihilationsakte in Leerstellen in kupferreicher Umgebung bestimmt. Oberhalb 200 "C wird ein Positroneneinfang durch semikoharente und inkoharente Ausscheidungen beobachtet.

Microstructure and vacancy-type defects of high-pressure torsion deformed Al-3 wt%Cu alloy

“…This lifetime is higher than the positron lifetime in bulk aluminium (s b $ 165 ps), but lower than the lifetime of positron trapped at aluminium vacancies (s v $ 246 ps) 25 or vacancy-copper complexes (s v-Cu $ 229-206 ps, depending on cooper environment). 26 This lifetime of $196 ps is often found in as quenched samples 12,[21][22][23] and is most likely a mixed lifetime of different vacancy-copper complexes with lifetimes reported as $205 ps 12,13,24,26,27 and 178 ps (found by Ferragut et al 22 after short heat treatment at 150 C for 1 min). The formation of Guinier-Preston zones in the quenched sample is possible but cannot be confirmed by the measurement results.…”

“…These facts can be attributed to the loss of the quenched in vacancycopper complexes at vacancy sinks as dislocations. The likely formation of multilayer GP-zones is not observed by positron lifetime, even tough these GP-zones trap positrons, 12 For the HPT-deformed sample isothermal annealing at 193 C for 1 h leads to the annealing out of the copper associated defects and the positrons are solely trapped at copper free defects, which correspond to dislocations and grain boundaries. This is indicated by the disappearance of the negative slope, associated with the copper environment ( Fig.…”

“…A similar trend upon prolonged annealing of Al-Cu is reported in the literature. 12,13,22,29 The HPT-deformed sample on the other hand, shows a slight evolution of the difference-ratio-curve towards annealed, pure aluminium ( Fig. 6(b)) upon further annealing at this temperature for up to 290 h. This is clearly supported by the DB-spectrum analysis, which shows a significant decrease of the fraction of deformed aluminium from w Al* $ 0.85 to w Al* $ 0.35 and the reduction of the copper contribution from w Cu* $0.12 to w Cu* $ 0 ( Fig.…”

“…11 In particular, this technique was successfully applied on age-hardenable Al-Cu alloys. 3,12,13 Especially, the two-dimensional Doppler broadening spectroscopy (2d-DB) proved its sensitivity to investigate the chemical environment of open volume defects in Al-Cu alloys. 3,14,15 In this study, the influence of HPT on a solution treated and quenched Al-3 wt%Cu sample on microstructure and the structural stability was determined.…”

The Influence of Trace Elements (In, Sn) on the Hardening Process of Al–Cu Alloys

“…In the base alloy the average positron lifetime steadily decreases over the course of the first 70 h at room temperature. This can be explained by the formation and coarsening of Cu clusters and GP‐I zones over time …”