No abstract

The coincidence Doppler broadening ͑CDB͒ technique is widely used to measure one-dimensional momentum distributions of annihilation photons, with the aim of obtaining information on the chemical environment of open-volume defects. However, the quantitative analysis of CDB spectra needs to include also purely geometrical effects. A demonstration is given here, on the basis of CDB spectra measured in quenched and in deformed pure aluminum. The comparison of the experimental results with ab initio computations shows that the observed differences come from the difference in free volume seen by positrons trapped in quenched-in vacancies or in vacancylike defects associated to dislocations. The computation reproduces accurately all details of CDB spectra, including the peak near the Fermi break, which is due to the zero-point motion of the confined positron.

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This is an initial report of a multi-technique study on the effect of Mg alloying on solute-vacancy interactions during the early stages of ageing of dilute 2xxx Al-Cu-Mg alloys so as to better understand the early rapid hardening (RH) that occurs in certain compositions of these alloys and the more general phenomena of secondary hardening (SH) at ambient temperatures. Therefore, RH at 150 °C and SH at room temperature from the as-quenched condition and after 60 sec ageing at 150 °C were studied in Al-1.1Cu and Al-1.1Cu-0.5Mg (at. %) variously by positron annihilation lifetime spectroscopy (PALS), coincidence Doppler broadening (CDB) spectroscopy and atom probe tomography (APT) and monitored by Vickers hardness measurements. The present results indicate that Cu-Cu, Mg-Mg and Cu-Mg clusters are formed in the ternary alloy already in the asquenched state and that they persist during ageing at 150 °C. The fraction of the solutes Cu and Mg that were associated with vacancies after ageing was increased 10-fold and double, respectively and the strength of the Cu clustering is enhanced greatly after 60 sec at 150 °C.