Hardening nanostructures in an AlZnMg alloy

Abstract

The formation of nanoscale and sub-nanoscale solute aggregates (clusters, Guinier–Preston zones and precipitates) in an AlZnMg alloy (Al–2.1 at.% Zn–1.5 at.% Mg) has been followed by a combination of experimental techniques with the aim of correlating the properties of the aggregates with their thermal history. The choice of thermal treatments was guided by the results of mechanical and calorimetric characterizations, supported by transmission electron microscopy for the identification of the morphology of the aggregates. Positron annihilation spectroscopy (using two variants of this technique, coincidence Doppler broadening and lifetime spectroscopy) was adopted for determining the local chemistry in the proximity of open volume defects. The geometrical parameters of the distribution (size, volume fraction, numerical density of the solute aggregates) were obtained by small-angle X-ray scattering. The results of the investigation provide new information regarding: two families of vacancy-rich clusters formed during or immediately after quenching; Guinier–Preston zones formed at 95°C after room-temperature pre-ageing; growth of η′ and η phases at 150°C; solute clusters formed at room-temperature in conditions of secondary ageing after preliminary heating at 150°C.

Acknowledgements

The authors acknowledge the support of ESRF for the SAXS measurements (beamline BM26B, experiment ME-504) and, in particular, the assistance of Dr. Wim Bras and Dr. Sven Hoffmann. This work was financially supported in Italy by Ministero dell’Istruzione, Università e Ricerca (project PRIN 2004023079 “Nanostructures in light alloys”) and in Argentina by Agencia Nacional de Promoción Científica y Tecnológica (PID N°435/2003 and PICT N°12-14376/2004), Comisión de Investigaciones Científicas de la Provincia de Buenos Aires and Secretaría de Ciencia y Técnica UNCentro. One of the authors (P.M.) also acknowledges the financial support of the Regione Marche (CIPE 20/04).