ScriptaMaterialia, Vol. 36, No. 5, pp. 517-Z-21,1997 Elsevier Science Ltd PII S1359-6462(96)00415-0 Copyright 0 1997 Acta Metallurgica Inc. Printed in the USA. All rights reserved 1359~6462/97 $17.00 + .OO CLUSTER HARDENING IN AN AGED Al-Cu-Mg ALLOY Simon P. Ringer *, Kazuhiro Hono+, Toshio Saksai and Ian J. Polmear* Institute for Materials Research, Tohoku University, Sendai, Japan *Department of Materials Engineering, Monash University, Clayton, 3 168, Victoria, Australia ‘INational Research Institute for Metals, 1-2-1 Sengen, Tsukuba, 305, Japan $ Emeritus Professor, Department of Materials Engineering, Monash University, Clayton, 3 168, Victoria, Australia (Received June 9, 1996) (Accepted October 9, 1996) Introduction The phenomenon of age hardening was fast discovered in an Al-Cu-Mg alloy by Wilm in Berlin in 1906, [l]. This work led to the development of the well known wrought alloy Duralumin which was quickly adopted in Germany for structural members of Zeppelin airships and for the Junkers F 13 pas- senger aircraft that first flew in 1919. These materials remain as one of the two major classes of alu- minium alloyis used for aircraft construction worldwide (2000 series). Here we present a new interpretation of the origin of hardening in these important alloys. Isothermal ageing of most Al-Cu-Mg alloys over a wide temperature range (100 to 24O’C) reveals several interesting features which are illustrated in Fig. 1 for the high-purity composition Al-l.l%Cu- 1.7%Mg (at. %): (i) hardening occurs in two distinct stages separated by a plateau during which time the hardness may remain constant for many hours [2,3], (ii) the first stage of hardening occurs very rapidly and is largely complete within 60 sec., (iii) some 60% of the total hardening during ageing (peak hardness minus the hardness measured immediately after quenching from the solution treatment temperature) occurs during this rapid first stage. The last two features are not widely realised, particularly in commercially heat treated alloys, because accurate values of as-quenched (AQ) hardness are often not determined. The first stage of hardening in Al-Cu-Mg alloys has generally been attributed to the formation of Guinier-Preston zones comprised of copper and magnesium atoms (often known as GPB zones), al- though the structure and chemistry of GPB zones remains poorly defmed. Evidence for the existence of these zones and their characteristics is based on interpretations of weak diffraction effects arising from diffuse X-ray scattering in alloys aged for times corresponding to various stages along the hardness plateau [4, 5, 61. Bagaryatsky [5] considered the zones to be associated with short range ordering 517