Explanations are presented for the origins of hardening in the artific
ially aged alloys Al-1.1Cu-1.7Mg-(0.1Ag) (at.%), which differ from tho
se hitherto accepted. These and other compositions which lie within th
e alpha + S phase field of the Al-Cu-Mg phase diagram, are known to ha
rden in two stages separated by a distinct, and often prolonged hardne
ss plateau. The first stage, which occurs very rapidly (e.g. within 60
s at 150 degrees C) and may contribute as much as 70% of the total ha
rdening in both alloys, is now attributed to solute clustering rather
than to the formation of GPB zones. This phenomenon has been termed cl
uster hardening. GPB zones were not detected in the ternary Ai-Cu-Mg a
lloy until later in the ageing process and they are considered to be t
he source of the second stage of hardening. The identity of another ph
ase, which nucleates and grows as laths on dislocation lines early in
the ageing process of the ternary alloy, was confirmed as the equilibr
ium phase S. Contrary to what is generally believed, precipitation of
this phase is not considered to have a significant effect on either st
age of hardening. In the Ag-containing alloy, the second stage of hard
ening occurs much more rapidly and is attributed to the presence of GP
B zones, plates of the hexagonal phase X' and finely dispersed particl
es of the S phase, which seem to form from GPB zones. This X' phase, w
hich is uniformly dispersed and nucleates at the sites of co-clusters
of Mg-Ag atoms, appears to have a composition close to that of the S p
hase (i.e. Al2CuMg), although it also contains significant amounts of
Ag. (C) 1997 Acta Metallurgica Inc.