Origins of hardening in aged Al-Cu-Mg-(Ag) alloys

Explanations are presented for the origins of hardening in the artificially aged alloys Al-1.1Cu-1.7Mg-(0.1Ag) (at.%), which differ from those hitherto accepted. These and other compositions which lie within the alpha + S phase field of the Al-Cu-Mg phase diagram, are known to harden in two stages separated by a distinct, and often prolonged hardness 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 hardening in both alloys, is now attributed to solute clustering rather than to the formation of GPB zones. This phenomenon has been termed cluster hardening. GPB zones were not detected in the ternary Ai-Cu-Mg alloy until later in the ageing process and they are considered to be the source of the second stage of hardening. The identity of another phase, which nucleates and grows as laths on dislocation lines early in the ageing process of the ternary alloy, was confirmed as the equilibrium phase S. Contrary to what is generally believed, precipitation of this phase is not considered to have a significant effect on either stage of hardening. In the Ag-containing alloy, the second stage of hardening occurs much more rapidly and is attributed to the presence of GPB zones, plates of the hexagonal phase X' and finely dispersed particles of the S phase, which seem to form from GPB zones. This X' phase, which 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 phase (i.e. Al2CuMg), although it also contains significant amounts of Ag. (C) 1997 Acta Metallurgica Inc.