Effects of Ag or Si on precipitation in the alloy Al-2.5 mass% Cu-1.5 mass% Mg

Calorimetric measurements and electron microscopy observations were performed on Al-2.5?mass% Cu-1.5?mass% Mg alloys containing also 0.4, 1 or 2% Ag or 0.5% Si, in order to improve understanding of the relationships between precipitation processes and age hardening. The analogous behaviour of calorimetric and hardness data confirms that the first hardening stage is initiated in all alloys by GPB zone formation which occurs via a nucleation and growth controlled mechanism. The vacancy-trapping effect of Mg is increased by Ag and Si additions and leads to slower precipitation kinetics. Consequently, refined GPB zones sizes are obtained leading to an increase in hardness with respect to the ternary alloy. During the second hardening stage, the formation of the more stable S′ phase increases the total amount of strengthening precipitates in the ternary alloy. Phases typical for binary Al–Cu alloys form additionally in the Si-containing alloy. In the Ag-bearing alloys, precipitation of the hardening X′ phase occurs the earlier the higher the Ag content; it is followed by S′ precipitation. During heating of the ternary alloy, the S′ phase forms after substantial dissolution of GPB zones and of the S′′ phase identified by high resolution electron microscopy; this contradicts the concept of a continuous precipitation sequence.