AlCuMg Alloys Research Articles

A study of ageing in an AlCuMgZn foundry alloy

A coupled differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) study of the ageing mechanism at 190 °C after solution treatment, quenching and room temperature annealing was carried out for an Al Cu4.5 Mg0.35 Zn3 foundry alloy of standard 249.0. It was found that three different precipitation sequences are present during the ageing process: ( i ) α sss → GPZ → θ " → θ ' → θ – CuA l 2 ( ii ) α sss → GPBZ → S ' → S – CuMgA l 2 ( iii ) α sss → GPZI → GPZII → η ' − η - MgZ n 2 There are no indications of possible interactions between the different sequences which seem to develop independently. The sequence based on the θ-CuAl 2 phase is clearly accelerated in comparison with AlCuMg alloys having the same Cu:Mg ratio already investigated and reported in literature. In particular GPZ, θ″ and θ′ precipitates are already detected after room temperature annealing, and a number of equilibrium θ-phase particles is present after 15 min ageing at 190 °C, which grow during the annealing. The acceleration of the precipitation kinetics is explained in terms of a decrease in the stacking fault energy induced by the presence of Zn atoms.

Investigation of the connection between the microstructure and thermomechanical properties in structured AlCu-based alloys

A description is given of the structure and precipitation sequence of AlCu4.5 alloys with a magnesium content between those of binary AlCu and quasi-binary AlCuMg alloys. For alloys with a small magnesium content (0.23 wt%) these are the coherent copper-like Guinier-Preston zones and the incoherent Θ' and Θ phases with an adequate content of dissolved magnesium, so the precipitation sequence can be essentially realized without any difficulty. In the case of higher magnesium concentration, a re-orientation occurs because it requires both time and energy. An undisturbed transition to the Θ (Mg) phase is impossible. The phase structure becomes more similar to that of the S phase but it is unsafe to describe it as a quasi-binary structure. The different sizes which develop between the AlCu4.5 and AlCuMg0.98 alloys are discussed.

The correlation between microstructure and thermomechanical properties studied on AlCuMg-based alloys

This paper deals with the description of the structure and precipitation sequence of AlCu4.5 alloys with a content of Mg between those of binary AlCu and quasi-binary AlCuMg alloys. For alloys with a small Mg content (0.23 wt%) these are the coherent Cu-like GP zones and the incoherent θ' and θ phases with the adequate content of dissolved Mg, so the precipitation sequence can be essentially realized without any difficulties. In the case of higher Mg concentration a re-orientation occurs because it requires both time and energy. An undisturbed transition to the θ(Mg) phase is impossible. The phase structure becomes more similar to that of the S phase but it is not safe to say that it is already a quasi-binary structure. The different size developments between the AlCu4.5 and AlCuMg0.98 alloys are discussed

Abnormal age hardening in an AlCuMg alloy containing silver and lithium

The S-phase is one of the most effective strengthening precipitates in the aluminum alloys for aerospace applications. Its precipitation mechanism upon thermal aging seems to have been well revealed and understood in the studies of AlCuMg alloys. Here we report an entirely different scenario of S-phase formation in an AlCuLiMg alloy. Using atomic-resolution imaging in transmission electron microscopy, it is shown that upon aging this alloy can form a precipitate microstructure dominated by Li-containing Guinier-Preston-Bagaryatsky zones and so-called S-like phase precipitates that are previously unknown. Upon formation a S-like precipitate initiates with a monoclinic structure of Al2CuMg2/3(MgLi)1/3, and may then genetically evolve with a dynamic composition of Al2CuMgLix (1/3 ≥ x > 0) towards the orthorhombic S-phase (Al2CuMg). This phase change is more a genetic structural phase evolution rather than a conventional phase transformation. Our study shows that it is possible for AlCuLiMg alloys to form a featured microstructure overwhelmed by S-like/S-phase and T1-phase precipitates, without the commonly observed δ′/θ′/δ′ composite precipitates.