Time evolution and thermal stability of precipitates in AA2618 and AA2024 alloys at elevated temperature studied by in situ small angle X-ray scattering

The strengthening effect of precipitates on the hardness of both 2618 and 2024 aluminum alloys aging at 190 °C for up to 96 h has been characterized by in situ high-energy laboratory small-angle X-ray scattering (SAXS) and micro-Vickers testing. The continuous time evolution of in situ SAXS profiles makes it possible to separate newly formed particles from existing ones. In this way, the time evolutions of four types of particles—Cu-Mg co-clusters, the intermediate phase, S phase, and coarse precipitates—have been evaluated separately. The strengthening contributions from four particles were evaluated using the Gerold–Haberkorn equation and the Orowan equation. In both alloys, the Cu-Mg co-clusters dominate the early age hardening, while the intermediate phase contributes most at peak hardness. Similarly, a decrease in hardness during the overaging state is mainly due to the reduction of Cu-Mg co-clusters and the intermediate phase. In the 2618 alloy, the decomposition of Cu-Mg co-clusters and intermediate phases, as well as the formation of the S phase, are delayed compared with the 2024 alloy. These results indicate that the precipitation kinetics of the 2618 alloy are slower than those of the 2024 alloy, and the slow kinetics maintain peak strength for a longer period during artificial aging at 190 °C in the 2618 alloy than in the 2024 alloy.