The precipitate structure and precipitation kinetics in an Al-Cu-Mg alloy (AA2024) aged at 190 °C, 208 °C, and 226 °C have been studied using ex situ TEM and in situ synchrotron-based, combined ultra-small angle X-ray scattering, small angle X-ray scattering, and wide angle X-ray scattering across a length scale from sub-Angstrom to several micrometers. TEM brings information concerning the nature, morphology, and size of the precipitates and the latter provides qualitative and quantitative information concerning the time-dependent size and volume fraction evolution of the precipitates at different stages of the precipitation sequence. Within the experimental time resolution, precipitation at these ageing temperatures involves dissolution of nanometer-sized small clusters and formation of the planar S phase precipitates. Using a three-parameter scattering model constructed on the basis of TEM results, we established the temperature-dependent kinetics for the cluster-dissolution and S-phase formation processes. These two processes, while occurring simultaneously, have different kinetic rates, with the cluster-dissolution rate approximately double the S-phase formation rate. We also identified a dissolution activation energy at (149.5 ± 14.6) kJ mol-1, which translates to (1.55 ± 0.15) eV/atom, as well as an activation energy for the formation of S precipitates at (129.2 ± 5.4) KJ mol-1, i.e. (1.33 ± 0.06) eV/atom. The SAXS/WAXS results show the absence of an intermediate GPB2/S" phase in the samples under the experimental ageing conditions. These results are further validated by precipitation simulations that are based on Langer-Schwartz theory and a Kampmann-Wagner numerical method.
Citation: ACTA Materialia
Pub Type: Journals
Al-Cu-Mg alloys, precipitation kinetics, microstructure characterization, thermodynamic modeling, in situ synchrotron X-ray scattering