In situ structural characterization of ageing kinetics in aluminum alloy 2024 across angstrom-to-micrometer length scales

Abstract

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 Transmission Electron Microscopy (TEM) and in situ synchrotron-based, combined ultra-small angle X-ray scattering, small angle X-ray scattering (SAXS), and wide angle X-ray scattering (WAXS) across a length scale from sub-Angstrom to several micrometers. TEM brings information concerning the nature, morphology, and size of the precipitates while SAXS and WAXS provide 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 simultaneously. These two processes are shown to have different kinetic rates, with the cluster-dissolution rate approximately double the S-phase formation rate. We 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. Importantly, the SAXS/WAXS results show the absence of an intermediate Guinier-Preston Bagaryatsky 2 (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.

Towards understanding the microstructural and structural changes in natural hierarchical materials for energy recovery: In-operando multi-scale X-ray scattering characterization of Na- and Ca-montmorillonite on heating to 1150 °C

“…One such technique involves combining ultra-small angle X-ray scattering (USAXS), small angle X-ray scattering (SAXS), and wide angle X-ray scattering (WAXS) characterization into a single set of measurements with one sample configuration. 18–20 In this study, we investigate the simultaneous microstructural and crystal structural changes in Na- and Ca-montmorillonites on heating to 1150 °C using USAXS/SAXS/WAXS measurements. USAXS, SAXS, and WAXS capture the clay morphology, the characteristic basal spacing, and the crystalline structures of clays collectively spanning a size range of µm to Å in one combined measurement.…”

Influence of Aerospace Standard Surface Pretreatment on the Intermetallic Phases and CeCC of 2024-T3 Al-Cu Alloy

“…A second group of particles of irregular appearance, whose dimensions range from a few micrometres up to 15 µm, are composed of Al-Cu-Mn-Fe-Si or Al-Cu-Fe-Mn; these are present on the surface as both isolated particles and clusters or clumps of several particles, Figure 2 (see black arrows). The stoichiometry of this second type is more difficult to assign [4,5], but it has been defined as Al-Cu-Mn-Fe-(Si). Conversely, some intermetallic particles have been removed after Pretreatment 2, giving rise to cavities that appear in positions in which there were Al(Cu,Mg) phases, see white arrows in Figure 4a.…”

“…In fact, the intermetallic particles provide sites on the substrate surface where O 2 reduction can take place, leading to corrosion such as pitting or filiform corrosion. The principal alloy phases in 2024 are Al(Cu,Mg) and Al-Cu-Fe-Mn-Si [1][2][3][4][5][6][7], with different stoichiometry depending on the author consulted. As a consequence, Al-Cu alloys surfaces are often treated in order to reduce the corrosion extension.…”

Modelling of Creep in Alloys Strengthened by Rod-Shaped Particles: Al-Cu-Mg Age-Hardenable Alloys

“…In the T3 state, the alloy is in an underaged condition, and even contains a relatively high dislocation density, introduced by stretching after solution treatment. Upon heating, this microstructure evolves, the Cu:Mg co-clusters and the other intermediate phases dissolve and are progressively replaced by the equilibrium phase [37]. Thus, overageing is somewhat delayed in the T3 state (20-30 h at 260 • C).…”

“…The agreement of the model curves with the data for the T6 state is remarkable, albeit one could wonder if the computed volume fraction of intragranular precipitates is realistic, or rather grossly underestimated. As a matter of fact, the volume fraction of the S-phase could be as high as 4% at 260 • C [37] and be accompanied by substantial amounts of other intermetallic phases. Experimental evidence indeed demonstrates that holding at high temperature results in a strong increase in the number of coarse particles at grain boundaries.…”