In Situ Structural Characterization of Ageing Kinetics in Aluminum Alloy 2024 across Angstrom-to-Micrometer Length Scales

Published: June 01, 2016

Author(s)

Fan Zhang, Lyle E. Levine, Andrew J. Allen, Carelyn E. Campbell, Adam A. Creuziger, Nataliya Kazantseva, Jan Ilavsky

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 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
Volume: 111
Pub Type: Journals

Keywords

Al-Cu-Mg alloys, precipitation kinetics, microstructure characterization, thermodynamic modeling, in situ synchrotron X-ray scattering
Created June 01, 2016, Updated November 10, 2018