The precipitation mechanism of the δ' (Al3Li) phase in Al-Li alloys has been controversially discussed in recent decades, specifically with respect to a conjectured congruent ordering process. However, kinetics in the Al-Li system does not allow to resolve the intermediate stages of precipitation and hence to experimentally clarify this issue. In this paper, we are revisiting the subject in ternary Al-Cu-Li alloys with pronouncedly slower kinetics, employing Transmission Electron Microscopy, High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy, Differential Scanning Calorimetry and Atom Probe Tomography. The results show clear evidence for congruent ordering in a selected compositional range, revealing an already strongly L12 ordered microstructure after natural aging with a chemically homogeneous Li distribution and a decomposition of the alloy upon annealing at elevated temperatures. The presented study of the δ' precipitation evaluates the reaction pathway of this process and compares it to the predictions of the Bragg-Williams-Gorsky model with respect to decomposition and ordering in this alloy system.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: Assistance from Moritz Stolpe and Ralf Busch with alloy preparation is gratefully acknowledged. The study was partially funded through the King Abdullah University of Science and Technology (KAUST) baseline funding program. P.N. acknowledges financial support from the Stiftung Industrieforschung and the Siemens Masters Program. P.N. and M.L. additionally acknowledge support from the German Research Foundation (DFG) via TRR 80 (Augsburg, München, Stuttgart). This work was supported by the German Research Foundation (DFG) and the Technische Universität München within the funding programme Open Access Publishing.