L1₂ ordering and δ′ precipitation in Al-Cu-Li

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.