A positron annihilation study of the formation and dissolution of L12precipitates in Al—Li alloys

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.

Fermi surface of an important nanosized metastable phase: Al3Li

Nanoscale particles embedded in a metallic matrix are of considerable interest as a route towards identifying and tailoring material properties. Al-Li alloys, which form ordered nanoscale precipitates of Al(3)Li for a range of concentrations, have been deployed successfully in the aerospace industry owing to their superior strength-to-weight ratio. The precipitates are metastable and their electronic structure has so far been inaccessible through conventional techniques. Here, we take advantage of the strong positron affinity of Li to probe the Fermi surface of nanoscale Al(3)Li precipitates.