The aging characteristics of an AI-2 Pct Li-3 Pct Cu-0.12 Pct Zr alloy at 190 °C

The aging characteristics of an Al-2 pct Li-3 pct Cu-0.12 pct Zr alloy at 190 °C have been examined using transmission electron microscopy. Specifically, the development of intragranular microstructures, and the interactions among the various precipitate species have been documented. Theδ′ phase (Al₃Li) is shown to be present, not only as small spherical particles, but also as a shell aroundβ′ (Al₃Zr) and as a film on the broad coherent faces of the θ′ (Al₂Cu). At peakage (18 hours), theδ′ is associated predominantly with theθ′. However, overaging leads to dissolution of theθ′ and a concomitant increase in the volume fraction of theT ₁. This latter phase can cut/grow through the δ’ and plays an increasing role in determining theδ′ distribution after prolonged aging. Finally, T₂ is found to precipitate within the matrix in the overaged condition. TheT ₂ precipitates display icosahedral symmetry. No evidence for the presence of theδ phase has been obtained.