First-principles insights into complex interplays among nano-phases in an Al-Cu-Li-Zr alloy

Multiple nano-phases and nano-complexes feature the typical microstructures of current commercial Al-Li alloys. We present a combined first-principles and scanning transmission electron microscopy study of various nano-features in an aged Al-Cu-Li-Zr model alloy, in attempt to elucidate the fundamental aspects of their formation and interaction. Our results suggest that highly-diffusive excess Li can segregate to the early-precipitated β’(L1 ₂-Al ₃Zr) nano-phase, to develop a tentative δ’(L1 ₂-Al ₃Li) shell at the early-aging stage. The δ’ shell promotes GP-zone precipitation along the (100) facets but is metastable only, and can eventually dissolve into β’ with time. Consequently, GP-zones can evolve into θ” and θ’ independently under the prolonged aging. Excess Li also weakly segregates to θ’ interfaces to develop an “in-phase” or “anti-phase” δ’ shell. The gained insights help clarify various experimental observations, and their implications for Al-Li alloy design are also discussed.