Revealing the intrinsic θ′-T₁ precipitation competition and the role of Mg in the multiphase evolution of LPBF Al-Li-Cu alloys

This study elucidates the intrinsic precipitation mechanisms regarding the inherent competition between the θ′-Al₂Cu and T₁-Al₂CuLi phases and the role of microalloying elements. Crystallographic strain analysis suggests that T₁ requires lower strain accommodation than θ′, indicating a lower strain-energy contribution to its nucleation, consistent with experimental observations showing T₁ dominance in Li-containing alloys even in the absence of microalloying additions. A distinct precipitation evolution pathway is identified in the LPBF-processed Al-Li-Cu-(Mg) alloy. The main Al-Li-Cu-Mg precipitation pathway evolves from SSS to δ′, then to S′ + T₁ and S′ + θ′ + T₁ + χ, and finally to a T₁-dominated state. During prolonged aging, Ti/Zr-rich L1₂-type Al₃X particles appear through a separate late-stage Ti/Zr-related pathway and are associated with the secondary hardness peak. During early aging, the incorporation of thermodynamically metastable S′ into δ′/θ′/δ′ composite structures increases interfacial energy, reduces the thermal stability of θ′, and accelerates its dissolution, thereby further favoring T₁ development. At prolonged aging stages, secondary β′-Al₃(Ti,Zr) particles precipitate, which contributes an additional strengthening effect and thus produces a distinct secondary hardness peak. These results establish a coherent mechanistic framework for precipitation competition and microstructure evolution in alloys.