Nucleation mechanisms and mechanical behavior of Al₃Li/Al₃Sc precipitates in Al alloys: a first-principles...

First-principles density functional theory (DFT) calculations were performed to investigate the nucleation and stability of ordered precipitates in Al-Li-Sc alloys. Three potential configurations of precipitates-Al₃Sc, Al₃Li, and Al₃Li(Al₃Sc) structures-were systematically examined. Atomic-scale structural models for Al/Al₃Li, Al/Al₃Sc, and Al₃Li/Al₃Sc were constructed along the (100), (110), and (111) crystallographic orientations. To ensure computational accuracy and convergence, specific slab thicknesses were adopted: 7, 5, and 9 layers for Al(100), Al(110), and Al(111), respectively; 9, 5, and 7 layers for Al₃Li(100), Al₃Li(110), and Al₃Li(111), respectively; and 5, 5, and 9 layers for Al₃Sc(100), Al₃Sc(110), and Al₃Sc(111), respectively. These models incorporated various stacking sequences, including bridge, top, hollow, and center configurations. Nucleation free energy calculations reveal that the nucleation free energy of the Al₃Sc(Al₃Li) structure is significantly higher than that of both the isolated Al₃Li + Al₃Sc precipitates and the Al₃Li(Al₃Sc) structure. This pronounced thermodynamic barrier fundamentally explains the absence of the inverse Al₃Sc(Al₃Li) architecture in most experimental observations.