Atomic-scale structure of T_1p and the T_1p-to-T₁ transformation mechanism in Al-Li-Cu alloy

This study resolves the atomic-layer structure of the T_1p precursor and establishes the complete T_1p-to-T₁ transformation mechanism in Al-Li-Cu alloys. Atomic-resolution HAADF-STEM imaging reveals that T_1p is a coherent five-layer modulation composed of alternating Cu-rich and Li-rich {111}_Al planes formed purely by solute diffusion. Comparative analysis with the seven-layer T₁ structure demonstrates that the T_1p → T₁ transition involves two sequential Shockley partial-dislocation slips on layers i and i-2. Thereafter, Al diffusion and layer-specific atomic rearrangement contribute to the completion of the transformation. These coupled processes convert the FCC stacking of the matrix into the HCP-like sequence of T₁ and generate its final lattice configuration. The findings provide a unified atomistic framework for understanding T₁ nucleation and growth in Al-Li-Cu alloys.