Formation mechanism of precipitate T₁ in AlCuLi alloys

Highlights: • We found that T₁-precipitates nucleate directly from GP_T1 zones. • We found that T₁-precipitates grow thicker in two possible paths. • We determined the structure of GP_T1 zones by advanced atomic imaging techniques. • We suggested a possible structural evolution from GP_T1 zones to precipitate T₁. • The suggested structural evolution was proved to be energetically reasonable. - Abstract: The formation mechanism of the plate-like strengthening T₁-phase in an Al–4.15Cu–1.15Li (wt.%) alloy has been revealed by means of advanced imaging techniques and the first-principle calculations. It is shown that T₁-precipitates nucleate directly from their own GP_T1 zones and grow thicker in two possible paths. One is regular by repeating T₁-cells, and the other is abnormal via T₁-variants. Structural model of GP_T1 zones has been suggested, and refined by image simulation. A possible structural evolution from GP_T1 zones to precipitate T₁ at atomic level is suggested and proved to be energetically reasonable