A precipitation pathway of T1 phase via heterogeneous nucleation on Li-rich particle in Al-Cu-Li alloy

The hexagonal T1 (Al2CuLi) phase is the main strengthening precipitate in Al-Cu-Li alloys and is usually considered to be formed heterogeneously at dislocations during artificial aging. The precipitation scenario in Al -Cu-Li-Mg alloy has been revisited using high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and in-situ heating inside TEM. Octahedral nano-complexes consisting of T1 precursors (T1p) on {111} facets of early-precipitated delta'(Al3Li) particles form at under-aging stage in the investigated alloy. This nano-complex with an average diameter of 5-10 nm is a transitional structure, which gradually dissolve to facilitate the transformation of a small portion of large precursors toward T1 precipitates. The T1p is determined to be isostructural to orthorhombic ohm phase based on atomic-resolution HAADF-STEM observations. The subsequent inward diffusion of Li is believed to promote the transition from ohm-like T1p to T1. An alternative precipitation path toward the key hardening T1 precipitate is thus proposed and could explain the formation of T1 precipitates apart from those at the dislocations. Our findings suggest complex multiple precipitation pathways for T1 phase should exist in Al-Cu-Li alloys.