New insights into multiple thickening mechanisms of T1 precipitates in Al-Cu-Li alloys

The thickening behavior of T1 precipitates in an over-aged Al-Cu-Li alloy was studied using high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy dispersive X-ray spectroscopy (EDX) and first-principles calculation. This study revealed four distinct T1 thickening mechanisms occur in the Al-Cu-Li alloy during long-term over-aging. The most prevalent mechanism, designated Type-I, is featured by a shared Al/Li layer in-between two T1 units. In contrast, Type-II and Type-IV involve two separated Al/Li layers with different misalignments, while the Type-III involves the insertion of a Cu-layer in-between two separated T1 units. These four different thickening processes can coexist and interconvert, resulting in diverse T1 precipitate configurations. The multiple T1 thickening mechanisms can regulate the lattice misfit of the T1/Al matrix interface, which potentially affect the strengthening and thermal stability of the T1 phase. © 2024