T1 precipitate stacks in an Al-Cu-Li-Mg-Ag alloy

• Alloys based on the Al-Cu-Li(-Mg-Ag) system can achieve ultrahigh tensile yield strength and excellent creep resistance at elevated temperatures, owing to the formation of thermally stable T 1 precipitate plates on {111} α planes of the α-Al matrix. In ternary Al-Cu-Li alloys, the nucleation of T 1 plates is difficult under the conventional isothermal ageing treatments. Therefore, micro-alloying additions of Ag and Mg are commonly employed to promote the T 1 formation. While many studies have been made in the past to uncover the segregation patterns of Ag and Mg to T 1 precipitates, debate still remains, which is due to the ambiguous distinction between a thicker T 1 plate of several unit-cell thickness and a T 1 stack comprising two or multiple T 1 plates. • In this work, a clear differentiation between a thicker T 1 plate and a T 1 stack is established. Furthermore, the atomic configurations and chemical compositions of various types of T 1 stacks were systematically characterised using atomic-resolution high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and near atomic-resolution energy-dispersive X-ray spectroscopy (EDX)-STEM. It is found that for the T 1 plates in the stacks, they can be either arranged directly on top of each other (S0) or separated by an additional stacking fault plane (S1). For the S0 stack and one type of S1 stack (S1-1), pronounced Ag enrichment is observed within the stack, specifically at the broad interface where the adjacent T 1 plates are in contact or at the stacking fault plane. In contrast, Mg is uniformly distributed throughout the entire stack. As for the other type of S1 stack (S1-2), Ag is absent, and instead, the stacking fault plane exhibits significant Cu enrichment. These experimentally observed elemental distributions were further analysed by density functional theory calculations. Alloys based on the Al-Cu-Li-Mg-Ag system can develop superior mechanical properties due to the formation of T 1 precipitate plates (Al 2 CuLi, space group: P6/mmm ) with large aspect ratios (> 50:1) on {111} α planes of the α-Al matrix. Individual T 1 precipitate plates of the same variant, each of ∼1 nm thickness, have been observed to form a stack after isothermal ageing of Al-Cu-Li-Mg-Ag alloys. These stacks are frequently—but ambiguously—described as “thicker” or “coarsened” T 1 precipitates. As a result, it becomes difficult to answer key questions regarding the distribution of micro-alloying elements, Mg and Ag, within the T 1 precipitate. In this work, atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy STEM were used to examine such stacks. It is found that individual T 1 plates within these stacks are arranged either directly on top of each other without any intervening atomic plane (S0) or are separated by an additional stacking fault plane (S1). Ag segregation is observed within the interior regions of S0 and one type of S1 stacks (S1-1), specifically where the two stacked T 1 plates make contact, whereas Mg appears to be uniformly distributed across the whole stack, without any noticeable enrichment in a specific atomic layer. In the other type of S1 stacks (S1-2), Ag is absent, while Cu is enriched in the stacking fault plane within the stack. The elemental distribution patterns observed experimentally were further elaborated by density functional theory calculations.