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

Alloys based on the Al-Cu-Li-Mg-Ag system can develop superior mechanical properties due to the formation of T₁ precipitate plates (Al₂CuLi, space group: P6/mmm) with large aspect ratios (> 50:1) on {111}_α planes of the α-Al matrix. Individual T₁ 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” T₁ precipitates. As a result, it becomes difficult to answer key questions regarding the distribution of micro-alloying elements, Mg and Ag, within the T₁ 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 T₁ 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 T₁ 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.