Interfacial structure evolution of the growing composite precipitates in Al-Cu-Li alloys

In high-performance Al-Cu-Li alloys with low Li-content, δ′-precipitates exist often by enveloping the Guinier-Preston zones and θ′-precipitates, forming so-called composite phases or precipitates. Using atomic-resolution Scanning transmission electron microscopy and energy calculations, we studied these composite phases for their interface structures and microstructure evolution in relation with the mechanical property of the alloys. It is found that all the composite phases have similar interface structures in terms of Cu-Li bonding: interfacial Li-atoms are required to occupy the second nearest neighbor sites to interfacial Cu-atoms. Two types of relations, “anti-phase” and “in-phase” relationships, exist between the two sideward δ′-precipitates in the δ′/θ′/δ′ composite precipitates, depending on the number of Cu-layers contained in the inward θ′-precipitate. Furthermore, it is shown that an “anti-phase” composite precipitate has to become an “in-phase” one when the Cu-layers in the θ′-precipitate increase by growing thick from even to odd number, and vice versa. Since thickening of the θ′-precipitates in these composite precipitates has to be accomplished through a layer-by-layer mechanism, their growth involves the switching of interface relationships with significant structure modifications (until the sideward δ′-precipitate adapts to the final optimized interface structure). As such, the coarsening of these precipitates can generally be depressed upon thermal heating, leading to fine precipitate microstructure of the alloys. It is also demonstrated by first-principles calculations that formation of the composite phases greatly lowers the total system energy, owing to significant decrement of interfacial energy as the θ′/Al interfaces are replaced by the Al/δ′/θ′/δ′/Al interfaces.

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