Origin of the modified orientation relationship for S(S″)-phase in Al-Mg-Cu alloys

The formation of S-phase with a modified orientation relationship (OR) has been previously observed in several Al-Cu-Mg alloys. In this paper, high-resolution transmission electron microscopy and Z-contrast imaging have been used to study the origin of the modified OR in an alloy with low Cu/Mg ratio and small Si addition. Based on the observations, and supported by ab initio simulations, the formation is governed by coherency at the (0 2 1)S//(0 1 4)Al S-phase/matrix interface, which is shown to coexist with the more commonly reported (0 0 1)S//(0 2 1)Al interface. This new (0 2 1)S//(0 1 4)Al S-phase/matrix interface explanation is compared with previously published explanations based on the invariant line concept and establishment of a different S-phase/matrix interface. Energy dispersive X-ray spectroscopy and atom probe tomography indicate that the S-phase is slightly enriched in Si. The role of Si as well as the overall alloy composition is discussed. Because of the similarities between our results and the early work of Bagaryatsky, the S″-phase notation is adopted for this early-forming, strained version of the S-phase.