Insights into precipitation characteristics and strengthening mechanisms in a low Cu/Mg ratio Al-Cu-Mg-Ag alloy

Using aberration-corrected scanning transmission electron microscopy, a new fully coherent precipitate platelet designated X′′, with a flexible atomic layer FCC structure, was identified in a low Cu/Mg ratio Al-Cu-Mg-Ag alloy. The emergence of the X′′ phase is primarily attributed to the preferential segregation behavior of Mg and Cu atoms towards the {111}α Ag layers. The strengthening mechanism of the X′′ phase is principally driven by the coherency strengthening, yielding efficient reinforcement (99.8 MPa increment per 1 % volume fraction). After dislocation cutting, the X′′ phase maintains coherence with the matrix, minimizing local strain accumulation at the interface. The mechanical tests show that the alloy with the precipitated X′′ phase, despite its lower strength compared to the alloy with the precipitated Ω phase, exhibits superior ductility and work hardening. The study holds promising implications for designing novel Al-Cu-Mg-Ag alloys with a desirable strength-ductility balance.