Phenomenological understanding of the contribution of bulk and grain boundary precipitates on strengthening in prolonged-aged Al-Zn-Mg-Cu aluminium alloys

This study is aimed at elucidating the mechanistic impact of structural evolution of bulk and grain boundary precipitates on the strength-ductility balance in prolonged artificial aged Al-Zn-Mg-Cu alloys. Combining aberration-corrected scanning transmission electron microscopy and first-principles calculations, the transition of bulk η-phase precipitates, which originated at Zn-terminated interfaces under tensile lattice strain field, was fundamentally explored. Intriguingly, as ageing progressed, significant partitioning of solute-Cu along the interfaces initiated a stacking transition in η-phase from hexagonal C14 to cubic C15 via di-hexagonal C36 Laves phase structures, leading to a reduction in lattice misfit strengthening. The driving mechanism behind this Laves phase transformation was found to link to interfacial lattice strain and Cu solute atom partitioning. Meanwhile, the aspect ratio of grain boundary S-phase precipitates that sporadically developed from the interconnected clusters present along the grain boundaries was progressively increased with ageing time, contributing to improved mechanical stability of grain boundary precipitates. Prolonged ageing led to a small decrease in tensile strength from 567 MPa to 526 MPa and minor increase in elongation from ∼11 % to ∼13 %. The new knowledge derived from the present study has the potential to transform the futuristic design and processing of next generation of aluminum alloys through tailoring of tensile strength and ductility, where the approach will be different from the conventional ageing process.