Tuning generalized planar fault energies to enable deformation twinning in nanocrystalline aluminum alloys

As deformation twins have a profound impact on the plastic flow and mechanical properties of metallic materials, enhancing deformation twinning in face-centered cubic (FCC) metallic materials has long served as a unique microstructure design strategy to attain an extraordinary strength-ductility synergy. Deformation twinning, however, rarely occurs in pure FCC Al and its alloys since its generalized planar fault energies (GPFEs) are almost unaffected by most soluble alloying elements such as Mg, Zn and Cu. Here we successfully tune the GPFEs of a nanocrystalline Al-Mg alloy by alloying with Zr, Fe or Y element, and enable deformation twinning in the Zr-, Fe- and Y-containing alloys. Based on a combined analysis of microscopic observations, modeling and ab initio calculations, we find a strong grain-size-dependent twinning (i.e., twinning occurs in preferable grains having sizes in the range similar to 20-40 nm), as well as only one single twinning plane (i.e., twinning occurs in single, parallel atomic planes) for twin formation rather than intersecting twinning planes (i.e., twinning occurs in multiple, unparallel atomic planes) usually observed in coarse-grained FCC materials. This interesting twinning behavior is further observed to be accompanied by grain rotations, producing defective twin boundaries. Our experimental results extend the current understanding of the plastic deformation mechanisms in nanograined metallic materials, and will guide microstructure design of twinnable nanograined Al alloys with an improved strength-ductility synergy.