Effect of stacking fault energy on mechanical behavior of bulk nanocrystalline Cu and Cu alloys

Twinning and dislocation slip are two major and competing modes of plastic deformation in metals and alloys. In addition to controlling the dislocation substructure in coarse grained materials, stacking fault energy (SFE) also affects the propensity to form deformation twins. However, the influence of SFE has not been fully explored in nanocrystalline materials. Here the role of SFE in deformation twinning and work hardening was systematically studied in bulk artifact-free, nanocrystalline (nc) Cu (SFE 55 mJ m−2), and a nc Cu–12.1 at.% Al–4.1 at.% Zn alloy (SFE 7 mJ m−2). The nc Cu (23 nm) and nc Cu alloy (22 nm) were synthesized using in situ consolidation during cryo and room temperature milling. Both materials showed ultra-high tensile strength, significant strain hardening, and good ductility. The nc Cu alloy exhibits a higher yield strength and lower uniform elongation (1067 ± 20 MPa, 6.5%) than that of nc Cu (790 ± 12 MPa, 14%). The SFE variation played a significant role in strengthening the nc Cu alloy. High resolution transmission electron microscopy analyses revealed that the low SFE of the nc Cu alloy alters the deformation mechanism from a dislocation-controlled deformation, which allows for the higher strain hardening observed in the nc Cu, to a twin-controlled deformation.