Effect of heterogeneous microstructures on mechanical properties of thin gradient nanotwinned copper foils

Strengthening of metals generally weakens ductility, but gradient strengthening is an exception. Heterogeneous microstructures, characterized by variations in grain size and twin spacing (structural gradients), were incorporated into 20-mu m thin nanotwinned copper (NT-Cu) foils to enhance their mechanical properties. Four distinct heterogeneous NT-Cu samples, including two-layer (2L) structures and three-layer (3L) structures, were produced through rotary electroplating. The mechanical properties, work hardening rates, and microstructural features of these samples were systematically analyzed and compared. The results showed that an increase of 9.5 %, 6.4 %, and 20.1 % in ultimate tensile strength, yield strength, and work hardening rate, respectively, could be achieved in 3L structures, and the elongation was maintained at 5.5 %. The significant structural gradients and two interfaces of 3L structures were confirmed to stimulate the production of geometrically necessary dislocations (GNDs), which increased strength and work hardening rate. Two interfaces of 3L structures also contributed to the efficient moderation of plastic strains and thus improved the ductility. This study significantly enhances the strength and ductility of thin electroplated NT-Cu foils by introducing heterogeneous microstructures, promoting the application of thin NT-Cu films in electric vehicles, batteries, and the semiconductor industry.