Direction-dependent mechanical strength of nanostructure-tuned copper

This study presents the direction-dependent mechanical properties of different nanostructures of nanotwinned copper (NT-Cu), utilizing advanced nano-scale analysis techniques. Nanoindentation tests were performed on the top and cross-sectioned surfaces of the electroplated Cu films. The ultrafine-grained plus nanotwinned (UFG + NT) Cu exhibited the highest hardness, reaching 2.57 and 2.03 GPa from its top and cross-sectioned surfaces, respectively, more than twice that of the coarse-grained (CG) Cu. In fine-columnar-grained plus nanotwinned (FCG + NT) Cu, hardness is higher (2.51 GPa) when the indentation is perpendicular to the twin planes than parallel to the twin planes (1.93 GPa). The measured Kernel average misorientation (KAM) increased with grain sizes, with the (UFG + NT) Cu showing the highest values: 0.61 on the top surface and 0.98 on the cross-sectioned surface. This resulted in the highest geometrically necessary dislocation density (ρGND) at 2.08 × 10¹⁵ and 3.34 × 10¹⁵. We also calculated strain rate sensitivity (M) and apparent activity volume (V) for each structure. The M value was found to be more sensitive to twin boundaries, while the V value is interdependent with the twin spacing. A confined layer slip model was adopted for an accurate cross-verification of theoretical predictions alongside experimental results. This comprehensive study enhances the understanding of essential parameters that contribute to the mechanical performance of Cu foils with various microstructures.