Revealing Extraordinary Intrinsic Tensile Plasticity in Gradient Nano-Grained Copper

TLDR

Nano‑grained metals are typically strong yet brittle, with free‑standing films showing only a few percent tensile elongation. In a gradient nano‑grained copper film confined by a coarse‑grained substrate, plasticity is achieved by suppressing strain localization and by a mechanically driven grain‑boundary migration that causes substantial grain growth, dominating deformation. The gradient film shows tenfold higher yield strength, tensile plasticity comparable to the coarse substrate, can sustain true strains over 100 % without cracking, and its extraordinary plasticity makes it a promising coating for bulk materials.

Abstract

Nano-grained (NG) metals are believed to be strong but intrinsically brittle: Free-standing NG metals usually exhibit a tensile uniform elongation of a few percent. When a NG copper film is confined by a coarse-grained (CG) copper substrate with a gradient grain-size transition, tensile plasticity can be achieved in the NG film where strain localization is suppressed. The gradient NG film exhibits a 10 times higher yield strength and a tensile plasticity comparable to that of the CG substrate and can sustain a tensile true strain exceeding 100% without cracking. A mechanically driven grain boundary migration process with a substantial concomitant grain growth dominates plastic deformation of the gradient NG structure. The extraordinary intrinsic plasticity of gradient NG structures offers their potential for use as advanced coatings of bulk materials.