Open Access Publications from the University of California

## Plastic Anisotropy and Kink Band Formation in Fine Grained Copper-Niobium Multilayers Produced by Accumulative Roll Bonding

• Author(s): Nizolek, Thomas Joseph
Metallic nanolaminates are a class of nanocrystalline materials composed of alternating layers of two or more dissimilar metals. These materials offer several advantages over traditional single phase nanocrystalline metals; their lamellar architecture and often immiscible constituent phases provide improved thermal stability and resistance to grain growth, while the nanocrystalline grain size and high interfacial density impart ultra-high strength and hardness. Cu-Nb nanolaminates synthesized via thin film deposition techniques have demonstrated extraordinary mechanical properties including strengths in excess of 2 GPa and significant plasticity prior to failure. Yet the limited volumes and film thicknesses (<40 $\mu$m) of these deposited materials severely limit both mechanical testing and the potential applications of these thin film nanolaminates.
The effects of layer thickness on strength, anisotropy, and deformability are investigated using bulk tensile and compression specimens of ARB Cu-Nb material with layer thicknesses ranging from 1.8 $\mu$m to 15 nm. A Hall-Petch type relationship is observed for the materials studied, however significant mechanical anisotropy is present in sub-100 nm nanolaminates. While the in-plane anisotropy is found to result largely from the effects of deformation processing induced crystallographic texture, the lamellar composite structure and grain aspect ratio provides a second source of anisotropy and cause the layer-parallel shear strength to diverge from that expected from a Hall-Petch analysis. The low layer parallel shear strength drives a form of strain localization known as kink banding during layer parallel compression.