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

  • Author(s): Nizolek, Thomas Joseph
  • Advisor(s): Pollock, Tresa M
  • et al.
Abstract

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.

In this work, an accumulative roll bonding (ARB) process was developed for synthesis of bulk Cu-Nb nanolaminates with layer thickness as small as 15 nm (containing over 200,000 individual layers through a 4 mm sheet thickness). The microstructure of these ARB processed nanolaminates differs significantly from previously investigated thin film nanolaminates, with dramatic changes in crystallographic texture, decreases in grain size, and increases in grain aspect ratio occurring throughout the ARB process. The bulk dimensions of these nanolaminates facilitate characterization of mechanical properties, material anisotropy, and deformation mechanisms using a suite of mechanical test methods.

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.

Kink bands are determined to be the dominant deformation mechanism during layer parallel compression of nanolaminates with layer thicknesses below 100 nm. A description of the kinematics of kink band formation is developed using observations from post-test microscopy and micropillar compression testing. An analytical model for kink band formation in perfectly plastic anisotropic materials is constructed based on these kinematics. The model predicts the experimentally observed kink band geometry and quantifies the driving force for continued initiation of new kink bands. It is confirmed that kink band formation can occur via volume preserving deformation, a result that indicates kinking is a general deformation phenomenon in plastically anisotropic materials. Finally, the phenomenon of kink band propagation is investigated using in situ SEM compression tests and digital image correlation (DIC) strain mapping during bulk compression testing. The DIC results provide a quantitative measure of the strain fields in front of a propagating kink band. Analysis of the strain fields indicates similarities between kink bands and mode II cracks, reveals the presence of a stress singularity at the tip of a propagating kink band, and points to a significant additional component to the total energy dissipation during kink band formation.

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