The flow stress, ductility, and in-plane anisotropy are evaluated for bulk accumulative roll bonded copper-niobium nanolaminates with layer thicknesses ranging from 1.8 μm to 15 nm. Uniaxial tensile tests conducted parallel to the rolling direction and transverse direction demonstrate that ductility generally decreases with decreasing layer thickness; however, at 30 nm, both high strengths (1200 MPa) and significant ductility (8%) are achieved. The yield strength increases monotonically with decreasing layer thickness, consistent with the Hall-Petch relationship, and significant in-plane flow stress anisotropy is observed. Taylor polycrystal modeling is used to demonstrate that crystallographic texture is responsible for the in-plane anisotropy and that the effects of texture dominate even at nanoscale layer thicknesses.

The ultra-high strengths of nanocrystalline materials have motivated their use over a broad spectrum of engineering and biomedical applications, [1] however these materials often fail at low plastic strains due to localized shear band formation. [2,3] Here, we demonstrate that metallic nanolaminates are capable of remarkable plasticity due to a deformation mechanism termed kinking. While coarse-grained cubic metals lack sufficient plastic anisotropy to drive kinking, this mechanism becomes active when the microstructure is refined to the nanoscale. The occurrence of kink bands in these cubic materials suggests that the oriented nanoscale microstructures have a profound effect on dislocation motion and slip system activity. This mode of deformation offers a means for improving toughness without sacrificing strength and appears applicable to a wide range of nanocrystalline materials.Grain size refinement below 100 nm imparts exceptionally high strengths to metals by dramatically increasing the number of intercrystalline grain boundaries. These boundaries serve as barriers to the motion of dislocations and thus increase the material's resistance to permanent deformation. However, an additional consequence of grain refinement to the nanoscale is an increased propensity for strain localization. [4][5][6][7] Nano-grained metals show a limited capacity for dislocation accumulation during straining and therefore lack the sustained high work hardening rates necessary to promote uniform plastic deformation. [6,8,9] Thus grain refinement, the very mechanism that allows these materials to attain ultrahigh strengths, also appears to be responsible for their tendency for strain localization and resultant low ductility.Shear banding is a prevalent and detrimental form of strain localization that is commonly observed in single-phase nanocrystalline materials [2,3,5,6] as well as metallic nanolaminates. [10][11][12][13] These bands consist of narrow planes of localized shear that often serve as sites for crack formation and occur even in traditionally ductile metals when the grain size is reduced below a few hundred nanometers. [2,3] For example, Jia and Ramesh [3] report that shear bands occur in iron at grain sizes below 300 nm and result in a compressive strain to failure of less than 6% at a grain size of 80 nm. While previous investigations have explored means of improving deformability by either delaying the initiation of shear bands or limiting the local strain along any single shear band, [14][15][16] an alternative approach is to promote a non-detrimental mode of strain localization, such as the formation of kink bands, in place of shear banding.Kink band formation has not been previously reported in nanocrystalline materials but is a recognized deformation mode in uniaxial fiber composites and low symmetry single crystals with a single dominant slip system. [17][18][19][20][21][22][23] If during compression the resolved shear stress along planes oriented nearly parallel to the compression direction reaches the stress ne...

Metallic nanolaminates, composed of alternating layers of two dissimilar metals, have attracted significant attention due to both their high strengths and their potential for excellent microstructural stability. While nanolaminates have traditionally been available only in thin-film form, advances in the severe plastic deformation process of accumulative roll bonding (ARB) have enabled the production of 4-mm-thick sheets of copper-niobium nanolaminates containing over 200,000 individual layers (a nominal layer thickness of\20 nm). The ability to produce bulk nanolaminates has greatly expanded the potential applications for these materials and has motivated investigations into formability, deformation behavior, and joining techniques. This paper presents an overview of both the ARB processing technique and recent investigations into the deformation behavior of these novel materials.

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