Mechanical behavior of nanocomposites
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Introduction Nanocomposites are composed of two or more phases engineered to have nanoscale architectures with a high density of interfaces, and often with nested, hierarchical geometries. By engineering the length scales, morphology, crystal structure, and chemistry of interfaces, nanocomposites can deliver mechanical properties that exceed what could be achieved from a single-phase material alone. Metal nanolayered composites consisting of alternating layers of two different metals with nanoscale repeat layer spacing exhibit ultrahigh strengths that greatly exceed the strength from simple volume fraction estimation.1 Using severe plastic deformation, interfaceengineered nanocrystalline steel has been produced with a far-from-equilibrium structure containing supersaturated carbon at iron subgrain boundaries and a tensile strength of 7 GPa, which is perhaps the strongest bulk ductile metallic nanocomposite reported.2 Biological systems such as nacre, a composite consisting of aragonite platelets separated by a thin organic layer, are known to have superior toughness.3 Bioinspired hierarchical and nanolaminated multiphase steels have been developed that exhibit superior fatigue resistance at ultrahigh flow strengths.4 The articles in this issue of MRS Bulletin highlight recent developments in the mechanics of nanocomposites that are composed of a combination of metallic, polymer, biological, and ceramic materials. They also outline the challenges and opportunities for future research directions. Advances in computational
and experimental approaches to guide the synthesis and elucidate novel and unusual aspects of nanomechanical behavior of composites are highlighted.
Metal-based nanocomposite systems Earlier work on metallic nanocomposites focused on simple geometries such as laminates, which contain metals with cubic crystal symmetries to explore size effects in plasticity following the “smaller is stronger” paradigm,5,6 and later on, expanded to other material systems such as metal–ceramic7,8 and metal–metallic glass.9–11 Recently, interest in this field has broadened to explore new forms of metal-based nanocomposites such as metal/nanocarbon, cubic/noncubic, and designed three-dimensional (3D) morphologies. A new paradigm of interface-dominant or interface-enabled mechanical behavior (“it is all about interfaces”) has emerged.12–16 Metallic nanolayered composites exhibit ultrahigh yield strengths, typically an order of magnitude higher than annealed bulk metals. This is due to interfacial confinement of dislocations and suppression of dislocation pileup-based behavior at the nanoscale. However, at the limit of a few nanometers, nanolaminates fail by shear bands initiated when the interfacial confinement breaks down and single dislocations, without the mechanical advantage of a pileup, transmit shear across the interphase boundaries.17 Suppression of shear bands in nanocomposites while taking advantage of the unprecedented high strengths enabled by nanostructuring has been a
Markus J. Buehler, Massachusetts Ins
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