Bimetallic Layered Nanocomposites: Synthesis, Structure, and Mechanical Properties
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Bimetallic layered nanocomposites, composed of alternating layers of soft metals, are shown to possess ultrahigh yield strengths, often approaching the theoretical limits defined for perfect crystals. For example, nanolayered composites made from ordinary soft metals such as Cu, Ni, Nb, etc. each with strength on the order of a few tens of MPa in the bulk can achieve strengths in excess of 2–3 GPa. More importantly, the conventional descriptions relating mechanical properties to the microstructural dimensions do not extrapolate to nanometer dimensions. The mechanisms that determine strength and failure limits of nanoscale materials lie within new realms of behavior. This overview starts with a description of the synthesis methods and the microstructures of bimetallic layered nanocomposites. The layer thickness dependence of the yield strengths of the nanocomposites, in the length scale range of submicrometer to ∼ 1 nm, is shown for a variety of material systems. Different functional dependences of layer thickness on strength are interpreted using dislocation models of strengthening. Dislocation pileup based Hall–Petch (H–P) model is applicable at submicrometer to a few tens of nanometer length scales. At smaller thicknesses, confined layer slip (CLS) of single dislocations provides the strength increase with decreasing layer thickness. A peak in strength is usually achieved at layer thickness of around 2–5 nm. This peak is determined by the stress needed for single-dislocation transmission across interfaces. Atomistic modeling results that explore the slip transmission stress for unit dislocations across coherent and incoherent interfaces are described. We show that the mechanical properties of bimetallic layered composites can be tailored via length scales as well as interface properties.