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Competition & Correlation Among Length Scales: Mesostructure & Mechanical Properties

Synthesis
Nanotwinned metals and alloys are emerging as a particular form of nanoscaled material that can exhibit high strength coupled with improved thermal stability, both of which are yet unexplained. We have developed an integrated experimental, modeling and simulation program to examine the underlying mechanisms of plasticity in nanotwinned samples. We employ a range of methods to create systems with differing twin morphologies and microstructures and characterize their structures using a range of techniques, from electron microscopy to synchrotron scattering to the use of an atom probe.  Mechanical testing of these samples is carried out in a novel tensile strain stage that enables accurate measurements of stress-strain behavior with concurrent in situ transmission electron microscopy (TEM) observations of evolving microstructures.  We will also use a temperature-controlled nanoindentor to characterize the thermal dependence of the mechanical properties.   The experiments are coupled with a modeling and simulation program that includes atomistics, dislocation dynamics and polycrystal plasticity simulations. The experiments provide both realistic validation of models and a deeper understanding of fundamental mechanisms, enhancing the development of new understandings of deformation in nanotwinned materials.  This program will not only shed new light on plasticity in nanotwinned materials by bridging the current gap between the experiments and modeling, but it will also greatly enhance our overall understanding of many collective and cooperative mechanisms of plasticity in these materials.

This research is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering.

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  • The anisotropic upper critical field as a function of temperature for a stoichiometric single crystal.

    A member of the complex family of iron-based superconductors has been newly synthesized, shown to be highly ordered, and exhibits nearly optimal properties.

  • Nanoscale twin boundaries — where one side of the boundary is a mirror image of the other — are not straight as thought, but instead have "kinks". Researchers used a newly developed transmission electron microscopy technique to resolve the orientation of features along these boundaries with 1 nanometer resolution. Twin boundaries that appear straight at lower resolution, actually contain many kink-like steps. These kinks are distributed non-uniformly from twin boundary to twin boundary.

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