On Achieving Large Uniform Tensile Ductility in Nanocrystalline Metals

Improving energy efficiency in transportation is directly linked to reducing weight, which in turn requires increased strength to compensate for reduced mass. Unfortunately, for most materials, high-strength means poor ductility and visa versa. Nanocrystalline (nc) metals are being heavily pursued due to their very-high strengths, but suffer from low ductility. Using real-time X-ray scattering during mechanical loading and high-resolution electron microscopy, we have discovered that large ductility can be achieved in nanostuctured metals when planar-defect mechanisms are present in the grains. By measuring the change in orientations of the atomic crystals (planes of atoms in repeating patterns) that make up the nano-sized grains, we show that plastic deformation in ductile nc-Co is accommodated by the atomic planes reorienting to specific angles relative to the applied stress. This reveals that large ductility can be achieved by mechanical twinning, a process in which the crystal planes are displaced in a manner so they mirror each other, creating the appearance of “twins.” Identifying this deformation mechanism responsible for enhanced ductility provides new pathways for designing nanostructured materials that exhibit high-strength with large ductility.

Contact: Ryan T. Ott, rtott@ameslab.gov

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