Time: 2:00 p.m.
Place: IA 1/155
Julia Greer, California Institute of Technology, Pasadena, USA
Creation of extremely strong yet ultra-light materials can be achieved by capitalizing on the hierarchical design of nanostructured metallic lattices which promise superb thermomechanical properties at extremely low mass densities (lighter than aerogels), making these solid foams ideal for many scientific and technological applications. Yet, the dominant deformation mechanisms in such “meta-materials”, where individual constituent size (nanometers to microns) is comparable to the characteristic length scale of the material, are essentially unknown. In order to harness the lucrative properties of 3-dimensional hierarchical structures, it is critical to assess mechanical properties at each relevant scale while capturing the overall structural complexity.
We present the mechanical properties of nano-sized cylinders, or nano-pillars, subjected to uniaxial tension and compression in an in-situ mechanical deformation instrument, SEMentor. We focus on the interplay between the internal critical microstructural length scale of materials and their external limitations in revealing the physical mechanisms governing the mechanical deformation, where competing material- and structure-induced size effects drive overall properties. Specifically, we discuss SMALLER is STRONGER phenomenon in single crystals in the framework of dislocation nucleation driven plasticity. Nano-twinned nano-pillars exhibit twinspacing-dependent ductile vs brittle transition and a lesser smaller-is-stronger trend, also explained through dislocation nucleation. We demonstrate the combined effects of a single grain boundary and free surfaces on dislocation behavior in bi-crystalline samples, and the effects of multiple grain boundaries within nanocrystalline metallic nano-pillars, which can exhibit SMALLER is WEAKER trend depending on grain size. Finally, we demonstrate that metallic glasses exhibit strength increase, brittle-toductile transition, and unprecedented fatigue resistance when reduced to nano-scale. Unlike in bulk, all of these nano-structured nano-sized samples exhibit highly stochastic, intermittent stress-strain relationships. We attribute these dissimilarities from bulk to the nano volume-induced unique dislocation interactions with internal interfaces - grain and twin boundaries – and shear transformation zones in metallic glasses - in the presence of free surfaces and, serving as fundamental reason for observed deformation mechanisms.
We also show that complex hierarchical materials like 50 micron-diameter cylindrical CNT form bundles deform via a series of localized folding events originating near the bundle base, upon which they sequentially propagate laterally and collapse from bottom to top. This unusual deformation mechanism accompanies a foam-like stress-strain relation having elastic, plateau, and densification regimes with the added feature of undulations in the stress throughout the plateau regime that correspond to the sequential folding events. These mechanisms and their effect on the evolved microstructure and the overall mechanical properties will be discussed.
Supporting information:Vortrag-Greer-MRD-13 07 12.pdf