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Enormous advances in fabrication, computations, and experimental characterization have the potential to catalyze hierarchical material design, where specific material properties will be attained through not only material choices but also architecture control of its constituents, where historically coupled traits no longer have to set the limits. Based on the exploitation of unique phenomena arising in nano-scale structures, it will be possible to define material design space with vastly superior properties than can currently be achieved. Utilizing architectural features as key elements in defining multi-dimensional material design space promises to enable independent manipulation of the currently coupled physical attributes and to develop materials with unprecedented capabilities.
Creation of extremely strong yet ultra-light materials can be achieved by capitalizing on the hierarchical design of nano structured hollow lattices which promise suberb 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. To harness the lucrative properties of 3-dimensional hierarchical structures, it is critical to assess mechanical properties at each relevent scale while capturing the overall structural complexity.
The Greer Group research focuses on the problems of unraveling the physical origins of size-dependent strength in nano-scale solids, where the presence of surfaces causes the emergence of unexpected deformation mechanisms in response to mechanical deformation.
It has been shown that when the sample size is reduced not only vertically (i.e. thin films) but also laterally, the mechanical properties of single crystals, for example, drastically differ from those of their bulk counterparts. They are thought to arise from the distinct defect behavior that emerges as a result of reducing material dimensions to the nano-scale and manifest themselves by causing unusual mechanical properties.
These characteristics include avalanche-like stochastic stress-strain signature, size-dependent strength, and tension-compression asymmetry - prevalent only in those structures where the surface area is significantly higher than their volume, i.e. sub-micron scale.
While these studies provide a powerful foundation for the fundamental deformation processes operating in these materials at small scales, they are a far reach from representing real materials, whose microstructure is often complex, containing boundaries and interfaces.
In fact, both homogeneous interfaces (grain boundaries, twin boundaries, etc.) and heterogeneous interfaces (phase boundaries, precipitate-matrix boundaries, and free surface) in size-limited features are crucial elements in structural reliability of most modern materials.
Establishing the link between the observed mechanical properties and microstructural evolution remains a grand challenge, and one of my major research goals is in establishing a more quantified, predictive relationship between the competing factors of intrinsic and extrinsic limitations on the overall material properties.
Our key research thrusts lie in the development of innovative experimental approaches that enable us to assess nano-scale mechanical properties, and in subsequent design and fabrication of new, innovative materials with tunable desired properties.