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Researchers: Zachary Aitken, Xun (Wendy) Gu, Dongchan Jang, Seok-Woo Lee (postdocs)
Collaborators: Chris Weinberger (Sandia NL), Huajian Gao (Brown University), Karin Dahmen (UIUC), and Dennis Kochmann (Caltech)
A long-standing problem in materials science is that of predicting mechanical properties from knowing the initial microstructure and the collective behavior of individual defects. The objective of this project is to develop a physical understanding of plasticity mechanisms operating in single-crystalline, amorphous, and nano-crystalline solids, when material dimensions are reduced to the nanometer scale (or to the size of their instrinsic microstructure) subjected to homogeneous deformation.
Our in-situ experiments are performed in a unique instrument "SEMentor" (SEM + Nanoindenter + Mentor = SEMentor), which is capable of conducting nano mechanical experiments at room temperature down to the cryogenic temperatures while simultaneously capturing video.
The stress-strain data obtained by the SEMentor, combined with the pre- and post- mortem TEM microstructural characterization and modeling efforts present a powerful method for gaining a more thorough understanding of plasticity at the nano-scale. This combination of techniques provides us with critical information for the development of constitutive laws that govern flow stress as a function of size and initial microstructure through an experimentally-based understanding of the underlying phenomena.
While "super-sizing" seems to be the driving force of our food industry, the direction of materials research has been quite the opposite: the dimensions of most technological devices are getting ever smaller. These advances in nanotechnology have a tremendous impact on parts of the economy as diverse as information, energy, health, agriculture, security, and transportation. Some of the examples include data storage at densities greater than one terabit per square inch, high-efficiency solid-state engines, single-cell diagnostics of complex diseases (e.g. cancer), and the development of ultra-light yet super-strong materials for vehicles, with the component sizes comprising these technological devices reduced to the sub-micron scale.
The functionality of these devices directly depends on their structural integrity and mechanical stability, driving the necessity to understand and to predict mechanical properties of materials at reduced dimensions. Yield and fracture strengths, for example, have been found to deviate from classical mechanics laws and therefore can no longer be inferred from the bulk response or from the literature. Unfortunately, the few existing experimental techniques for assessing mechanical properties at that scale are insufficient, not easily accessible, and are generally limited to thin films. In order to design reliable devices, a fundamental understanding of mechanical properties as a function of feature size is desperately needed; with the key remaining question whether materials really are stronger when the instrumental artifacts are removed, and if so then why and how.