Experimental Characterization of Gold Single Crystals and Bicrystals at the Nanoscale with Emphasis on Interaction Between Dislocations and Grain Boundaries

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TECHNICAL: Multiscale simulations of dislocation-mediated plasticity with a truly predictive capability have the potential to dramatically reduce the development time of new alloys as well as to enhance the reliability of components made from existing and new alloys. If, however, the potential of the multiscale simulations are ever to be achieved, they must be validated against experiment at all the pertinent length scales. The most difficult length scales at which to perform experiments are the atomic and nanometer length scales simply because of the small magnitude of the quantities involved. It is precisely the experiments at the smallest length scales which are the most important because the ramifications of the phenomena at the smaller length scales cascade out to all larger length scales. The thrust of this transformative project is to perform a definitive set of experiments at the nanometer length scale which can serve as a baseline for critical evaluation and validation of numerical simulations at the nanometer length scale. The PI has developed methods and techniques to fabricate free-standing nanoscale single crystals and bicrystals of gold which have a well-defined crystallographic orientation as well as a well-defined geometry and size (100 nm by 250 nm by 7000 nm). The mechanical properties of the free-standing specimens were probed by deflecting the specimens using a nano-indenter. A continuum analysis of the resulting force-displacement data suggests that the yield strength can be as high as several hundred MPa, and that slip localization by avalanches of dislocations are common in specimens at this small length scale. PI will extend these studies so that it is possible to quantitatively characterize the deformation of single crystals and bicrystals with nanoscale dimensions in terms of the discrete dislocation activity within the specimens. This will be done by developing a MEMS-based actuator to load the nanoscale specimens in uniaxial tension. Detailed characterization will be done by scanning electron microscopy and also transmission electron microscopy in situ during loading. This will give the opportunity to shed insight into the predominance of bulk, grain boundary or surface dislocations sources at that length scale. In addition the detailed interaction of dislocations in nanoscale specimens can be investigated under carefully controlled conditions. Also of critical importance, the conditions for the transmission or non-transmission of dislocations through grain boundaries can be probed as well. The Intellectual Merit of the project is that the ability to bridge the gap between experiment and theory/simulation at the nanoscale would allow for direct validation of concepts and models at the smallest length scales of multiscale models. The enhanced robustness of the models would then cascade out to the larger length scales. Some of the specific items of interest include the determination of location of dislocation sources in nanoscale components, the detailed interaction between dislocations, as well as the detailed interactions of grain boundaries and dislocations. NON-TECHNICAL: One of the main Broader Impacts of this work would include validated physics-based material models which have a true predictive capability. This would significantly shorten the product development of new metal alloys with enhanced strength and toughness. Several undergraduates and at least one graduate student would gain important experience in nanoscale research and technology. An outreach program through the NSF Research Experience for Teachers (RET) program will be established with the science department of a public New York City high school. There will be broad dissemination of the results though peer-judged publication as well as scientific conferences.