Rapid progress in nanotechnology is currently under way in that small structures and devices are being fabricated at the micrometer to nanometer scales. The reliable design of these structures and devices calls for an understanding of the mechanical properties of materials at small length scales. Metallic nanostructures like nanowires have been shown to exhibit ultra-high yield strength, on the order of one tenth of their elastic moduli. However, these metallic nanostructures usually have limited hardening, causing low tensile strain to failure. Such low ductility can severely affect the mechanical integrity of the constituent nanostructures in nanomechanical devices and other technological applications. There is currently a critical need to understand the fundamental deformation mechanisms governing the strain hardening and tensile ductility in metallic nanostructures. The proposed research synergistically integrates the in situ nanomechanical experiment and computational modeling to investigate the nearly unexplored strain hardening behaviors in metallic nanostructures. The results are expected to advance our fundamental understanding of deformation mechanisms governing the tensile ductility in metal nanowires and provide a mechanistic basis for the design of strong and ductile metallic nanostructures. Undergraduates will be recruited for summer research on this project. Collaborative research between the graduate students working on this project in Georgia Institute of Technology and North Carolina State University will promote their scientific exchange, increase team-work experience, and develop interdisciplinary expertise.
The metallic nanostructures such as nanowires usually exhibit ultra-high strength, but low tensile ductility, owing to their limited strain hardening capability. The objective of this proposal is to elucidate the deformation mechanisms governing the strain hardening and tensile ductility of an interesting type of metallic nanostructures - five-fold twinned Ag nanowires - which exhibit significant strain hardening in our preliminary experimental measurements. The proposed research involves three thrusts: (i) to perform the in situ nanomechanical testing to measure the tensile stress-strain responses and mechanical properties of individual nanowires; (ii) to perform the transmission electron microscopy characterization of pristine and deformed nanowires for investigation of the underlying dislocation mechanisms and particularly the effects of surface and twin boundary mediated defects; (iii) to conduct the molecular dynamics and transition state theory based atomistic modeling to elucidate dislocation mechanisms that control the strain rate and temperature effects on strain hardening and tensile ductility. The five-fold twinned nanowires studied in this project are different from the single-crystal nanowires, bulk nanocrystalline and nanotwinned metals in that the synergetic effects of free surfaces and coherent internal interfaces (i.e., twin boundaries with unique orientation parallel to the nanowire axis) can be critically important for controlling the dislocation mechanisms of hardening and related mechanical properties. The mechanistic insights gained from this project will be valuable to develop means to enhance the strength without a severe loss of ductility in a range of small-volume metallic materials.