SGER: Detailed Interaction of Dislocations and Grain Boundaries in Nanoscale Gold Bicrystals

Columbia University, New York NY

Investigators

Abstract

TECHNICAL: The ultimate goal of the experimental capability herein is to quantify individual interactions between dislocations and grain boundaries. The PI and coworkers have developed the ability to measure mechanical properties of freestanding gold nanocrystals of dimensions 100 nm x 250 nm x 7 micron using a deflection method based upon a nanoindenter. The method induces both a tensile and a bending load in the specimen. The experiments suggest that the tensile stress necessary to cause plastic deformation in the nanocrystals is as high as 500 MPa, which is two orders of magnitude higher than that for bulk single crystals. The detailed behavior of individual dislocations cannot be measured with the current experimental capabilities, though. The thrust of this SGER is to extend the experimental capability, which the PI and coworkers have developed in order use a microelectromechanical (MEMS) device to apply a pure tensile force to the nanocrystals. This will significantly simplify the interpretation of the experiments. In addition, the MEMS device will operate in electron microscopes in order to precisely quantify the overall deformation, localization of plastic slip as well as the behavior of individual dislocations under well defined loading states. This experimental capability will be amenable to testing individual bicrystals at the nanoscale in order to observe and quantify directly the interactions between dislocations and grain boundaries. This research is exploratory and high risk in that the development of the experimental MEMS loading fixture will be fraught with difficulties and success is uncertain. The potential high payoff reward of this transformative, yet high risk research is the ability to bridge the gap between experiment and theory/simulation at the nanoscale that would allow for direct validation of concepts and models at the smallest length scales of multiscale models. The enhanced robustness of the model would then cascade out to the larger length scales. NON-TECHNICAL: Multiscale modeling of materials has revolutionized the way engineers and scientists think about understanding material properties as well as designing materials with specific properties. The potential of multiscale simulations is yet unfulfilled in part because of a lack of concomitant experimental guidance and validation. In order to establish a one-to-one comparison between experiment and theory/simulation it is necessary that numerical simulations become less ideal and that experiments become more ideal until the defined geometry, loading conditions, strain rates and measured variables are the same. Many phenomena considered in the simulations have perhaps never been measured or have never been quantified with sufficient accuracy, including geometrically necessary dislocation density, dislocation interactions in small volumes, as well as dislocation interactions with grain boundaries. Of critical importance is to develop experiments that have a controlled geometry and a known set of very small dimensions. This research includes 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. In addition, one graduate student and an undergraduate student will be involved in this high-risk, high payoff, transformative research.