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

技术：实验能力的最终目标是量化位错和晶界之间的个体相互作用。PI及其同事已经开发出了使用基于纳米压头的偏转方法测量尺寸为100 nm x 250 nm x 7微米的独立金纳米晶体的机械性能的能力。这种方法在试样中产生拉伸和弯曲载荷。实验表明，纳米晶体中引起塑性变形所需的拉应力高达500 MPa，比块状单晶高两个数量级。然而，个体位错的详细行为无法用目前的实验能力来测量。这个SGER的重点是扩展实验能力，PI和同事们已经开发了一个使用微机电（MEMS）设备对纳米晶体施加纯拉伸力的实验能力。这将大大简化对实验的解释。此外，MEMS器件将在电子显微镜下操作，以便精确量化整体变形，塑性滑移的局部化以及在明确定义的加载状态下单个位错的行为。这种实验能力将适用于在纳米尺度上测试单个双晶体，以便直接观察和量化位错和晶界之间的相互作用。本研究具有探索性和高风险，实验MEMS加载夹具的研制将充满困难，成功与否也不确定。这种变革性的高风险研究的潜在高回报是，它能够在纳米尺度上弥合实验和理论/模拟之间的差距，从而允许在多尺度模型的最小长度尺度上直接验证概念和模型。增强的模型鲁棒性随后会扩展到更大的长度尺度。非技术：材料的多尺度建模已经彻底改变了工程师和科学家理解材料特性以及设计具有特定特性的材料的方式。多尺度模拟的潜力尚未实现，部分原因是缺乏相应的实验指导和验证。为了在实验和理论/模拟之间建立一对一的比较，有必要使数值模拟变得不那么理想，而实验变得更理想，直到定义的几何形状、加载条件、应变率和测量变量相同。模拟中考虑的许多现象可能从未被测量或从未以足够的精度量化，包括几何上必要的位错密度，小体积中的位错相互作用以及与晶界的位错相互作用。至关重要的是发展具有可控几何和已知的一组非常小的尺寸的实验。这项研究包括经过验证的基于物理的材料模型，它具有真正的预测能力。这将大大缩短具有增强强度和韧性的新型金属合金的产品开发时间。此外，一名研究生和一名本科生将参与这项高风险、高回报、变革性的研究。

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.