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

技术：位错中介塑性的多尺度模拟具有真正的预测能力，有可能极大地缩短新合金的开发时间，并提高由现有和新合金制成的部件的可靠性。然而，如果要实现多尺度模拟的潜力，必须根据所有相关长度尺度的实验对它们进行验证。最难进行实验的长度尺度是原子和纳米尺度，这仅仅是因为所涉及的量很小。最重要的恰恰是在最小长度尺度上的实验，因为在较小长度尺度上的现象的分支级联到所有较大长度尺度上。这一变革性项目的主旨是在纳米尺度上进行一系列确定的实验，这些实验可以作为在纳米尺度上对数值模拟进行关键评估和验证的基线。PI已经开发了方法和技术来制备具有明确的晶体取向以及明确的几何和尺寸(100 nm×250 nm×7000 nm)的独立的纳米级金单晶和双晶。利用纳米压头对试件进行偏转，对试件的力学性能进行了测试。对由此产生的力-位移数据的连续分析表明，屈服强度可以高达数百兆帕，在这种小长度的试件中，位错雪崩导致的滑移局部化是常见的。PI将扩展这些研究，这样就有可能根据样品中离散的位错活动来定量表征纳米尺度的单晶和双晶的变形。这将通过开发一种基于MEMS的致动器来实现，以加载纳米级样品的单轴拉伸。在加载过程中，将通过扫描电子显微镜和透射电子显微镜进行详细的表征。这将使我们有机会深入了解体位错源、晶界位错源或表面位错源在这种长度尺度上的优势。此外，在严格控制的条件下，可以研究纳米样品中位错的详细相互作用。同样重要的是，还可以探讨位错通过晶界传输或不传输的条件。该项目的智力价值在于，在纳米尺度上弥合实验和理论/模拟之间的差距的能力将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。然后，模型的增强的稳健性将级联到更大的长度尺度。一些特别感兴趣的项目包括确定纳米级组件中位错源的位置，位错之间的详细相互作用，以及晶界和位错的详细相互作用。非技术性：这项工作的主要更广泛影响之一将包括经过验证的基于物理的材料模型，这些模型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金属合金的产品开发。几名本科生和至少一名研究生将在纳米研究和技术方面获得重要经验。通过NSF教师研究经验(RET)计划，将与纽约市一所公立高中的科学系建立一个外展计划。结果将通过同行评议的出版物和科学会议广泛传播。

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.