Collaborative Research: Investigation of Deformation Mechanisms Governing the Tensile Ductility of Twinned Metal Nanowires

纳米技术目前正在迅速发展，在微米到纳米尺度上制造小的结构和装置。这些结构和设备的可靠设计要求了解材料在小长度尺度上的机械性能。像纳米线这样的金属纳米结构已经显示出超高的屈服强度，相当于其弹性模量的十分之一。然而，这些金属纳米结构通常具有有限的硬化，导致低拉伸应变失效。在纳米机械器件和其他技术应用中，这种低延展性会严重影响组成纳米结构的机械完整性。目前迫切需要了解金属纳米结构中控制应变硬化和拉伸延展性的基本变形机制。本研究将原位纳米力学实验和计算模型相结合，研究金属纳米结构中几乎未被探索的应变硬化行为。研究结果有望促进我们对控制金属纳米线拉伸延展性的变形机制的基本理解，并为设计高强度和延展性的金属纳米结构提供机制基础。本项目将招收本科生进行暑期研究。佐治亚理工学院和北卡罗莱纳州立大学的研究生在这个项目上的合作研究将促进他们的科学交流，增加团队工作经验，并发展跨学科的专业知识。金属纳米结构，如纳米线，由于其有限的应变硬化能力，通常表现出超高的强度，但低拉伸延展性。本提案的目的是阐明一种有趣的金属纳米结构的变形机制，即五重孪生银纳米线的应变硬化和拉伸延展性，在我们的初步实验测量中表现出显著的应变硬化。该研究包括三个方面：(i)进行原位纳米力学测试，以测量单个纳米线的拉伸应力应变响应和力学性能；（ii）对原始和变形的纳米线进行透射电子显微镜表征，以研究潜在的位错机制，特别是表面和孪晶界介导缺陷的影响；（iii）进行基于分子动力学和过渡态理论的原子模拟，阐明控制应变速率和温度对应变硬化和拉伸延性影响的位错机制。本项目研究的五重孪晶纳米线不同于单晶纳米线、大块纳米晶和纳米孪晶金属，因为自由表面和相干内部界面（即平行于纳米线轴线的独特取向的孪晶界）的协同效应对于控制硬化的位错机制和相关力学性能至关重要。从该项目中获得的机械见解对于开发在不严重损失延展性的情况下提高强度的方法非常有价值，适用于一系列小体积金属材料。

Non-technical summaryRapid 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.Technical summaryThe 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.