Exploring Deformation Mechanisms in Metallic Nanostructures Under Extreme Conditions of Temperature and Strain Rate

非技术描述：了解纳米晶体金属固体在极端压力和低温条件下的力学行为，对于涉及聚变反应堆(替代能源)、爆炸载荷和装甲(国防)、小行星撞击(科学进步)等工程应用具有重要意义。在这样的载荷下，典型的工程材料，特别是金属，会以类似玻璃的脆性方式失效，即使在环境条件下，它会通过吸收大量能量以塑性方式失效。金属的力学行为取决于共同构成其结构的颗粒的大小。通过用孤立的纳米管模拟单个颗粒，我们将研究这些柱子(最终是单个颗粒)是否会以延展性的方式变形，即使它们受到上述极端条件的影响。如果它们保持与环境条件下相同的延展性，那么这项研究将迈出开发新金属材料的第一大步，这种材料可能会彻底改变吸能防爆民用、核和防御结构的设计，包括用于减少创伤性脑损伤的个人防护装备(头盔和防弹衣)。这项研究还应该导致高能材料物理领域的基础科学进步。这里提出的工作是材料科学家使用先进的纳米制造和显微技术；物理学家使用最先进的多尺度建模策略(包括控制原子间结构和力的基本原理)和机械工程师使用最先进的光学和实验技术来表征工程固体的机械行为之间的真正合作。因此，它为研究生和本科生在跨学科科学和技术领域提供了极好的培训。随着这些学生进入劳动力大军，他们将比他们的同龄人更有能力跨越国界，将基础科学和高水平工程结合起来。为了将研究的想法和成果更广泛地带给社会，由这笔资金资助的研究生还将参加加州大学洛杉矶分校的高中暑期研究计划。因此，该项目将支持鼓励和培训青年人才进入科学和工程领域的社会需求。技术描述：该项目将加深对纳米结构金属固体在极端压力、高加载速率(爆炸和爆炸)和低温(低于冰点)条件下的机械行为的理解。上述目标将通过开展一系列新颖的实验来实现，这些实验以多尺度建模和透射电子显微镜(TEM)分析为支持，在应力(大于20 Gpa)、应变速率(高于108s-1)和温度(低温)的极端条件下，利用亚纳秒上升时间的激光产生的应力波加载不同长度(50 nm到100 nm)和长宽比(50 nm到100 nm)的fcc(铜)和bcc(Mo)金属单晶纳米管样品。提出了一种在均匀张力下直接加载纳米棒的新方法。这应该会消除压缩下存在的晶格摩擦和局部压力效应。当与低温试验相结合时，均匀拉伸下的加载将显著增加材料中的内应力。这将导致更新的位错成核和迁移机制，并为进一步了解这些金属目前的动态性能极限提供了进一步的见解。由于非常高的内应力，这项研究很可能为冲击固体中的无位错塑性提供第一个实验证据。为了将研究的想法和成果更广泛地带给社会，由这笔资金资助的研究生还将参加加州大学洛杉矶分校的高中暑期研究计划。因此，该项目将支持鼓励和培训青年人才进入科学和工程领域的社会需求。

Non-Technical Description:Understanding the mechanical behavior of nano-crystalline metallic solids under extreme conditions of pressure and low temperatures is of great interest to engineering applications involving fusion reactors (for alternative energy), blast loadings and armors (national defense), and asteroid impacts (progress of science), among others. Under such loading a typical engineering material, notably a metal, fails in a brittle glass-like manner even though at ambient conditions it fails in a plastic fashion by absorbing substantial energy. The mechanical behavior of metals is dependent upon the size of the grains that collectively form its structure. By simulating individual grains by isolated nanopillars we will study if these pillars (and eventually individual grains) will deform in a ductile fashion even when they are subjected to aforesaid extreme conditions. If they retain the same ductility as under ambient conditions then this research would have taken the first major step to develop new metallic materials that could revolutionize the design of energy-absorbing blast resistant civil, nuclear, and defense structures, including personal protective equipment (helmet and body armors) for reducing traumatic brain injuries. This research should also lead to fundamental scientific advances in the area of high energy materials physics. The work proposed here is a true collaboration between material scientists employing advanced nano-fabrication and microscopy techniques; physicists using state of the art multi-scale modeling strategies that encompass basic principles which govern the inter-atomic structures and forces; and mechanical engineers employing the most sophisticated optics and experimental techniques for characterizing the mechanical behavior of engineering solids. As such, it provides excellent training for graduate students and undergraduates in the area of interdisciplinary science and technology. As these students go into the work force, they will be more able than their peers to cross boundaries and combine basic science and high level engineering. To bring the research ideas and results more broadly to the community, graduate students funded under this grant will also participate in the High School Summer Research Program at UCLA. This project will thus support the societal needs of encouraging and training young talents into the fields of science and engineering. Technical Description: This project will develop understanding the mechanical behavior of nano-structured metallic solids under extreme conditions of pressure, high rates of loading (blasts and explosions), and low temperature (below freezing). The above goal will be accomplished by carrying out a series of novel experiments, backed by multiscale modeling and transmission electron microscopy (TEM) analysis, by loading TEM-ready single crystal nanopillar samples of fcc (Cu) and bcc (Mo) metals of varying lengths (50 nm to 100 nm) and aspect ratios (50 nm to 100 nm in diameter) by laser-generated stress waves of sub-nanosecond rise times, under extreme conditions of stress (greater than 20 GPa), strain rate (higher than 108s-1), and temperature (cryogenic). A new method is proposed to load the nanopillars directly under uniform tension. This should eliminate the lattice friction and local pressure effects present under compression. When combined with cryogenic testing, loading under uniform tension should substantially increase the internal stress in the material. This should result in newer dislocation nucleation and mobility mechanisms and provide further insights into the present dynamic performance limits of these metals. Because of very high internal stress, this study is likely to provide the first ever experimental evidence for dislocation-free plasticity in shocked solids. To bring the research ideas and results more broadly to the community, graduate students funded under this grant will also participate in the High School Summer Research Program at UCLA. This project will thus support the societal needs of encouraging and training young talents into the fields of science and engineering.