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

非技术描述：了解纳米晶金属固体在极端压力和低温条件下的机械行为对于涉及聚变反应堆（替代能源）、爆炸载荷和装甲（国防）以及小行星撞击（科学进步）等工程应用非常有意义。 在这样的负载下，典型的工程材料，尤其是金属，即使在环境条件下，也会通过吸收大量能量而以塑性方式失效，但会以类似玻璃的方式失效。金属的机械行为取决于共同形成其结构的晶粒的尺寸。通过用孤立的纳米柱模拟单个晶粒，我们将研究这些柱（以及最终的单个晶粒）是否会以延性方式变形，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步，这种材料可以彻底改变吸能防爆民用、核和国防结构的设计，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）。这项研究还将带来高能材料物理领域的基础科学进展。 这里提出的工作是采用先进纳米制造和显微镜技术的材料科学家之间的真正合作；物理学家使用最先进的多尺度建模策略，其中包含控制原子间结构和力的基本原理；机械工程师采用最先进的光学和实验技术来表征工程固体的机械行为。 因此，它为跨学科科学技术领域的研究生和本科生提供了良好的培训。 当这些学生进入劳动力市场时，他们将比同龄人更有能力跨越界限，将基础科学和高水平工程结合起来。为了将研究想法和成果更广泛地推广到社区，获得这笔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项目。因此，该项目将支持鼓励和培养年轻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、高负载率（爆炸和爆炸）和低温（冰点以下）条件下的机械行为的理解。上述目标将通过进行一系列新颖的实验来实现，以多尺度建模和透射电子显微镜 (TEM) 分析为支持，通过在极端应力条件下（大于 20 GPa)、应变率（高于 108s-1）和温度（低温）。提出了一种在均匀张力下直接加载纳米柱的新方法。 这应该消除压缩下存在的晶格摩擦和局部压力效应。 当与低温测试相结合时，均匀张力下的负载会大大增加材料的内应力。 这应该会产生更新的位错成核和迁移机制，并为这些金属目前的动态性能极限提供进一步的见解。 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。为了将研究想法和成果更广泛地推广到社区，获得这笔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项目。因此，该项目将支持鼓励和培养年轻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.