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）等工程应用具有极大的兴趣。在这种载荷下，典型的工程材料，特别是金属，会以脆性玻璃状的方式失效，即使在环境条件下，它也会因吸收大量能量而以塑性方式失效。金属的力学行为取决于共同构成其结构的晶粒的大小。通过用孤立的纳米柱模拟单个颗粒，我们将研究这些柱（以及最终单个颗粒）是否会在上述极端条件下以延展性的方式变形。如果它们在环境条件下保持相同的延展性，那么这项研究将迈出重要的第一步，开发新的金属材料，这将彻底改变吸能防爆民用、核和国防结构的设计，包括减少创伤性脑损伤的个人防护设备（头盔和防弹衣）。这项研究也将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之间的真正合作，采用先进的纳米制造和显微镜技术；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则；机械工程师使用最复杂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。随着这些学生进入工作岗位，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。为了将研究理念和成果更广泛地带到社区，获得该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）条件下的力学行为的理解。上述目标将通过开展一系列新颖的实验,得到多尺度建模和透射电子显微镜(TEM)分析,通过加载TEM-ready单晶nanopillar fcc(铜)和bcc的样本(Mo)不同长度的金属(50 - 100 nm)和纵横比(直径50 - 100 nm) sub-nanosecond激光产生压力波的上升时期,在极端条件下的压力(大于20 GPa),应变率(高于108 s - 1),温度（低温）。提出了在均匀张力下直接加载纳米柱的新方法。这应该消除晶格摩擦和局部压力的影响下存在的压缩。当与低温试验相结合时，均匀拉伸下的加载应大大增加材料的内应力。这将导致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，这项研究可能为受冲击固体的无位错塑性提供第一个实验证据。为了将研究理念和成果更广泛地带到社区，获得该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.