Nanocrystalline Metals and Thin Films: Quantized Plasticity, Internal Stress, and Grain Boundary Strength

技术摘要：拟议的研究旨在通过研究变形的基本物理学来了解纳米晶金属的不寻常特性。这是通过开发一个新的连续描述称为量化晶体可塑性，并告知它与原子模拟和机械测试的结果。原子论研究通过测量触发滑移事件的临界强度分布来实现这一点。他们还探讨了控制位错吸收，排斥和成核的晶界的局部特征的基本描述。从这个意义上说，两种规模的模拟？连续统和原子论是耦合的。通知量化晶体塑性模型可以解决实验相关的样品大小和应变速率，是不可能与原子模拟。具体而言，该模型耦合到拉伸实验，研究应力辅助晶粒生长和双峰晶粒尺寸分布对数控塑性的影响。这项工作有两个主要的预期技术成果。第一部分是对纳米晶金属临界强度分布和内应力分布的基本描述。这包括理解这些分布如何随着宏观变形而演变。第二个是基于有限元的模型，捕捉这些功能，并允许预测纳米晶金属的各种应力或变形路径的机械响应。这些结果为纳米晶金属的结构-力学性能关系提供了基础。非技术性总结：纳米晶金属体现了一种极端的策略，使高强度金属？也就是把单个晶体的大小缩小到只有几百个原子。这可以减少或消除传统上削弱金属的缺陷，并引入增强金属的屏障。事实上，纳米晶金属在非凡的抗屈服性、低温下的大断裂能以及在振荡载荷（疲劳）下的改进的抗失效性方面显示出前景。拟议的工作将使来自美国和国际机构的研究人员了解基本的强化过程。一个关键的成果是一个计算工具，预测纳米晶金属的基础结构的机械性能。这使得能够优化这种材料的性能。两名研究生将接受使用先进计算和实验技术的培训。他们还将协助开发，实施和完善基于项目的学习模块，用于新的科学，技术，工程和数学（STEM）中学。这一努力的一个副产品是新颖的，基于网络的界面，以提高材料工程概念的理解，并从代表性不足的群体的学生的预期参与。

TECHNICAL SUMMARY:The proposed research aims to understand unusual characteristics of nanocrystalline metals through study of the underlying physics of deformation. This is accomplished by developing a new continuum description called quantized crystal plasticity and informing it with results from atomistic simulations and mechanical testing. The atomistic studies achieve this by measuring distributions of critical strength to trigger slip events. They also explore fundamental descriptions of the local features of grain boundaries that control dislocation absorption, repulsion, and nucleation. In this sense, two scales of simulation?continuum and atomistic?are coupled. The informed quantized crystal plasticity model can address experimentally relevant sample sizes and strain rates that are not possible with atomistic simulations. Specifically, the model is coupled to tensile experiments to study stress-assisted grain growth and the effect of bimodal grain size distributions on nc ductility. There are two primary expected technical outcomes of this work. The first is a fundamental description of the distribution of critical strengths and distribution of internal stress state in nanocrystalline metals. This includes an understanding of how these distributions evolve with macroscopic deformation. The second is a finite element based model that captures these features, and permits the prediction of the mechanical response of nanocrystalline metals to various stress or deformation paths. These outcomes provide a basis for structure-mechanical property relations for nanocrystalline metals.NON-TECHNICAL SUMMARY:Nanocrystalline metals embody an extreme strategy to make a high strength metal?namely to shrink the size of individual crystals to only a few hundred atoms across. This can reduce or eliminate defects that traditionally weaken metals and also introduce barriers that strengthen the metal. Indeed, nanocrystalline metals show promise in terms of extraordinary resistance to yielding, large fracture energy at low temperature, and improved resistance to failure under oscillating loads (fatigue). The proposed work will team researchers from US and international institutions to understand the fundamental strengthening processes. A key outcome is a computational tool to predict the mechanical properties of nanocrystalline metals based on their underlying structure. This enables optimization of the performance of such materials. Two graduate students will be trained in the use of advanced computational and experimental techniques. They will also assist in the development, implementation, and refinement of project-based learning modules for a new Science, Technology, Engineering, and Mathematics (STEM) secondary school. A spin off of this effort is novel, web-based interfaces to improve comprehension of materials engineering concepts, and the anticipated involvement of students from under-represented groups.