Collaborative Research: Experimentally Informed Modeling of Structural Heterogeneity and Deformation of Metallic Glasses

大多数金属是晶体，原子排列成三维周期性阵列，它们的结构-性能关系已经建立，这使得它们作为许多现代技术中不可或缺的结构材料得到广泛应用。相比之下，金属玻璃是无定形的，原子不会排列成周期性阵列。尽管在金属玻璃中发现了许多特殊的特性，如高强度和高弹性，但缺乏对其变形机制的基本理解阻碍了它们在许多技术领域的潜在应用。该项目旨在促进我们对成分如何影响结构的基本理解，特别是纳米尺度下金属玻璃的结构非均质性，以及这种结构非均质性如何影响其变形行为。假设不同类型的中量程有序（即纳米尺度上的局部有序原子排列）及其在机械加载过程中的运动是金属玻璃不同变形行为的原因。本项目采用先进的电子显微镜和多尺度计算机模拟相结合的方法，研究了不同金属玻璃中存在的中量程有序原子结构，并建立了中量程有序结构与变形行为之间的关系。在教育推广方面，该项目整合了(i)为具有不同背景的当地社区大学生提供实习计划，为他们提供研究经验并促进他们过渡到四年制大学，以及（ii）在线研讨会，传播金属玻璃和其他非晶态材料的实验和计算方法。本项目通过定量分析使用四维扫描透射电子显微镜（4D-STEM）获得的纳米衍射图，在纳米尺度上表征金属玻璃（mg）的结构非均质性，从而提供了构成mg结构非均质性的局部中程有序（MRO）的信息。利用实验结构数据，结合原子间势，基于遗传算法的StructOpt优化在MRO结构与实验数据完全一致的约束下，提供了势能最小的原子结构。得到的mro的原子构型用于随后的势能景观分析，以定量了解mro如何影响剪切转变区（STZs），包括不同剪切模式、活化能势垒、活化体积、软化行为和局部弹性模量的关键信息。这些量化基本塑性事件的关键STZ参数，以及直接来自4D-STEM的与结构非均质性相关的微观结构信息，被纳入非均质随机STZ动态模型，在与现实世界实验相关的长度和时间尺度上进行变形模拟。本文通过对模拟结果的分析，探讨了不同MRO类型及其在实验表征下的分布，以了解两种主要MG体系（即zr基MG和la基MG）中延展性和β-弛豫显著变化的结构根源。这些发现可以显著推进对mggs结构-性能关系的基本理解，并影响非晶材料的设计和发展。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-Technical SummaryMost metals are crystalline where atoms arrange into periodical arrays in 3-dimensions and their structure-property relationships have been well-established, which have enabled their widespread applications as indispensable structural materials in numerous modern technologies. In contrast, metallic glasses are amorphous where atoms do not arrange into periodical arrays. Despite many exceptional properties found in metallic glasses such as high strength and elasticity, the lack of fundamental understanding of their deformation mechanisms has hindered their potential applications in many technical areas. This project seeks to advance our fundamental understanding on how composition affects the structure, in particular, structural heterogeneity at the nanoscale, in metallic glasses and how such structural heterogeneity affects their deformation behavior. The hypothesis is that the presence of different types of medium range ordering, i.e., locally ordered atomic arrangements at the nanometer scale, and their motion during mechanical loading are responsible for different deformation behaviors of metallic glasses. Using a combination of advanced electron microscopy and multiscale computer simulations, this project investigates the atomic structures of medium range ordering present in different metallic glasses and establish a relationship between medium range ordering structures and deformation behavior. For educational outreach, the project integrates (i) an internship program for local community college students with diverse backgrounds, which provides them with research experience and facilitate their transition to 4-year universities, and (ii) online workshops for dissemination of the experimental and computational approaches for metallic glasses and other amorphous materials.Technical SummaryThis project characterize structural heterogeneities in metallic glasses (MGs) at the nanometer scale by quantitatively analyzing nanodiffraction patterns acquired using 4-dimensional scanning transmission electron microscopy (4D-STEM), which provides the information on local medium-range ordering (MRO) that constitutes the MGs’ structural heterogeneities. Using the experimental structure data, together with interatomic potentials, the genetic-algorithm-based StructOpt optimization provides atomic structures that minimize the potential energy under the constraint of MRO structures being fully consistent with the experimental data. The resulting atomistic configurations of MROs are used for subsequent potential energy landscape analysis to gain quantitative understanding on how the MROs affect shear transformation zones (STZs), including the key information on distinctive shear modes, activation energy barrier, activation volume, softening behavior, and local elastic moduli. These crucial STZ parameters that quantify the elementary plastic events, along with the microstructural information related to the structural heterogeneity directly from 4D-STEM, are incorporated into a heterogeneously randomized STZ dynamic model to perform deformation simulations at the length and time scales relevant to real-world experiments. The results from the simulations, which explore various MRO types and their distribution revealed by the experimental characterization, are analyzed to gain understanding of the structural origins of the substantial variations in ductility and β-relaxation observed in two major MG systems, i.e., the Zr-based and La-based MGs. These findings could advance significantly the fundamental understanding of structure-property relationship in MGs and impact the design and development of amorphous materials.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.