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

大多数金属是晶体，其中原子在三维中排列成周期性阵列，并且它们的结构-性质关系已经很好地建立，这使得它们能够作为许多现代技术中不可或缺的结构材料而广泛应用。相比之下，金属玻璃是无定形的，其中原子不排列成周期性阵列。尽管在金属玻璃中发现了许多特殊的性质，如高强度和弹性，但对其变形机制缺乏基本的了解，阻碍了其在许多技术领域的潜在应用。该项目旨在推进我们对组成如何影响结构的基本理解，特别是金属玻璃在纳米尺度上的结构异质性，以及这种结构异质性如何影响其变形行为。 假设存在不同类型的中程有序，即，纳米尺度上原子的局部有序排列及其在机械加载过程中的运动是导致金属玻璃不同变形行为的原因。利用先进的电子显微镜和多尺度计算机模拟相结合，该项目研究了不同金属玻璃中存在的中程有序的原子结构，并建立了中程有序结构和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学生提供实习计划，为他们提供研究经验，并促进他们向四年制大学过渡，和（ii）在线研讨会，用于传播金属玻璃和其他非晶材料的实验和计算方法。通过定量分析使用4维扫描透射电子显微镜（4D-STEM）获得的纳米衍射图案，在纳米尺度上测量MG），其提供关于构成MG的结构异质性的局部中程有序（MRO）的信息。使用实验结构数据，连同原子间的势，基于遗传算法的StructOpt优化提供的原子结构，最小化的势能下的约束下的MRO结构是完全一致的实验数据。由此产生的原子结构的MRO用于随后的势能景观分析，以获得定量的了解MRO如何影响剪切转变区（STZ），包括独特的剪切模式，活化能垒，活化体积，软化行为和局部弹性模量的关键信息。这些关键的STZ参数，量化的基本塑性事件，沿着直接从4D-STEM的结构异质性相关的微观结构信息，被纳入一个异质随机STZ动态模型进行变形模拟的长度和时间尺度相关的真实世界的实验。模拟的结果，探索各种MRO类型及其分布所揭示的实验表征，进行了分析，以了解在两个主要的MG系统中观察到的延性和β松弛的实质性变化的结构起源，即，Zr基和La基MG。这些发现可以大大推进对MG结构-性能关系的基本理解，并影响非晶材料的设计和开发。该奖项反映了NSF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.