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

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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.