Ductility and Deformation in Bulk Metallic Glass

Date of Award

Spring 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering & Materials Science (ENAS)

First Advisor

Schroers, Jan

Abstract

Bulk metallic glasses (BMGs) are a unique engineering material that combine excellent strength and elasticity, often paired with high fracture toughness and high precision formability for manufacturing. These attributes have made them valuable in a range of applications, including biomedical, energy, aerospace, and consumer products. However, these materials exhibit a range of fracture toughness values, a material property associated with flaw tolerance and often approximated by ductility, posing a hurdle to technological adoption. High ductility and, importantly, the accurate prediction of ductility, are necessary for load-bearing structural materials, as they prevent the unexpected, catastrophic failure of materials. Despite this importance and much on-going research, a complete understanding of ductility in BMGs is presently lacking, in large part due to the complexity of the structure-process-property relationship in amorphous materials. This thesis addresses the control and predictability of ductility in bulk metallic glasses, rooted in the structure of the material itself, via three projects: 1) the formulation of a framework for predicting ductility based on characterizations of deformation, 2) the technological development of a thermo-mechanical process to improve ductility, and 3) a mechanistic study of the brittle-to-ductile transition with temperature and strain rate. These are presented in chapters 2, 3, and 4 respectively. Chapter 2 proposes a definition of ductility that would enable prediction of BMG plasticity. This definition and framework are based on the material's tolerance to non-uniform applied stresses, and characterizations are performed to determine these tolerances for multiple compositions at room temperature. To accomplish this, a novel method is demonstrated for measuring ductility in compression and tension. These measurements are described in terms of the stability of the shear band, wherein the applied stress is competing with the strength of the material to either accelerate or halt the growth of the shear band. This framework offers the prospect of predicting the material’s ability to accommodate plastic strain via stable shear bands, i.e. ductility, and suggests a route towards improving the ductility in service of any composition of BMG by precisely controlling the gradients of applied stresses. In chapter 3, an experimental method is introduced and developed, referred to as excited liquid cooling, to improve the ductility of a BMG. There, a BMG is heated to the visco-plastic supercooled liquid temperature range before being simultaneously pulled at a high strain and cooled to below the glass transition temperature. In doing so, changes are introduced to both the macroscopic geometry of the workpiece and the microscopic structure of the alloy. Macroscopically, the workpiece, initially a rod, is drawn out into a wire, with a reduction in diameter of > 90%. Of greater scientific interest, however, the microstructure of the BMG is altered, with a significant increase in free volume within the material, a measure of the average spacing between atoms in the amorphous solid. This occurs because the material dilates due to the mechanical strain during pulling, and this dilated structure, although energetically unfavorable, is frozen into the structure via the simultaneous cooling. In turn, the higher free volume material exhibits greater ductility in bending. The process is described systematically in terms of temperature and strain rate, the resultant effects on the material are characterized via thermal and mechanical methods, and the underlying theory is described in terms of competing time scales and potential energies. Finally, strategies are outlined for scaling this technique to larger, more complex geometries. Chapter 4 describes ductility and deformation of BMGs across temperatures and strain rates in uniaxial tension. Specifically, it looks at bridging the two extreme ends of deformation and failure: brittle failure where only a small fraction of atoms undergo deformation (those within the shear band) and ductile necking failure where approximately all atoms undergo the same deformation in the necking region prior to failure. Using both experimental and computational methods, the combined effects of temperature and strain rate on this brittle-to-ductile transition are observed and parameterized. The transition is described at the particle scale, and an intermediate deformation mode is observed. In this sense, it serves to inform processing strategies like those described in chapter 3 but also serves to better predict service behavior of BMGs in higher temperature, dynamic loading conditions. Finally, it offers a new mechanistic insight of dilation driven deformation, suggesting that constitutive modeling of BMG deformation at high temperatures should work to include this facet to most robustly capture real behavior. In summary, this dissertation advances our collective understanding and control of ductility in BMGs through experimental, computational, and theoretical work. Efforts are made to ground these findings in the structure-process-property relationship of BMGs, where the processing history of the material governs the structure of the atoms which in turn dictates the properties and performance of the material under characterization. These continuous advancements in BMG ductility will serve to open new applications for these materials in service.

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