Deformation Mechanisms of Bulk Metallic Glasses Investigated Using Small-scale Mechanical Testing

Date of Award

Spring 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering & Materials Science (ENAS)

First Advisor

Schwarz, Udo

Abstract

Many bulk metallic glasses (BMGs) have exceptional mechanical, physical, and chemical properties, which allows for applications like surgical tools and sports goods that are not possible with conventional materials. In contrast to traditional polycrystalline metals and alloys that have defined atomic structures, the amorphous nature of BMGs endows them with only short- to medium-range order. However, although BMGs have better ductility than traditional glasses due to the non-directional metallic bonds that keeps the constituents together, their inherent brittleness and flaw sensitivity still limit their use in many applications. To improve on these shortcomings, an understanding of how shear transformation zones (STZs) and shear bands form is of crucial importance, since both play an important role in the deformation of BMGs. In this thesis, the deformation behavior of BMGs has been analyzed using macro-, micro-, and nanoscale testing methods. First, the role of external mechanical stimulations in improving the ductility and strength of BMGs has been studied through both tensile cycling and thermal cycling methods. A rejuvenation to relaxation transition has been observed with the increase of cycling number. Additionally, a novel nanoindentation-based mechanical cycling approach was developed to overcome the throughput limitations of traditional tension-compression testing. This method enables precise control over cycling amplitude and frequency, facilitating high-throughput analysis of deformation responses. Furthermore, BMGs have been analyzed under micro-/nanopillar compression, where the respective pillars have been prepared using a novel method based on thermoplastic forming. This approach minimizes structural flaws and ensures geometrically uniform pillars, enhancing experimental reproducibility while enabling rapid, large-scale sample production. By controlling the strain rate, the critical load for the formation of shear transformation zones and pillar yielding behavior has been analyzed. A universal three-phase deformation model is proposed to describe structural evolution during compression, spanning initial elastic response, intermittent plasticity, and shear banding regimes.

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