Shear-band structure in metallic glasses

Liu, Chaoyang

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https://hdl.handle.net/2142/107923 Copy

Description

Title

Shear-band structure in metallic glasses

Author(s)

Liu, Chaoyang

Issue Date

2020-04-29

Director of Research (if dissertation) or Advisor (if thesis)

Maass, Robert

Doctoral Committee Chair(s)

Maass, Robert

Committee Member(s)

• Zuo, Jian-Min
• Bellon, Pascal
• Shim, Moonsub

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Metallic glass
• shear band

Abstract

Metallic glasses (MGs) are amorphous alloys that possess exceptional mechanical properties compared to their crystalline counterparts. The fundamental problem that currently prevents the use of MGs as reliable structural materials is the lack of understanding of how their structure evolves as a function of macroscopic strain. At low homologous temperatures (e.g., operating temperatures of common structural materials), strain localization into shear bands make these defects play a decisive role in controlling the plastic behavior of MGs. However, the characterization of shear bands has not yet succeeded in connecting their structural evolution and damage accumulation with any macroscopic engineering strain or local shear strain parameters. This dissertation work aims to understand such connection by studying the structure of shear bands in Zr-based MGs from the nano-scale shear-band core to the micro-scale shear-band region. First, the nano-scale structural properties, such as morphology, thickness, and density variation, of a single system-spanning shear band that has carried all plastic flow were sampled along its millimeter-length path with high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). It was found that those properties sensitively depend on the position along the shear band, offering a more complex view of shear-band structure than the commonly considered simple nano-scale planar defect. Next, the origin of the nano-scale structural change was evaluated by correlative analytical methods, including TEM-based energy dispersive X-ray spectroscopy (EDX) and scanning electron nano-beam diffraction (SEND), as well as synchrotron-based nano-beam X-ray fluorescence (NB-XRF) and atom probe tomography (APT). These methods not only confirmed the validity of using the HAADF-STEM technique to quantify the shear-band density difference with respect to the matrix material, but also unambiguously revealed the structural and compositional changes in the nano-scale shear-band core of the Zr-based MG alloys studied here. Then the evidence of structural damage in shear bands was investigated as a function of shear strain. To this end, a large number of shear bands were extracted from bulk samples and analyzed with HAADF-STEM, revealing a general positive correlation between shear-band volume dilatation and shear strain. The structural dependency on shear strain suggested that the shear-driven disordering in shear bands dominated over thermally-activated relaxation during plastic flow at low homologous temperatures over the probed shear-strain regime. Finally, extending studies beyond the nano-scale shear-band core on MG samples unloaded prior to fracture using synchrotron-based X-ray tomography (XRT) yielded the reconstruction of large amounts of micron-size cavities (internal micro-cracks) in the micro-scale shear-band region. The shear-band cavities significantly reduced the true load-bearing area, leading to an apparent strain hardening phenomenon in those samples. We proposed that the formation of internal shear-band cavities proceeded gradually during plastic straining, which questioned the meaningfulness of stress determinations that were solely based on externally measured dimensions of MG samples. Overall, the experimental findings in this dissertation work are at the forefront of MG research and have expanded our knowledge on the structure of shear bands, which agree well with recent advances in molecular dynamics simulations work that strongly suggest the existence of structural heterogeneities in MG shear bands. The evolution of shear-band structure was proven here to have a strain dependency. Future work may include in-situ observations of such structural evolutions, and the challenge remains at linking the damage accumulation mechanism across different length scales both experimentally and theoretically.

Graduation Semester

2020-05

Type of Resource

Thesis

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http://hdl.handle.net/2142/107923

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Copyright 2020 Chaoyang Liu

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