Improving the mechanical properties of metallic glasses through structure and stress engineering: A molecular dynamics simulations study

Xudong Yuan

Research output: ThesisDoctoral Thesis

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Abstract

This thesis has further explored the atomic-level deformation mechanism of metallic glasses under various conditions using molecular dynamics simulations and, additionally, exposed the effect of structure rejuvenation/relaxation on the mechanical properties of metallic glasses. In the end, we provided a systematic guide on the design of high-performance metallic glasses which combines strain hardening together with enhanced tensile ductility. The effects of cooling rate, temperature, and applied strain rate on the tensile deformation behavior of a Cu64Zr36 metallic glasses are investigated. An increase in the quenching rate during sample preparation, as well as an increase in the temperature or the applied strain rate, causes a brittle-to-ductile transition in the deformation behavior of metallic glasses. High quenching rates lead to lower energy barriers for activation of local atomic rearrangements as compared to those metallic glasses obtained at low quenching rates. The kinetic energy of the atoms increases dramatically with the increase of loading temperature, which allows the homogeneous activation of shear events. As for the strain rate, faster loading can store a large amount of elastic energy in the glassy matrix and induce a high density of shear events and, therefore, results in a low probability for strain localization and formation of critical shear bands. Structural rejuvenation is an excitation process that can bring metallic glasses to a higher energy state and usually improve their plasticity. In this work, by using a dilution procedure conducted by randomly removing atoms from the glass matrix, the degree of rejuvenation was systematically controlled and the maximum rejuvenation threshold of Cu 64 Zr 36 metallic glasses are identified. The structural relaxation is activated during the rejuvenation process and the dynamic balance between free volume creation and annihilation defines the rejuvenation ability of metallic glasses. Furthermore, loading-unloading tensile tests reveal that stress-induced relaxation of the highly rejuvenated metallic glasses provides strain-hardening, but, can never make them exceed the strength of their initial as-cast state. To overcome the brittleness of metallic glasses, their structure and chemistry are usually modified. Additionally, we also proved that the mechanical properties of MG can be controlled by introducing residual stress. During loading, the designed stress modulated metallic glasses heterostructures show enhanced ductility together with strain hardening. The stress heterogeneity leads to shear band multiplication that consequently enhances the macroscopic ductility of metallic glasses. In addition, the residual compressive stress significantly increases the strength of the glass and is responsible for the observed strain hardening during tensile deformation.
Translated title of the contributionVerbesserung der mechanischen Eigenschaften von metallische Gläser durch Struktur und Stress Engineering: Eine Molekulardynamik Simulationen studieren
Original languageEnglish
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Holec, David, Co-Supervisor (internal)
  • Eckert, Jürgen, Supervisor (internal)
  • Lekka, Christina, Assessor B (external), External person
  • Romaner, Lorenz, Assessor A (internal)
DOIs
Publication statusPublished - 2023

Bibliographical note

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Keywords

  • metallic glasses
  • shear bands
  • molecular dynamic simulations
  • relaxation
  • rejuvenation
  • residual stress

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