Doctoral Dissertations

Date of Award

5-2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Wojciech Dmowski, Takeshi Egami

Committee Members

Yanfei Gao, Hahn Choo, Steven Zinkle

Abstract

Bulk metallic glasses are promising structural materials due to some prominent mechanical properties, such as high strength. While, the limited RT plasticity is the Achilles' heel of BMGs that impedes their application. Therefore, the study of the room temperature (RT) plasticity of bulk metallic glasses (BMGs) is essential and challenging. Due to the intrinsic disordered structure, the “defects” in metallic glasses (MGs) are only qualitatively described. It’s well established that “elastic” deformation of BMG is locally heterogeneous and non-affine because of the local topological rearrangement (LTR) which is related to the emerging shear transformation zones (STZs) that control the nature of the plastic deformation beyond yielding. To understand the large compressive plasticity of Zr-based BMGs with high Zr atomic fraction and some noble-metal based alloys, the structure of BMGs during mechanical deformation should be investigated. This dissertation addresses three critical issues related to the plastic behavior of metallic glasses and the α to ω phase transition of Zr during high pressure torsion (HPT): Quantification of the deformation “defects” in metallic glasses. We have conducted in-situ high energy X-ray diffraction experiments and anisotropic pair distribution function (PDF) analysis to quantify the “defects” activated by external stresses in metallic glasses with a proposed strain ratio parameter, ε_0/ε_∞ . Our results based on this characterization method reveal the microscopic origin of plasticity in some metallic glasses. Correlation of the microstructure to the plasticity of BMGs. Through the compression mechanical tests and literature review, we found that the proposed parameter is consistent with the plasticity measured for various BMGs and provides a method to predict the plasticity of BMGs. Since the parameter is obtained from the structure characterization during deformation, the correlation between the parameter and the plasticity is well established. Elucidation of the α to ω phase transition of HPT-Zr. Using the high energy X-ray diffraction and the Rietveld refinement, the structure change of the Zr with increasing shear strain under high-pressure torsion is investigated, including the phase fractions and texture evolution. Finally, the α to ω phase transformation pathway is clarified.

Comments

Portions of this document were previously published in journal Applied Physics Letters

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