Date of Award
Doctor of Philosophy (PhD)
Despite intense interest in the discovery and design of metallic glasses, the efficient a priori identification of novel glass-formers without the need for time-consuming experimental characterization has remained an unattained goal due to the inability to fully understand why certain metallic alloys are better glass-formers than others. Using geometric alignment and density-based clustering algorithms to quantitatively describe the short-range atomic structure in simulated metallic liquids reveals that each liquid is comprised of a surprisingly small number of geometrically similar atomic clusters (6-8 characteristic motifs) with the variance in the population distribution (relative sizes of the associated groups) providing an a priori first-order predictor of GFA in a variety of metallic systems.Considering the similarity of these liquid motifs to equilibrium and metastable crystal structures reveals evidence of crystal embryos in Cu-Zr, Ni-Nb, Ni-Zr, and Al-Ni-Zr metallic liquids. A detailed analysis of these embryonic structures in Cu-Zr liquids suggests that glasses are observed either where the overall embryo fraction in the liquid is low (Cu50Zr50), providing few templates to facilitate crystal nucleation, or where the fractions of multiple types of embryos are high (Cu64Zr36), causing competition that frustrates the nucleation process. Furthermore, analysis of the “crystal embryo fraction” in metallic liquids improves the a priori prediction of experimental GFA when combined with the variance in the Cu-Zr, Ni-Nb, and Al-Ni-Zr alloy systems.We also show that the short-range clusters present in various Cu-Zr liquids (1450 K), undercooled liquids (700 K), and simulated metallic glasses (350 K) can predominantly be described by one of 12 primary “building blocks” regardless of composition or temperature with each being structurally distinct and characterized by icosahedral (<0,0,12,0>) or quasi-icosahedral (i.e. <0,1,10,2>, <0,2,8,1>) geometries. As a final analysis, we investigate the correlation of interconnectivity of geometrically similar short-range structures in 1800 K simulated liquids and 300 K metallic glasses to measured mechanical properties in the Al-Ni-Zr, Cu-Zr-Al, and Cu-Zr-Ti systems. While preliminary in nature, this final chapter represents the first foray into describing the medium range order of representative structure types, representing an exciting future direction for the research.Most of the analysis and subsequent results stem from our ability to use geometric alignment and density-based clustering to effectively quantify structure and order within macroscopically amorphous materials (i.e. metallic liquids, metallic glasses). The results suggest that an a priori predictor of GFA can be achieved by considering the population of characteristic atomic clusters in the simulated liquid and the fraction of the liquid that is similar to crystallizable structures. Moreover, the later analyses suggest that deeper analysis of these structures could similarly be used to understand the fundamental building blocks that make up metallic glasses and/or the trends in other relevant material properties.
Katharine M. Flores
Roman Garnett, Kenneth Kelton, Rohan Mishra, Shankar Sastry,
Available for download on Friday, April 26, 2024