Date of Award
Doctor of Philosophy (PhD)
Materials Science & Engineering
Daniel S. Gianola
Robert W. Carpick
Metallic glasses (MGs) exhibit both high yield stresses and elastic strain limits owing to their metallic bonding character and lack of long-range order. Yet the structural state (i.e. local atomic packing), and the corresponding elastic and plastic mechanical response, of MGs is nuanced and dependent on processing history. Moreover, the interplay between small length scales and glass processing routes have produced seemingly conflicting results. Here, the influence of processing on MG mechanical behavior at sub-micron length scales is explored, revealing extreme sensitivity to ion irradiation, enhanced control over the mechanical response, and an underpinning of yield strength in thermodynamic properties.
Using in situ testing methods, the deformation response of individual thermoplastically molded MG nanowires is studied. In contrast with previous literature reports the nanowire behavior is observed to be consistent with bulk deformation, exhibiting brittle fracture and shear banding at room temperature. To determine the role of processing at the nanoscale, ion irradiation is used to systematically alter the glassy structure of molded nanowires, leading to enhanced tensile ductility and reduced strength. A model for MG strength and ductility rationalizes the observations based on the glass transition temperature and a structure-dependent excess energy term. In addition, studying deformation at elevated temperature provides insight into the role of size and processing history on the mechanical properties in MGs. The Newtonian to non-Newtonian flow transition occurs at higher strain rates in nanoscale specimens. This suggests a more relaxed nanowire structural state, potentially owing to thermal processing, and a wider range of thermally accessible structures at the nanoscale. Finally, the range of structural states in MG thin films is explored by sputter deposition at different substrate temperatures. The maximum hardness increases more than 30% with deposition temperature, revealing a wide range of achievable glass structures. Together, the nanowire and thin film results emphasize the need to quantify glass structural state and suggest a potential processing -- structural state -- property relationship in MGs, correlating mechanical properties with thermodynamic quantities.
Magagnosc, Daniel J., "Elucidating the Mechanical Response of Metallic Glasses Prepared in Different Structural States at Sub-Micron Length Scales" (2016). Publicly Accessible Penn Dissertations. 1869.