Transmission Electron Microscopy Study of Deformation Processes in Metallic Glasses

Amorphous metallic alloys known as metallic glasses exhibit remarkable mechanical strength, elasticity, and resistance to wear when compared to their crystalline counterparts. These intriguing attributes have generated considerable interest in utilizing them for engineering materials over the past decades. Nevertheless, a significant drawback of metallic glasses lies in their limited ductility, which causes them to undergo abrupt yielding when undergoing plastic deformation. This plastic deformation of metallic glasses primarily occurs through the creation of shear bands, brought by work-softening nature of glasses. As regions within the material experience plastic deformation, they become softer, promoting localized strain accumulation within a narrow band-like zone, so-called shear band. Unfortunately, the sudden emergence of shear bands contributes to the premature failure of metallic glasses and hinders their toughness. Hence, comprehending the mechanisms that give rise to shear band formation becomes pivotal in constructing the theory of glass deformation and enhancing the mechanical stability of metallic glasses. However, recent advancements in the deformation mechanisms of metallic glasses have predominantly leaned on simulations, as experimentally characterizing the amorphous phases and nanoscale volumes within shear bands comes with substantial challenges. ... mehrThe lack of experimental observations concerning the structures implicated in the deformation of metallic glasses has restricted research findings to a hypothetical level, stalling the progress in novel material development.
This thesis focuses on an experimental investigation of deformed structures of metallic glasses using transmission electron microscopy (TEM) techniques, particularly four-dimensional (4D) scanning-TEM (STEM). The study incorporates methodological advancements, such as developing correlative mapping of nanoscale strain fields and atomic packing structure of glasses using 4D-STEM and Lorentz 4D-STEM, enabling the correlation of atomic structure and magnetic information. Machine learning analysis is applied to extract principal and correlated information from the 4D-STEM dataset. This development allows for direct experimental observations and detailed examination of the deformed structures in metallic glasses. The research outcomes establish an experimental foundation for understanding the formation of an individual shear band and the multiplication of shear bands. This is achieved through direct observations of strain concentrations, shear bands, shear band-affected zones (SBAZs), and local heterogeneity within a deformed glass matrix. Structure-property correlations in metallic glasses are discussed based on these microscopic observations. This new methodology is expected to open up extensive research possibilities for addressing questions in amorphous materials.