Defect dynamics and plasticity in metallic nanoparticles

Enhancing the mechanical properties of structural materials requires the ability to control defect nucleation, propagation, and interactions. Defects, such as dislocations in crystalline metals or shear transformation zones in amorphous materials, are the primary drivers of plasticity and dictate a wide variety of mechanical properties such as yield strength, strain hardening, and hardness. One common approach to modify defect behavior is to introduce microstructural features which interact with defects. However, due to the complexity of bulk materials, it is difficult to isolate individual features of interest for mechanical testing using current experimental approaches. Rather than looking at bulk materials, I propose the use of metallic nanoparticles as model systems to probe defect behavior. Nanoparticle synthesis allows for control of a variety of microstructural features, such as crystallographic orientation, defect density, twinning, chemical composition, surface properties, and shape. Through tailored synthesis and in situ testing, I aim to identify defect-microstructure interactions that can inform the design of nano/microstructured materials. In the first half of this dissertation, I explored dislocation nucleation and dislocation-precipitate interactions in crystalline metal nanoparticles. Fcc defect-free Ag and Cu metallic nanocubes (NCs) are ideal systems for understanding dislocation nucleation given their controlled shape, size, and surface features. In addition, I investigate the mechanical behavior of defect-free Al NCs. Unlike the aforementioned metals, compression of Al NCs leads to pronounced strain hardening, which I relate to the formation of prismatic dislocation loops. Building upon the studies of single metal NCs, I extend this work to core@shell NCs as models for precipitate hardening. Two types of core@shell NCs, Au@Ag and Au@Cu, were tested as models for coherent and semi-coherent precipitates, respectively. Au@Ag NCs show no dislocation interaction at the interface and the mechanical behavior matches pure Ag NCs. In contrast, Au@Cu NCs show extensive strain hardening. Using computational and analytical techniques, strain hardening was attributed to threading dislocations looping around the core, leading to increasing Orowan and back stress with each additional loop. In the second half, I investigated the behavior of metallic glass (MG) nanoparticles. MGs, characterized by their lack of long range order, possess many unique and useful properties such as a high elastic limit and corrosion resistance but are brittle due to the formation of shear bands. To address this problem, I compressed in situ colloidally synthesized 90-260 nm Ni-B MG particles. The particles exhibited decreasing propensity for shear band formation with increasing size, which was attributed to the increase in metallic bonding in larger MG particles. Furthermore, colloidally synthesized MG coatings of crystalline Au nanocubes exhibited enhanced ion radiation resistance due to a low barrier for defect diffusion to a free surface

Keywords: Materials Science and Engineering.
Advisor: Gu Wendy (Professor of mechanical engineering) 
Prinz F. B.
Cui Yi