Date of Award

2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Materials Science & Engineering

First Advisor

Daniel S. Gianola

Abstract

Defect-scarce nanowires exhibit unique and often superior material properties compared to their bulk counterparts. They can reach their theoretical strength limit by requiring nucleation of dislocation for the onset of plasticity. Surface is suggested as the most important feature that controls the activation of fresh dislocation in these material class. Despite the rich understanding for the activities of pre-existing dislocations, a comprehensive physical description of the nucleation-controlled dislocation-mediated plasticity has not been fully addressed. Therefore, experimentally elucidating the strength and rate-limiting deformation mechanisms of defect-scarce materials is demanding to utilize their superior properties.

In this dissertation, novel experimental methods were applied to understanding the mechanisms of plasticity of defect-scarce FCC nanowires. The methods include quantitative in situ tensile test of individual defect-scarce nanowire in combination with cryostat, scanning electron microscopes (SEM), transmission electron microscopes (TEM), and synchrotron x-ray diffraction imaging techniques. The first realization of in situ Bragg coherent diffraction (BCD) tensile test directly reveals full trajectory of tensile response of defect-scarce FCC nanowires with atomic lattice level resolution. Nucleation of partial dislocations is governing mechanism, which facilitates plastic flow if proper boundary conditions are met. Introducing multiple planar boundaries lead to higher yield strengths by hindering the glide motion of nucleated partials, whereas consideration of importance of surface diffusional activities leads to a novel approach for control of nucleation strength. The surface of the defect free Au nanowires was modified by atomic layer deposition in order to control the surface diffusional activities. The presence of coating increases both the activation energy and activation volume, which are displayed as increase in the yield strength and decreases in the scatter of strength distribution. The experimental findings suggest new avenue for nanostructure modification strategies.

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