Degree Type

Dissertation

Date of Award

2016

Degree Name

Doctor of Philosophy

Department

Materials Science and Engineering

Major

Materials Science and Engineering

First Advisor

Richard A. LeSar

Second Advisor

Krishna Rajan

Abstract

Nanotwinned materials exhibit high strength combined with excellent thermal stability, making them potentially attractive for numerous applications. When deposited on cold substrates at high rates, for example, silver films can be prepared with a high-density of growth twins with an average twin boundary spacing of less than 10 nm. These films show a very strong {111} texture, with the twin boundaries being perpendicular to the growth direction. The origins of superior mechanical and thermal properties of nanotwinned materials, however, are not yet fully understood and need further improvements.

The aim of this research is to develop a connected experimental and theoretical/modeling study to elucidate the fundamental mechanisms that control the strength and stability of nanotwinned materials. To that end, we employed in-situ high-temperature nanoindentation to examine the mechanical behavior of nanotwinned materials. The hardness and strain rate were determined as a function of temperature, from which activation energies, activation volumes and strain rate sensitivities –which are fingerprints of dominant deformation mechanism- were determined. Furthermore, to better understand the physical phenomena that leads to their high strength, we have used the phase field dislocation dynamics (PFDD) model to study the effect of twin boundary spacing, grain size, applied stress on the stress driven emission and interaction of leading/trailing partial dislocations from grain boundary. Understanding both the mechanical properties of nanotwinned materials as well as how to control their structures will allow us to design better materials with desired properties.

Copyright Owner

Hakan Yavas

Language

en

File Format

application/pdf

File Size

125 pages

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