Date of Award

2-2017

Document Type

Thesis

Publisher

Santa Clara : Santa Clara University, 2017.

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

First Advisor

Dr. Panthea Sepehrband

Second Advisor

Dr. Niaz Abdolrahim

Abstract

The topic of this research is self-diffusion along a 1⁄2 〈110〉 screw dislocation core in face-centered cubic metals. Using molecular dynamics, self-diffusion along a screw dislocation core in four FCC metals, aluminum, copper, nickel and silver is investigated. In all metals under study except in Ag, the results show high diffusivity along the core even in the absence of any preexisting point defects (intrinsic diffusion). Enhanced self-diffusion due to screw dislocations is more remarkable in Al and Ni than in Cu. This behavior has been related to the stacking fault energy and dissociation width of partial dislocations.

The simulations show generation of point defects (e.g. vacancy and interstitial) in core regions at high temperatures, which leads to high diffusivity along the core. Formation of point defects is suggested to be due to reversion of partial dislocations into a full dislocation, where a point defect forms and starts to migrate. The reversion of partials into a full dislocation is observed to occur more frequently in Al and Ni, due to their higher stacking fault energy and lower dissociation width compared to Cu.

Moreover, by introducing a single vacancy in the core regions of Cu, the vacancy mechanism contribution to diffusion was measured and found to be negligible compared to the contribution of intrinsic diffusion. This indicates that the dislocation core becomes an effective source of point defect generation at high temperatures to an extent that the effect of pre-existing point defects on diffusion becomes negligible.

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