Date

2016

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Materials Science and Engineering

First Adviser

Harmer, Martin P.

Other advisers/committee members

Vinci, Richard P.; Strandwitz, Nicholas C.; Gilchris, James F.

Abstract

A grain boundary complexion is a distinct structure/composition that is in thermodynamic equilibrium with the abutting phases. Grain boundary complexions can undergo transition with a change in temperature, pressure, or composition, resulting in discontinuous changes in the interface properties. The first part of this present work aims to link the grain growth kinetics to the structures of grain boundary complexions in Eu-doped spinel. This system is chosen for this purpose because it exhibits a strong propensity for abnormal grain growth (AGG). It was found that Eu-doped spinel exhibits three different types of segregation behaviors in a temperature range of 1400 °C-1600°C. The different types of complexions result in different transport kinetics, leading to a dramatic change in grain boundary mobility. The temperature where such a transition occurs is in the range of 1500 °C. The complexion underwent a transition from sub-monolayer (considering type I) below the transition temperature to a bilayer (type III) above the transition temperature. Moreover, it was found that the grain boundary character anisotropy is increased as a result of the complexion transition. The change in grain boundary character has a significant influence on anisotropy in grain-boundary energy, which is also consistent with a parallel study of the change in grain boundary energy in tri-crystal samples. As a result of complexion transitions, the relative grain boundary energies in Eu-doped spinel decrease 33-38% in different boundaries compared with undoped samples. The kinetics of the complexion transitions was further investigated. A systematic study of the time and temperature dependence of grain boundary complexion transitions was conducted at a temperature range of 1400-1800 °C and used to produce a grain-boundary complexion TTT diagram. The Johnson-Mehl-Avrami fitted curve method was used to determine the start and finish lines in the complexion TTT diagram.Complexion TTT diagrams summarize the kinetics of complexion transitions and can therefore be used to design optimized heat treatments. To obtain coarsening-limited microstructures (i.e. nanocrystalline material), complexion transition should be avoided in the heat treatment schedule. To produce bimodal microstructures, partial complexion transition is favorable in the heat treatment pathway. Finally, single crystal conversion may be achieved via controlled abnormal grain growth to nucleate a single abnormal grain and grow at the optimal heat treatment. It is believed that complexion TTT diagrams have the potential to be a useful tool to engineer material properties by controlling complexion transition, which will be critical to advancing the state of the art in the field of grain-boundary engineering.

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