Identifier

etd-08202005-215619

Degree

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Document Type

Dissertation

Abstract

Grain-boundary migration controls grain growth and is important in materials processing and synthesis. When a grain boundary ends at a free surface, a groove will develop at the tip to reduce the combined surface and grain-boundary energies. This groove may hinder the grain boundary movement and its effect needs to be understood. Previous studies have solved the groove profile for stationary or migrating grain boundaries by assuming small groove slopes with isotropic surface free energy. This work extends the analysis to finite slopes with isotropic or anisotropic surface free energy.
We first study migrating grain-boundary grooves with isotropic surface energy and finite slopes. The groove translates by surface diffusion and the quasi-steady profiles are computed by shooting methods. It is found that the groove turns the grain boundary (by angle thata) away from being perpendicular to the free surface. We include the tilting effect into the "quarter-loop" and Sun-Bauer methods of measuring grain-boundary mobility and obtain better agreement with the measured grain-boundary profiles.
We next study the effect of strong surface energy anisotropy on the migrating grooves. A newly developed delta-function facet model is used to prescribe the surface energy. We find that most bicrystals show faceted grooves. However, a few anisotropic bicrystals can form smooth grooves. We also show that a migrating groove profile measured on a polycrystalline alumina surface can be well fitted by our model.
In the fourth chapter, we consider a vertical grain boundary that ends at a horizontal free surface. The anisotropic surface energy is asymmetric about the grain boundary. We show that the asymmetric groove grows with time t as t1/4. We solve the self-similar groove profile numerically by shooting methods. We find that the asymmetric surface energy tilts the grain-boundary tip sideways, which induces migration of the grain boundary. This asymmetry induced migration is revealed for the first time. Anisotropic groove profiles measured in SrTiO3 and Ni are fitted by our model and the profiles agree well.
Finally, we study the Rayleigh instability of Lennard-Jones nanometer scale liquid threads by the classical molecular dynamics. We show that Rayleigh's stability criterion is obeyed even at the molecular scale.

Date

2005

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Harris Wong

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