Degree Type

Dissertation

Date of Award

2009

Degree Name

Doctor of Philosophy

Department

Materials Science and Engineering

First Advisor

Ralph E. Napolitano

Abstract

The cooperative or diffusively coupled growth of multiple phases during solidification is one of the most widely observed and generally important classes of phase transformations in materials. Technologically, due to having lower melting points compared with their pure components, and small freezing ranges, these alloys have excellent fluidity exhibited during casting and favorable properties offered by the fine composite structures both of which contribute to the wide application of eutectic alloys in the casting, welding, and soldering of engineered components. Academically, the topic of eutectic solidification offers a wealth of rich problems involving multicomponent thermodynamics, solid-liquid and solid-solid interfacial phenomena, morphological stability, chemical and thermal diffusion, and nucleation phenomena. Surprisingly, despite the broad-based technological and academic importance, many fundamental questions regarding eutectic solidification remain unanswered. This lack of understanding severely limits our ability to employ computational methods in the prediction of microstructure for the effective design of new materials and processing techniques through simulation.

In this study, pattern selection dynamics in rod eutectics are investigated by performing systematic directional solidification experiments with succinonitrile-(D) camphor transparent organic metal analog material and phase field simulations. Significant influence of the slide geometry on the rod morphology, even for specimen thicknesses which are many times greater than the characteristic eutectic spacing, has been observed. A new geometrical model is determined to accommodate the array distortion which exhibits very good agreement with the experimental observations, indicating that the principal effect of slide geometry that must be accounted for is array distortion. These results are confirmed with the phase field simulations. Two distinct influences of specimen slide geometry were observed. First, geometrical constraints give rise to the selection of array order, amounting (in the present case) to the selection of array orientation. Two particular states were observed, defined here as P1 and P2, where the 1st and 2nd nearest neighbors are aligned along the slide plane, respectively. Second, for relatively thin specimens (δ ≤ 30ym), a transition from 3D to quasi-3D growth was observed at very low velocities, where δ/λ∼ 1. Furthermore, by using phase field simulations, stability maps of different rod arrangements are obtained in addition to observation of rods to lamellar at very thin specimen thicknesses. Lastly, mechanisms of eutectic onset in directional growth is investigated and concluded that the formation of eutectic structure is very dependent on both external and internal parameters such as specimen thickness, thermal field, prior eutectic structure, single-phase formation and growth.

DOI

https://doi.org/10.31274/etd-180810-1632

Copyright Owner

Melis Serefoglu

Language

en

Date Available

2012-04-29

File Format

application/pdf

File Size

185 pages

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