Degree Type

Dissertation

Date of Award

1992

Degree Name

Doctor of Philosophy

Department

Materials Science and Engineering

First Advisor

R. W. McCallum

Second Advisor

A. I. Goldman

Abstract

The phases and microstructures in rapidly solidified Al-Cu-Ru alloys were investigated in this study. A chemically and topologically disordered icosahedral (i) phase grows dendritically from the liquid as the primary solidification product over the entire compositional region studied. The as-solidified i-phase is metastable and transforms to crystalline products at ≈500°C. The i-phase was not found as a product of the exothermic transformation for any composition, indicating that it is not the low temperature stable phase in the Al-Cu-Ru system;A chemically and topologically ordered i-phase was found to be an equilibrium phase at temperatures above ≈670°C and exists over a compositional region of several atomic percent. Once formed, this phase was easily retained at lower temperatures because of kinetic limitations of the transformation to the low temperature crystalline phase;Crystalline phases which from diffraction results appear structurally similar to the i-phase were also found in the Al-Cu-Ru system. These "approximant" phases aid in the determination of the atomic structure of i-phases by having common structural units. A simple cubic structure (a = 12.38 A, Pm3) containing a bcc network of icosahedral clusters was discovered. Comparisons of this phase with the i-phase indicated that strong similarities exist between the two structures;A rhombohedral approximant phase was also found. It exists as a transition state between the low-temperature crystalline phase and the high-temperature i-phase. This approximant phase also contains local icosahedral symmetry. The strong presence of icosahedral clusters in approximant phases in the Al-Cu-Ru system points to the distinct possibility that the i-phase is a quasiperiodic packing of icosahedral clusters of atoms.

DOI

https://doi.org/10.31274/rtd-180813-11307

Publisher

Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/

Copyright Owner

Jeffrey E. Shield

Language

en

Proquest ID

AAI9223965

File Format

application/pdf

File Size

79 pages

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