Microstructural characteristics of gold-rich gold-iron alloys
Chou, Chen-Chia
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https://hdl.handle.net/2142/21968
Description
Title
Microstructural characteristics of gold-rich gold-iron alloys
Author(s)
Chou, Chen-Chia
Issue Date
1990
Doctoral Committee Chair(s)
Wayman, C. Marvin
Department of Study
Materials Science and Engineering
Discipline
Materials Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Engineering, Metallurgy
Engineering, Materials Science
Language
eng
Abstract
As a typical spin-glass alloy system with diverse controversies on its magnetic properties and atomic arrangements, Au-rich Au-Fe alloys with Fe content from 10.7 to 33at% were studied to clarify their microstructural characteristics. As a consequence and in comparison with previous work, some different interpretations are proposed.
"At an early stage, a static concentration wave approach is used to interpret the diffraction patterns generated by Au-Fe alloys. The atomic arrangement of short range ordered alloys is described by a (1 1/2 0) concentration wave which corresponds to an ""average"" tetragonal structure with a $\approx$ a$\sb{\rm fcc}$ and c $\approx$ 2a$\sb{\rm fcc}$. In the meantime, rod-shaped and/or lobe-like strain contrast images whose visibility was sensitive to specimen preparation and diffraction conditions were seen after aged specimens were further cleaned using an ion miller. Trace analysis as well as other evidence indicates that the images correspond to small iron clusters rather than defects generated by ion-milling. Using high resolution electron microscopy no evidence was found for Fe-rich platelets lying on $\{$420$\}$ planes in the lobe-like contrast images, which are thought to be related to the $\{$1 1/2 0$\}$ diffuse reflections and the $\langle$210$\rangle$ streaks in this alloy system. Aging experiments also show that $\{$1 1/2 0$\}$ special point reflections have nothing to do with the zones."
Microstructural evolution in Au-Fe alloys was traced through morphological, compositional and structural changes as functions of temperature and time, using various techniques. Transformation-Temperature-Time curves were constructed for the alloys used. It is found that the solvus in the composition range studied is lower than the estimated value in the current phase diagram. The precipitation process is simply that BCC $\alpha$-iron enriched by Au atoms forms in the matrix. Characteristics of microstructural evolution have been described and discussed.
An oxide layer was found on the surface of the precipitates in aged Au-Fe alloys after the specimen was perforated for transmission electron microscopic investigation. By tilting through the reciprocal lattice, using bright field and dark field image analysis and double diffraction considerations, a general picture of the surface layer was depicted. Some microstructural characteristics of the oxide layer were also described and discussed.
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