Atomic short-range order determination in gold-25 atomic percent iron and nickel-12.5 atomic percent silicon: A synchrotron x-ray diffuse scattering study
Anderson, Jeffrey P.
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https://hdl.handle.net/2142/21774
Description
Title
Atomic short-range order determination in gold-25 atomic percent iron and nickel-12.5 atomic percent silicon: A synchrotron x-ray diffuse scattering study
Author(s)
Anderson, Jeffrey P.
Issue Date
1992
Doctoral Committee Chair(s)
Chen, Haydn
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)
Engineering, Metallurgy
Engineering, Materials Science
Language
eng
Abstract
The short-range order (SRO) structure of a Gold-25 atomic percent Iron single crystal was studied using wide-angle diffuse synchrotron x-radiation scattering at room temperature. Two heat-treatments were investigated; a 400$\sp\circ$C aging treatment for two days and a second at 440$\sp\circ$C for five days, both preceded by solution treatment in the single-phase field and water-quenched to room temperature. Evolution of the SRO structure with aging was determined by fitting the two sets of Cowley-Warren SRO parameters to a pair of 140,608-atom models. The microstructures, while quite disordered, showed a trend with aging for an increasing volume fraction of an Fe-enriched and an Fe-depleted environment. The Fe-enriched environment displayed a slight preference for Fe segregation to the $\{$110$\}$ and $\{$100$\}$ fcc matrix planes. A major portion of the Fe-depleted environment was found to contain elements (and variations of these elements) of the D1$\sb{\rm a}$ ordered superstructure. The SRO contained in the Fe-depleted environment may be best described in terms of the standing concentration wave packet (SCWP) model. This study provides, for the first time, a quantitative real-space view of the atomic arrangements which make up both environments in the SRO structure of Au-25at.%Fe.
An equivalent study was performed on a water-quenched Ni-12.5at.%Si single crystal. The 140,608-atom model for this system did not display any long-range ordered domains of the equilibrium $\gamma$$\sp\prime$ (L1$\sb2$) superstructure. Rather, nearest-neighbor (nn) configurations C16, the basic structural element of $\gamma$$\sp\prime$, and C17, a faulted variant of C16, were found to combine to form small domains resembling disordered embryos of the $\gamma$$\sp\prime$ phase.
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