A Study of the Kinetics of Phase Separation in a Nickel-12.5 at.percent Silicon Alloy
Polat, Seyda
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https://hdl.handle.net/2142/71849
Description
Title
A Study of the Kinetics of Phase Separation in a Nickel-12.5 at.percent Silicon Alloy
Author(s)
Polat, Seyda
Issue Date
1987
Department of Study
Metallurgy and Mining Engineering
Discipline
Metallurgical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Metallurgy
Abstract
The kinetic behavior of $\gamma\sp\prime$ precipitation in a supersaturated Ni-12.5 at.% Si alloy single crystal has been studied by means of in situ wide-angle x-ray diffraction (XRD), small-angle neutron scattering (SANS) and high-resolution wide-angle XRD techniques. The kinetic parameters, i.e. rate constants, governing the growth of the particle size and the volume fraction of the $\gamma\sp\prime$ precipitate are determined from the in situ XRD measurements. The interfacial free energy, $\gamma$, the solute diffusivity, D, and the equilibrium solubility, C$\sb{\rm e}$, are deduced from this kinetic data. SANS experiments, performed on anneal-then-quench specimens, have revealed the presence of an interference peak in the scattering function. The mean interparticle distance is found from the peak position and the particle size is determined according to the Guinier approximation. Both in situ XRD and SANS studies show that $\gamma\sp\prime$ precipitate in this alloy grows by diffusion controlled coarsening. The experimental observations agree well with the Ostwald ripening theory of Lifshitz, Slyozov and Wagner. The particle growth kinetics shows a transition from an earlier growth stage, where a parabolic growth rate is in effect, to a classical coarsening stage. Enhanced diffusion, due to quenched-in excess vacancies, is noted at lower aging temperatures. The dynamical scaling of the SANS spectra and power law behavior of peak positions and intensities are investigated. Agreement with the existing scaling theories at especially high temperatures and long annealing times has been satisfactory, implying that the process of phase separation is entirely controlled by a unique characteristic length. The nonrandom arrangement of precipitates is studied also by high-resolution XRD utilizing synchrotron radiation. Diffraction side bands (satellites) in the vicinity of the fundamental Bragg peaks are observed and their asymmetry suggests that there is a simultaneous variation of concentration and lattice spacing. The origin of these satellites about the fundamental reflections and the 000 peak (SANS) is attributed to the presence of elastic interactions, short range in nature.
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