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Microstructure and crystallization behavior in bulk glass forming alloys

Citation

Bossuyt, Sven Cecile Rene (2001) Microstructure and crystallization behavior in bulk glass forming alloys. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-07022001-164944

Abstract

The solidification microstructure in wedge-shaped castings of Cu-Ni-Ti-Zr glass forming alloys is investigated, while the composition was systematically varied. Near the critical thickness for glass formation, a spatially inhomogeneous dispersion of nanocrystals is observed, where spherical regions contain a much higher density of nanocrystals than the surrounding material. This microstructure is inconsistent with the prevalent theories for crystallization in metallic glasses, which predict a spatially uniform distribution of crystals. The spatial localization of the nucleation density is attributed to a recalescence instability. Linear stability analysis of the equations for heat flow coupled with crystal nucleation and growth reveals that at low temperature recalescence can occur locally, triggered by a small fluctuation in the early stages of the crystallization process, because in deeply undercooled liquids the nucleation rate increases with temperature. The localized recalescence events and their interaction accelerate crystallization; consequently they are important in determining the glass forming ability as well as the microstructure of these alloys. The composition dependence of the critical thickness for glass formation, determined from the observed microstructures, and in situ small angle scattering results indicate that the crystallization occurs in several steps, involving competing crystalline phases

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:crystallization kinetics; localized recalescence; metallic glass; nanocrystals
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Applied Physics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Johnson, William Lewis
Thesis Committee:
  • Johnson, William Lewis (chair)
  • Fultz, Brent T.
  • Ustundag, Ersan
  • Atwater, Harry Albert
  • Haile, Sossina M.
Defense Date:3 November 2000
Author Email:sven (AT) hyperfine.caltech.edu
Record Number:CaltechETD:etd-07022001-164944
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-07022001-164944
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:2801
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:13 Jul 2001
Last Modified:28 Jul 2014 17:40

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