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Reaction of hydrogen with crystalline and amorphous alloys--crystal to amorphous transformation induced by hydrogen

Citation

Yeh, Xian-Li (1987) Reaction of hydrogen with crystalline and amorphous alloys--crystal to amorphous transformation induced by hydrogen. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-03042008-144844

Abstract

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Some aspects of turbulence in sediment-laden open-channel flows are examined. A conceptual model based on similarity hypotheses rather than the traditional mixing-length closures is proposed. It is argued that, over a wide range of laboratory conditions, the main effect of the suspended sediment on the flow is confined to a layer near the bed. If such a distinct layer can be discerned, then this is separated from the outer flow by an inertial subregion in which the mean-velocity profile is approximately logarithmic, with an associated von Karman constant of [...] 0.4, i.e., the same value as in single-phase flows. It is further shown that power-law profiles may be derived from general similarity arguments and asymptotic matching. These implications contrast with those of previous models in which changes in the mean-velocity profile are supposed to occur throughout the flow or primarily in the flow far from the bed. Length and concentration scales appropriate to sediment-laden flows are suggested.

An experimental study was also undertaken. Both the saturated case, in which a sand bed was present, and the unsaturated case, in which a sand bed was absent, were investigated. The study was restricted to nominally flat beds, composed of three well sorted sands (median grain diameters ranged from 0.15 mm to 0.24 mm). A two-component laser-Doppler-velocimetry system was used for velocity measurements. Suction sampling was used to measure local mean concentrations. The major points of the conceptual model are supported by the experimental results. Higher-order statistics of the velocity field were found to exhibit little evidence of any effect on the outer flow, supporting the view that the effect of the suspended sediment is felt primarily in the inner region. This contrasts with the predictions of recent models that propose an analogy between sediment-laden flows and weakly stable density-stratified flows.

Metastable polycrystalline [...] alloys having a fcc structure are reacted with hydrogen gas at temperatures ranging from 25[...]C to 250[...]C. It is demonstrated for the first time that an amorphous phase can be formed during such a solid state reaction when the temperature lies below 220[...]C. Such a reaction is possible only if the following requirements are satisfied: The existence of a thermodynamic driving force (i.e., the amorphous phase must have a lower free energy than the free hydrogen and the crystalline phase from which it forms) and the existence of a kinetic constraint (i.e., the formation of thermodynamically preferred equilibrium phases or phase mixtures must be kinetically suppressed).

X-ray diffraction and TEM studies show that the amorphous phase grows at the expense of the crystalline phase during hydrogen absorption by these metastable fcc alloys. The formation of the amorphous hydride phase is observed by TEM to begin at grain boundaries of the polycrystalline alloys much in the same manner that "melting" nucleates at grain boundaries. X-ray analysis indicates that the Zr-Zr distance increases as hydrogen is absorbed, suggesting that hydrogen atoms prefer to stay in tetrahedral sites surrounded by four Zr atoms. This provides evidence as to why the amorphous hydride phase is more stable than the fcc hydride phase. The thermal behavior of amorphous hydrides obtained by hydriding metallic glasses and that obtained by hydriding metastable crystalline alloys are compared and found to be similar. The hydrogen distribution and surface effects are investigated using hydrogen depth profiling, SEM and Rutherford backscattering. Hydrogen permeation through the sample surface has been found to be the rate limiting step in the hydriding reaction.

Based on the present experiments and an analysis of the relevant free energy curves, we discuss the thermodynamic and kinetic aspects of this phase transformation to explain why an amorphous phase is formed. The mechanism for this can be viewed as melting in the solid state. A simple "chemical frustration" model is proposed to explain the kinetics of amorphization via hydriding.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Applied Physics
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Johnson, William Lewis
Thesis Committee:
  • Unknown, Unknown
Defense Date:18 September 1986
Record Number:CaltechETD:etd-03042008-144844
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-03042008-144844
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:873
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:14 Mar 2008
Last Modified:26 Dec 2012 02:33

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