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|Title:||Electrical Resistance Studies of Hydrogen Diffusion and of Hydride Precipitation in Niobium|
|Author(s):||Clark, Elliot Andrew|
|Department / Program:||Metallurgy and Mining Engineering|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||Diffusion of hydrogen in niobium was studied by using thermotransport to establish a concentration gradient in a periodic width modulated sample. A sensitive Kelvin double bridge allowed the kinetics of return to uniform concentration to be monitored. In undeformed Nb, the Arhennius plot exhibited a curvature, in agreement with previous results. Deformation indiced by hydride precipitation and by cold work removed the curvature, and increased the fitted values of Do and Q in D = D(,o)e('-Q/kT). These effects are considered to be due to dislocation trapping of H. The undeformed data were fit to the Flynn-Stoneham quantum tunneling model of diffusion. The value for the lattice Debye temperature found was unphysically high (900 K); the validity of the model is therefore in question.
The precipitation of hydride in Nb was studied by balancing the Kelvin double bridge such that the bridge output was proportional to the concentration of unprecipitated hydrogen, and following the concentration of free hydrogen with temperature. Repeated cycles of precipitation and reversion showed at first a decreasing, and then constant hysteresis (difference between precipitation and reversion temperatures). Prior cold work did not affect the precipitation in any way. The formation of a local dislocation structure for hydride precipitation may explain this behavior. Increasing the heating/cooling rate increased the precipitation/reversion temperatures. Fits of the curves to an equation of the solvus give values for the constrained solvus that differ from previous investigations. The amount of isothermal hydride precipitation decreases with cycling, but the isothermal rate is much less affected. The mechanism of isothermal hydride precipitation is unclear: dislocation processes are postulated.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1986.
|Date Available in IDEALS:||2014-12-16|
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Dissertations and Theses - Metallurgy and Mining Engineering
Graduate Dissertations and Theses at Illinois
Graduate Theses and Dissertations at Illinois