Effect of dislocation trapping on deuterium diffusion in deformed, single-crystal Pd
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INTRODUCTION
The diffusion of hydrogen and hydrogen isotopes in metals has been an actively investigated area of research for decades. The diffusivities of hydrogen, deuterium, and, to a lesser extent, tritium in several metals and alloys have been characterized under a variety of conditions with a variety of techniques.[1] Beyond these basic studies, the effect of defect interactions, such as those associated with grain boundaries[2,3] and dislocations,[2,4] on diffusion has been a subject of major interest. And while it is recognized that under certain circumstances dislocation cores can act as diffusion short cuts,[5] the more typical effect of the trapping interaction is to reduce the apparent diffusivity. This is, in fact, the basis of the well-known set of equations developed by McNabb and Foster describing the effect of defect trapping on the measured permeability or evolution.[6] The McNabb–Foster equations, typically in the thin- or thick-sample limit, are used to analyze thermal desorption experiments of hydrogen permeation through, or evolution from, metal membranes. The purpose of thermal desorption analysis (TDA) is not so much to study diffusion, but to measure binding energies of the diffusing species at trapping sites. This is done by measuring the permeation, or evolution, rate as a function of temperature and correlating apparent diffusivities, or outgassing peaks, to a characteristic trap binding energy.[7,8,9] The strength of different trapping interactions can then be determined, for example, in hydrogen embrittlement studies. In this article, we present small-angle neutron scattering (SANS) measurements of deformed, deuterium-loaded Pd single crystals. Coherent small-angle scattering results from nonrandom density fluctuations (specifically, nonrandom fluctuations in the bound atom nuclear scattering-length BRENT J. HEUSER, Assistant Professor, is with the Department of Nuclear Engineering, University of Illinois, Urbana, IL 61801-2984. JOHN S. KING, Emeritus Professor, is with the Department of Nuclear Engineering, University of Michigan, Ann Arbor, MI 48109. Manuscript submitted February 17, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
density) of the sample extend over distances upward from a few times the neutron wavelength (generally, distances ˚ for thermal neutrons). greater than approximately 10 A Nonrandom density fluctuations can take many forms in metals; internal surfaces, such as those associated with voids and second-phase precipitates, scatter strongly at small angles, whereas dislocations and grain boundaries typically are weak scatterers. Dislocations in deformed Pd act as trapping sites for deuterium (and hydrogen) and essentially become ‘‘decorated’’ with the interstitial solute. In the case of deuterium (positive scattering length), this decoration represents an increase in the local scattering-length density and additional SANS results from the spatial correlation of the trapped deuterium along the length of the dislocation. The deuterium-deuterium spatial correlatio
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