Solubility and diffusion of hydrogen in vanadium-oxygen alloys
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I.
INTRODUCTION
A number of investigators have observed that the solubility and diffusion of hydrogen in Group VB metals, vanadium, niobium, and tantalum, are influenced by the presence of small amounts of other interstitial solutes, carbon, nitrogen, and oxygen. In general, an increase in the hydrogen solubility and a decrease in the hydrogen diffusivity have been reported. Trapping, an attractive interaction between lattice defects (dislocations, interstitial, or substitutional impurities) and hydrogen atoms has often been discussed as an explanation for these observations. Hydrogen solubility investigations, by pressure-composition isotherms, in Group VB metals with interstitial solutes have been only qualitative and at temperatures above 500 K where the trapping effect is diminished. J'2'3 Hydrogen diffusion studies have been most extensive in the niobium-nitrogen or oxygen systems. *-7 Additional experimental data are needed in other systems in order to establish the thermodynamics and kinetics of the trapping phenomenon. In this investigation, the solubility as a function of hydrogen pressure and the diffusion of hydrogen were measured in vanadium-oxygen alloys. The vanadium-oxygen system has been reported to show hydrogen trapping s and the vanadiumoxygen phase diagram is well known. 9 Furthermore, the rapid diffusion and relatively high solubility of hydrogen in vanadium allowed these experiments to be done in a reasonable amount of time and over a wide hydrogen concentration range. Pressure-composition isotherms at 297 and 373 K were obtained indirectly by an isopiestic technique reported by Peterson and Nelson. m The hydrogen diffusion coefficients as a function of hydrogen concentration were measured at 227, 297, and 373 K by a Boltzmann-Matano method, u.12 A number of trapping models have been proposed.~'~4'~5 A local deep trapping effect is implicit in these models; the interstitial atom is tenaciously bound to the lattice defect. The depth of the trap is given by the energy difference between a normal lattice site and a trap site, called the binding energy. A summary of reported binding energies of D.T. PETERSON is Professor, Department of Materials Science and Engineering and Senior Metallurgist, Ames Laboratory-Department of Energy, Iowa State University, Ames, IA 50011. B.J. SCHLADER, formerly a Graduate Student, Department of Materials Science and Engineering, Iowa State University, is a Metallurgist at Texas Instruments, Inc., Sherman, TX 75090. Manuscript submitted January 12, 1987.
METALLURGICALTRANSACTIONS A
hydrogen to various interstitial impurities in Group VB metals is tabulated in an article by Rosan and Wipf. ~6 These values, approximately 9.6 M/mole, were calculated from changes in diffusion or terminal solid solubility behavior by various models. Binding energies of this magnitude represent a very strong local trap. The pressure-composition isotherms and the hydrogen diffusivity vs hydrogen concentration data should both directly reflect the strong local trapping by the oxygen atom
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