Calculations of the binding of hydrogen to fixed interstitial impurities in nickel
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I.
INTRODUCTION
HYDROGEN diffusion in metals can be dominated by traps such as dislocations, vacancies, impurities, and inclusions when the binding energy to these traps is a significant fraction of the activation energy for hydrogen motion in the bulk lattice. In fcc metals, the activation energy for motion, Ea, is of the order of 0.4 to 0.6 eV (9 to 14 kcal/mole) while in bcc metals, Ea is of the order of 0.05 eV (1 kcal/mole). Therefore, weak traps are a great deal more important in bcc's than in fcc's. Since hydrogen is a chemically active species which forms strong bonds with a number of elements known to be impurities in metals (C, O, N . . . . ), it is natural to question the nature and magnitude of the trapping behavior of hydrogen to these elements within the metal. These impurities also produce electronic and lattice distortions by themselves, and these effects may dramatically alter the trapping behavior. It is reasonable to ask whether chemically active impurities provide deeper binding sites for hydrogen than, for example, inert impurity atoms. With this motivation in mind, we have calculated the binding energy of hydrogen to a number of fixed interstitial impurities in fcc nickel. The calculations were performed on inert (He, Ne, Ar, Kr, Xe) impurities as well as chemically active species (C, O, N) for comparison purposes. The calculation is based on the quasiatom 1'2'3 (or effective medium4'5) approach which assumes that the energy of an impurity atom in a metal is a function of the electronic charge density at its position in the lattice. This evaluation involves calculating the energy of the atom in a homogeneous electron gas as a function of the density of that gas. 6 In this work, we also modify the quasiatom method to include lattice relaxations by allowing the host atom to move via pairwise forces obtained from experimental data. Changes in the electronic charge density at an impurity atom site due to these relaxations are therefore taken into account, but local electronic distortions are neglected beyond those in the quasiatom approach. Further details of the method of calculation are presented in Section II. Section III contains the results and Section IV a brief summary and conclusion.
II.
METHOD
In the quasiatom (or effective medium) approximation, MURRAY S. DAW, Physicist, C.L. BISSON, Member of Technical Staff, and W. D. WILSON, Physicist and Supervisor, are all with Sandia National Laboratories, Livermore, CA 94550. Manuscript submitted October 11, 1982. METALLURGICALTRANSACTIONS A
the energy of a light impurity in a metal is determined by the local host environment according to 1-5 AE (1~) = F, (p(I~))
[11
where I~ is the impurity position, p(l~) is the host density at the site prior to the introduction of the impurity, and Ft is the immersion energy function, independent of host, for the impurity type I. For hydrogen in nickel, for example, the density of electrons at a perfect Oh site is computed and the energy of inserting the hydrogen there is evaluated from the function FH.
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