The effect of hydrogen on the surface energy of nickel
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1. I N T R O D U C T I O N W H I L E hydrogen has demonstrably large effects on the mechanical properties of metals at both high and low temperatures, its detailed mechanistic role has not been clearly established in m a n y systems. In m a n y cases solute hydrogen or high hydrogen fugacity environments cause a transition in the fracture mode from ductile to brittle or a decrease in the amount of ductility prior to ductile fracture by microvoid coalescence. For systems in which hydrides do not form, these effects have often been interpreted j-6 as due to decreases in the surface energy or the "cohesive energy" at the crack tip or to decreases of the interfacial energies at second phase particles 7 which are postulated to result from hydrogen in solid solution. Hydrogen is known to strongly adsorb on clean metal surfaces 8 thereby reducing the surface energy. However, in the above phenomena the important question is whether hydrogen in solid solution decreases the "cohesive energy" and correspondingly the surface energy of metals leading to hydrogen embrittlement. In a recent review of the available experimental data Birnb a u m ~has concluded that little evidence in support of the postulated decrease in cohesive energy due to hydrogen exists. However, most of the data is available for G r o u p V b hydride formers as these have relatively high hydrogen solubilities while the decohesion theories are generally applied to nonhydride formers having low hydrogen solubilities. In the present paper, a direct determination of the effect of hydrogen in solid solution on the surface energy of nickel is reported. Since nickel exhibits hydrogen embrittlement in the absence of hydride formation it is believed that this result has general applicability to nonhydride forming systems. 2. E X P E R I M E N T A L M E T H O D S The present experiments utilize the technique of zero creep which has been recently reviewed by Linford. 9 For the present purpose (assuming an elastically isoE. A. CLARK is Research Assistant and H. K. BIRNBAUM is Professor at the University of Illinois, Department of Metallurgy and Mining Engineering. R. YESKE is Research Scientist, Westinghouse Research and Development Center, Pittsburgh, PA. Manuscript submitted February 4, 1980.
tropic solid, high temperatures and neglecting the variation of the surface energy with orientation) it can be shown that the surface energy free 3', i . e . the partial Helmholtz free energy with respect to area, is numerically equal to the surface tension which is the magnitude of the diagonal terms of the two-dimensional surface stress tensor. The surface free energy can be determined at elevated temperatures by equilibrating the surface tension with the gravitational force acting on a weight supported by a thin wire or sheet specimen. At a weight W 0 such that the creep rate of the specimen is zero, the force balance can be written 9 for a b a m b o o grain structure as follows; W o = wry
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n_ qrr2" GB l r
[1]
where r is the wire radius, n is the number of grain boundari
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