Abstract
A theoretical investigation of the trapping of hydrogen by oxygen and nitrogen impurities in the b.c.c. metals niobium, vanadium and tantalum is described. The method of lattice statics is used to calculate hydrogen-impurity binding energies as functions of the locations and orientations of the hydrogen and impurity atoms. Based on these results, two hydrogen-impurity defect structures (a third nearest-neighboring (300) pair and a fourth- and fifth-nearest-neighboring (212)–(232) ring structure) are proposed in order to account for anelastic relaxations observed in these systems. In both cases it is assumed that hydrogen atoms occupy tetrahedral sites and that impurity atoms occupy octahedral sites. The binding energies calculated for the two structures are 0.11 and 0.06 eV respectively, in good agreement with the experimental observations of Zapp and Birnbaum. A transition path for the observed relaxations is developed and lower bounds to the relaxation activation enthalpies are calculated which are in agreement with experiment.
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