TRAPPING OF HYDROGEN BY METALLIC SUBSTITUTIONAL IMPURITIES IN NIOBIUM, VANADIUM, AND TANTALUM

Abstract

The elastic interaction of hydrogen with substitutional metallic impurities in niobium, vanadium and tantalum has been calculated using lattice statics. The elastic interaction is found to be attractive at the 1st and 2nd nearest neighboring sites to the impurity if the impurity causes the host lattice to expand and is repulsive at these distances if the impurity contracts the lattice. The electronic interaction energy between hydrogen and substitutional impurities is also estimated using a crude phenomenological model. The electronic interaction is found to be attractive if the metal impurity has an electron deficit with respect to the host, and is repulsive if the impurity has an electron excess. Because of the general relationship between atomic size and number of valence electrons, the two interactions tend to occur in a complementary fashion, giving the “rule of thumb” that impurities to the left of the host in the periodic table trap hydrogen and those to the right do not. It is concluded that elastic and electronic interaction energies both play an important role in determining whether substitutional impurities trap, in marked contrast to the case of interstitial impurities where it has been found that trapping is due primarily to elastic interactions. The effect of substitutional trapping on hydrogen solubility in the host is discussed. It is concluded that trapping has only a secondary effect on solubility changes; rather the primary effect appears to be changes in the host's elastic properties upon addition of the substitutional impurity.