The clustering of oxygen in solid solution in niobium—II. Interpretation

Abstract

Anelastic measurements on niobium-oxygen alloys containing 0.1–1.5 at. % oxygen in solid solution are quantitatively interpreted as showing that oxygen is present partly as single, unassoeiated atoms and partly, in more than random numbers, as clusters of two atoms (pairs), three atoms (triplets), and possibly four atoms (quadruplets). The interpretation is based on (1) the constancy of the relaxation times of the relaxation processes at a fixed temperature and over a wide range of oxygen concentrations and (2) the behavior of the component relaxation strengths according to the law of mass action when the strengths are used as measures of the concentrations of the clusters. From the variation of the component relaxation strengths with temperature, binding enthalpies (enthalpies required to dissociate the clusters) are determined. These quantities are 0.07 ± 0.03 eV for pairs, 0.18 ± 0.04 eV for triplets, and 0.27 ± 0.05 eV for quadruplets. From the variation of the component relaxation strengths with total oxygen concentration, the specific relaxation strength of each cluster (relaxation strength of oxygen atoms per unit concentration of cluster) is determined. These quantities are used to estimate values of the standard free energies and entropies of dissociation of the clusters.