Change in solute oxygen level and loss of ductility of niobium during oxidation in imperfect vacuum conditions

The oxidation behaviour of Nb held at a temperature of 7250C and an air pressure of 1x10-4Torr for 1-100 h was investigated. The changes of the oxygen solute concentration, the lattice parameter, the Vicker's microhardness and the bend ductility during the oxidation were followed. The oxygen solute concentration estimated by the internal friction method exceeded the solid solubility, and the degree of supersaturation reached about 50%. The supersaturation disappeared rapidly after 50 h exposure, with the formation of an oxide which was identified as (NbO+NbO2). The ductility decreased sharply in the sample oxidized for only 5 h; in this sample the oxygen content was well below the solubility limit. This embrittlement was caused by localized oxide precipitation along the grain boundary. (Auth.)