Oxidation behavior of a Zr--0.5Y alloy

A Zr--0.5Y alloy was found to oxidize about 6 times faster than pure zirconium at 400 to 5650C. The activation energies were nearly identical (approximately 32 kcal/mole) and correspond to grain boundary diffusion of oxygen through the scale. The higher oxidation rate of the alloy was attributed to a higher anion vacancy concentration and the assumption that diffusion sites in the lattice and boundaries were in local equilibrium. Measurements on yttria-doped zirconia showed that ionic conductivity was increased markedly by yttrium and extended over a wide range of oxygen pressure. The defect structure of the doped oxide was changed to one of oxygen vacancies, even at the high end of the oxygen pressure range, 10-8 to 0.2 atm, over which pure zirconia contains oxygen interstitials. The doped oxide was found to be extrinsic over the entire range of oxygen pressure, and, although ionic conductivity predominated, electronic conductivity was still appreciable. The electronic conductivity, however, was still sufficiently high so that electron transport was not rate-controlling in the predominantly ionic-conducting scale. (U.S.)