Actes du 3ème Colloque International sur la Corrosion et la Protection des Matériaux à Haute Température
J. Phys. IV France 03 (1993) C9-75-C9-84
Internal oxidation of Nb-Zr alloys over the range 1555-1768°C at low oxygen pressuresD.L. Douglass1, D.L. Corn2 and Fernando Rizzo3
1 Department of Materials Science and Engineering, UCLA, Los Angeles, CA 90024-1595, U.S.A.
2 Ralph M. Parsons Company, 100 West Walnut, Pasadena, CA 91124, U.S.A.
3 Pontifica Universidade Catolica de Rio de Janeiro, Rio de Janeiro, Brazil
Three Nb alloys, containing 1 w/oZr, 2.5 w/oZr, and 10W-2.5Zr, were internally oxidized in the range of 1555 to 1768°C at oxygen pressures ranging from 5 x 10-5 to 1 x 10-3 torr. Linear kinetics were measured suggesting that oxygen arrival at the surface and not oxygen diffusion in the substrate was rate controlling. Both tetragonal and monoclinic ZrO2 formed, the tetragonal form being favored by high nucleation rates (lower temperatures), lower alloy content, and location in the reaction zone (small particles near the surface). Monoclinic ZrO2 formed at higher temperatures and deeper within the reaction zone where larger precipitates formed. The high solubility product of ZrO2 in Nb-Zr alloys gives rise to non-Wagnerian behavior so that the solute is not precipitated out at the reaction front, additional precipitation occurring after the reaction front has passed. This causes a variation in the precipitate volume fraction with distance in the zone. Experimental observations are discussed in terms of various models for non-classical internal oxidation.
© EDP Sciences 1993