Internal oxidation of Nb-Zr alloys over the range 1555-1768°C at low oxygen pressures

Abstract
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.