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

Three Nb alloys, containing 1 w/oZr, 2.5 w/oZr, and IOW-2.5Zr, were internally oxidized in the range of 1555 to 1768 O C at oxygen pressures ranging from 5 x to 1 x 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 ZrOn 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 ZrOp formed at higher temperatures and deeper within the reaction zone where larger precipitates formed. The high solubility product of Zr02 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.