Solid state deoxidation of zirconium-oxygen solid solution using a combination of metallic Ca and Y in CaCl2 flux

Abstract

Aiming at the deficiencies of the three typical "external gettering" systems: metal-oxide equilibrium deoxidation (MED), halide flux deoxidation (HFD) and electrochemical deoxidation (ECD), a novel deoxidation method based on the combination of oxytropic rare earth metal Y and Ca for in-depth deoxidation of Zr-O solid solution was proposed. The thermodynamic properties of the deoxidation method were analyzed, and it was found that the addition of metallic Y could significantly improve the deoxidation ability of the Ca-CaCl2 system, because Y could form Y-Y2O3 equilibrium to "anchor" the activity of CaO (aCaO) at a very low level. When the activity of Y2O3 (aY2O3) and aCaO in the flux conforms to aY2O3< 18.9a3CaO at 1173 K, the deoxidation mode is thermodynamically feasible. Experiments were conducted to verify the method. The results showed that even if CaO was saturated in CaCl2 flux, the oxygen content of Zr-O solid solution could still be controlled at the level of less than 100 ppm, thus the "anchoring effect" of Y on aCaO was verified. The oxygen content, phase and microstructure of the reacted Y specimens were analyzed, which suggested that when the ratio of CaO was less than 40 % of its solubility in CaCl2 flux, the oxygen potential of the system was controlled by Y-O solid solution, and when it was greater than 40 %, the oxygen potential of the system was controlled by Y-Y2O3 equilibrium.