Stress measurements and structural studies during oxidation of zirconium base alloys

Stress measurements in zirconium oxide at various stages of oxidation were carried out to study its variation with film thickness and to correlate the results with the transition. The results have shown initial high apparent stress levels ranging from 5 to 7x105 psi for thin films of 0.2 to 0.5 microns (studied for Zr-1Nb,Zr-1Cr, zircaloy-2 and Zr.0.5 Nb-1Cr alloy). These high stress levels dropped rather sharply to about 1.0 x 105 psi at an oxide thickness of about 3.5 microns, which was also found to be the thickness at which the oxidation kinetics showed the transition. For still thicker films, the decrease in stress was gradual, reaching about 0.4 x 105 psi at an oxide thickness of about 6 microns. During the oxidation, the initial high stress levels were retained for a longer time in these alloys as compared with Kroll Zr. This stress relief during the oxide growth has been correlated with the changes in the structural characteristics of the oxides of various stages as determined by X-ray diffraction studies. These studies indicated a reorientation of the oxide crystallites in the (202) plane during the pretransition film growth and attainment of the regular texture in the post-transition region. This has been observed from the plots of pole density (psup(*)) versus thickness of the oxide. (author)