OXYGEN DIFFUSION IN MASSIVE Cr_2O_3 AND IN Cr_2O_3 SCALES STUDIED BY SIMS AND RELATION WITH OXIDATION KINETICS : THE EFFECT OF Y IN Cr_2O_3 SCALES

摘要

The lattice and grain boundary diffusion coefficients of ~(18)O at pO_2=0.1atm and at 900-800℃ were determined in massive Cr_2O_3 (single crystals and massive palycrystals) and in Cr_2O_3 scales which were developed by oxidation of a Ni_(70)Cr_(30) alloy either unimplanted or implanted with Y (10~(16)atoms/cm~2). With our analysis for the concentration profiles established by SIMS, the oxygen lattice diffusion coefficients determined in single crystals, in polycrystals and in scales are close each other. Oxygen grain boundary diffusion coefficients determined in Cr_2O_3 scales are greater than those determined in massive polycrystals. The oxygen diffusion coefficients determined in the scales containing Y are smaller than those determined in the scales without Y. From oxidation kinetic tests, it appears that the alloy implanted with Y has a lower oxidation rate than the unimplanted alloy. With some assumptions, our diffusion data lead to a calculated parabolic oxidation constant in good agreement with the experimental one. The scale growth is ensured by counter-current diffusion of oxygen and chromium for the scales formed on the implanted and unimplanted alloys, mainly by grain boundary diffusion.