Yttrium diffusion in a chromia scale grown on a Ni-30 wt% Cr alloy

摘要

Diffusion coefficients of yttrium in chromia (Cr2O3) scales grown on a Ni-30 wt% Cr alloy by high-temperature oxidation have been determined in the temperature range 800-1000 degrees C in air. A thin yttrium-containing film on the oxide surface was produced by the deposition of a liquid solution of YCl3 and, after a diffusion treatment, the penetration profiles were established by secondary-ion mass spectrometry. The apparent diffusion coefficients were calculated from the thin-film solution of Fick's equation and, at the same time, the bulk and grain-boundary diffusion coefficients were obtained according to a model recently proposed in Orsay, in which the oxide surface roughness and the relationship between bulk, grain-boundary and apparent diffusion coefficients was considered. The results showed that yttrium diffusion in Cr2O3 scales obeys the relationships D-B(cm(2)s(-1))=1.2x10(-11) exp(-144kJ mol(-1)/RT) for bulk diffusion and D-gb(cm(2)s(-1)=2.56x10(-4) exp(-190 kJ mol(-1)/RT) for grain-boundary diffusion. The difference between the values for bulk and grain-boundary diffusion coefficients is about five orders of magnitude in the temperature range studied. These results are discussed in comparison with the diffusivity of other elements in Cr2O3 scales and of yttrium in bulk alumina. Both the bulk and the grain-boundary diffusion coefficients for yttrium have been found to be lower than those for oxygen and chromium diffusion in the same Cr2O3 scale.