The diffusivity of Y in chromia scales which had grown on a Ni-30wt%Cr alloy during high-temperature oxidation was determined in air at temperatures ranging from 800 to 1000C. A thin Y-containing film was produced at the oxide surface by depositing a liquid solution of YCl3. Following the diffusion treatment, penetration profiles were measured by means of secondary-ion mass spectrometry. The apparent diffusion coefficients were calculated by using thin-film solutions to Fick's equation. The bulk and grain-boundary diffusion coefficients were obtained by using a new model which took account of oxide surface roughness and of the relationship between bulk, grain-boundary and apparent diffusion coefficients. The results (tables 2 and 3) showed that Y diffusion in Cr2O3 scales obeyed the relationships:

bulk:      D (cm2/s) = 1.2 x 10-11exp[-144(kJ/mol)/RT]

grain boundaries:      D (cm2/s) = 2.56 x 10-4exp[-190(kJ/mol)/RT]

The difference between the bulk and grain-boundary diffusion coefficients was of the order of 5 orders of magnitude within the temperature range studied. The bulk and grain-boundary diffusion coefficients for Y were both lower than the equivalent values for O and Cr diffusion in the same oxide scale.

J.Li, M.K.Loudjani, B.Lesage, A.M.Huntz: Philosophical Magazine A, 1997, 76[4], 857-69

Table 2

Bulk Diffusivity of Y in Cr2O3

Table 3

Grain-Boundary Diffusivity of Y in Cr2O3