Study by SIMS of the 54Cr and 18O Diffusion in Cr2O3 and in Cr2O3 Scales
The diffusivities, in lattice and along grain boundary, of 54Cr and 18O were determined at 900 °C and pO2 = 104 Pa in Cr2O3 (single and polycrystals) and in Cr2O3 scales formed on a Ni70Cr30 alloy by oxidation. All the concentration profiles were establis
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Study by SIMS of the 54Cr and 18 0 Diffusion in Cr 203 and in Cr 2 0 3 Scales S. C. Tsai 1 ,*, A. M. Huntz 1 , C. Dolin z, and C. M onty 3 1
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Laboratoire de Metallurgie Structurale, CNRS URA 1107, Bat 413, Universite Paris XI-Orsay, 91405 Orsay Cedex, France Laboratoire de Physique du Solide, Ecole Superieure de Physique et de Chimie Industrielles de la Ville de Paris, 75231 Paris Cede x 05, France Institut de Science et de Genie des Materiaux et Pro cedes, B. P. 5 Odeillo, 66125 Font-Romeu Cedex, France
Abstract. The diffusivities, in lattice and along grain boundary, of 54Cr and 180 were determined at 900 °C and pOz = 104 Pa in Cr Z03 (single and polycrystals) and in Cr Z03 scales formed on a Ni 70 Cr 30 alloy by oxidation. All the concentration profiles were established by SIMS which allows an accurate determination of the diffusion profiles in materials such as oxides which are characterized by a very low diffusivity, even with an important surface roughness as in oxide scales. With our analysis for the concentration profiles and thefvalue, fraction of atomic sites associated with grain boundaries modified by a ridge model, it appears that the lattice diffusion coefficients of Cr and determined in Cr Z03 scales are close to those determined in Cr Z03 single or polycrystals. However, the grain boundary diffusion coefficients of Cr and determined in polycrystals are smaller than those determined in scales. The lattice diffusion coefficients of Cr and are close to each other. But, the chromium grain boundary diffusion coefficient is slightly greater than that of oxygen.
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Key words: chromium, oxygen, lattice diffusion, grain boundary diffusion, Cr Z0 3 , Cr Z 0 3 scales.
Cr Z03 scale is generally regarded as a barrier to protect metallic alloys at high temperatures. As a consequence, the transport properties of Cr Z03 are of considerable interest. Many attempts have been made to measure the diffusion coefficients of chromium and oxygen in Cr Z 0 3 , and to determine from diffusion data the rates at which Cr Z 0 3 scales grow on chromia-forming materials. From early studies of the self-diffusion of oxygen and chromium summarized by Kofstad [1J, it was shown that oxygen diffusion was lower than chromium diffusion
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To whom correspondence should be addressed
D. Benoit et al. (eds.), Microbeam and Nanobeam Analysis © Springer-Verlag Wien 1996
S. C. Tsai et al.
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by about 3 orders of magnitude. Later, from oxidation studies, it was concluded that 3 scales grow on chromium metal by counter-current diffusion of chromium and oxygen, mainly via grain boundaries [2, 3]. Nevertheless, Kofstad [4J and Atkinson [5J suggested that outward cationic diffusion is more important for the growth of undoped Cr 203 scales. On the contrary, Sabioni et al. [6-8J recently determined the lattice and grain boundary self-diffusion coefficients of oxygen and chromium in single crystals and in polycrystals of Cr 203 in the range of temperatures 1100- 1450°C and showed that the oxygen diffusion coefficients are greater t
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