Atsushi Takei1 and Kazuyoshi Nii1
1National Research Institute for Metals, Tokyo
The Ni-20Cr alloy was oxidized at 1100°C at various oxygen partial pressures, ranging from 10-12 to 10-1 atm, which were controlled by Ar-O2 mixtures. A large difference was observed between the oxidation behavior in a high PO_2 (≥qq10-3 atm) range and that in a low PO_2 (≤q10-5 atm) range. Therefore, the detailed experiments were carried out in PO_2 of 10-1 and 10-5 atm, the former being typical in high PO_2 and the latter in low PO_2. The weight gain-time curves showed that the growth rate of scale in low PO_2 lay between the parabolic and linear law, while that in high PO_2 was close to the parabolic law, which indicates that the scale formed in low PO_2 is less protective than that in high PO_2. It was found, on the other hand, that the oxide scales formed in low PO_2 were more adherent to the alloy than those formed in high PO_2. In high PO_2 a large amount of spalling was observed on cooling even in short time oxidation. The oxide scales formed in low PO_2 was uniform in thickness and rather porous, while those formed in high PO_2 was non-uniform in thickness and dense. In low PO_2 there were found a number of small voids at the oxide-alloy interface, but in high PO_2 large voids were found, which suggest the occurence of plastic deformation of oxide and alloy by induced stress in the growing scale. The above difference in oxidation behavior can be ascribed to the difference in growth mechanism of oxide scale due to the high and low PO_2.
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