LAUSR

The oxidation rates of Ni–Al and Ni–Al–Cr alloys in an Ar–12 vol%H2O atmosphere during heating were determined by a hydrogen sensor and an oxygen pump sensor installed at the subsequent stage. The effect of Cr on the Ni–Al alloy was studied. As a result, the Ni–5 wt%Al showed a sharp increase in oxidation rate at 900 °C. Thereafter, a linear increase in the oxidation rate was observed during its heating. On the other hand, the Ni–5 wt%Al–5 wt%Cr alloy showed a sharp increase in the oxidation rate at 900 °C, but the value was lower than that of Ni–5 wt%Al. During the heating from 900 to 1250 °C, it showed a discontinuous behavior. In the cross-sectional structure, a thick oxide and thin oxide were formed. For the thin part, NiAl2O4 was formed on the surface and Al2O3 was formed below it. The oxidation rate of Ni–5 wt%Al–10 to 20 wt%Cr alloys when the temperature reached 900 °C was very low. For the cross-sectional structure, Cr oxide was formed on the surface, and Al2O3 was formed below it. When Cr oxide was formed on the surface, a sharp decrease in the oxidation rate was observed during the heating. It is considered that the oxidation rate decreases by the transformation of Al2O3 from θ to α during the heating. Furthermore, the rate of oxidation during the heating decreased more when chromium oxide was formed than when Ni oxide was formed on the surface. This revealed the relationship between the oxidation rate and the effect of Cr on the Al2O3.