The oxidation of Ni3Si-base alloys

The isothermal oxidation behavior in air of Ni-19at.%Si and Ni-19at.%Si-8at.%Cr alloys, which are based on the Ll2 structure of Ni3Si, was studied in the oxidation temperature range from 500 °C to 900 °C for exposures up to 20 h. The maximum oxidation rate for Ni-19at.%Si was observed at 700 °C. The oxides which formed in this alloy at temperatures up to 700 °C consisted of external NiO layers and internal oxides that preferentially formed in the two-phase (Ll)2 + f.c.c.) regions of the microstructure adjacent to the sample surface. The increase in oxidation rate with temperature below 700 °C is attributed to these oxides. For oxidation temperatures above 700 °C a sharp decrease in oxidation rate is observed and is attributed to the formation of a continuous SiO2 protective oxide film at the base of the NiO layers. The addition of 8 at.% Cr significantly modified the oxidation behavior of the alloy with a monotonic increase in weight gain observed as a function of oxidation temperature. The formation of Cr2O3 appears to dominate the oxidation behavior of the Ni-19at.%Si-8at.%Cr alloys. At temperatures below 800 °C, the oxidation rates for the Cr-containing alloy were found to be significantly lower than those for the Ni-19at.%Si base alloy. However, at temperatures of 800 °C and above the oxidation rates were larger than in the base alloy, suggesting that the Cr2O3 scale is less protective than the SiO2 scale which forms in the base alloy in this temperature range. The formation of a continuous Ni3Si layer by Cr depletion that is separated from the external Cr2O3 by an Ni-rich oxide region is observed.