The oxidation of Ni 3Si-base alloys

Abstract

The isothermal oxidation behavior in air of Ni-19at.%Si and Ni-19at.%Si-8at.%Cr alloys, which are based on the Ll 2 structure of Ni 3Si, 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 SiO 2 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 Cr 2O 3 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 Cr 2O 3 scale is less protective than the SiO 2 scale which forms in the base alloy in this temperature range. The formation of a continuous Ni 3Si layer by Cr depletion that is separated from the external Cr 2O 3 by an Ni-rich oxide region is observed.