THE OXIDATION OF NI3SI-BASE ALLOYS

The isothermal oxidation behavior in air of Ni-19at.%Si and Ni-19at.%S i-8at.%Cr alloys, which are based on the L1(2) structure of Ni3Si, was studied in the oxidation temperature range from 500 degrees C to 900 degrees C for exposures up to 20 h. The maximum oxidation rate for Ni- 19at.%Si was observed at 700 degrees C. The oxides which formed in thi s alloy at temperatures up to 700 degrees C consisted of external NiO layers and internal oxides that preferentially formed in the two-phase (L1(2) + f.c.c.) regions of the microstructure adjacent to the sample surface. The increase in oxidation rate with temperature below 700 de grees C is attributed to these oxides. For oxidation temperatures abov e 700 degrees 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 significantl y modified the oxidation behavior of the alloy with a monotonic increa se 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 degrees C, the o xidation rates for the Cr-containing alloy were found to be significan tly lower than those for the Ni-19at.%Si base alloy. However, at tempe ratures of 800 degrees C and above the oxidation rates were larger tha n in the base alloy, suggesting that the Cr2O3 scale is less protectiv e than the SiO2 scale which forms in the base alloy in this temperatur e range. The formation of a continuous Ni3Si layer by Cr depletion tha t is separated from the external Cr2O3 by an Ni-rich oxide region is o bserved.