Oxidation behavior of Ni3Al and FeAl intermetallics under low oxygen partial pressures

Selective oxidation and formation of protective oxide layers are critically important for engineering alloys used at high temperatures. In this project, a solid-state electrochemical oxygen pump system was used to establish oxidizing atmospheres with controlled oxygen partial pressures, in the range of Po2 ≅ 10−15–10−22 atm, and selective oxidation behavior of nickel- and iron-aluminides under these low oxygen partial pressures was studied, special attention was paid to early oxidation stages. The results indicated that under Po2 > 10−18 atm, pure nickel particles formed on the Ni3Al specimen surface, while Al2O3 could not completely cover the surface after short time oxidation. It was believed that the outward diffusion of nickel was related to the formation of the internal Al2O3, which generated extra volume in the alloy substrate, resulting in compressive stresses. After a long time oxidation, external oxidation dominated the process; and the specimen was completely covered by an Al2O3 scale. Under the lowest tested oxygen partial pressure (Po2 ≅ 10−22 atm), formation of nickel particles was not observed, while an external Al2O3 scale formed on the surface. For FeAl samples, no outward diffusion and formation of iron particles were observed, and an Al2O3 scale formed under the present conditions.