High-temperature oxidation of directionally solidified Ni--Cr--Nb--Al (γ/γ'--delta) eutectic alloys

The oxidation of Ni--23.1Nb--4.4 Al and Ni--19.7Nb--6 Cr--2.5 Al alloys in air at temperatures in the range 870 to 11000C has been studied for times up to 168 hr, in the as-cast, slowly cooled, and directionally solidified forms. The oxidation rate decreases with increasing temperature for the ternary alloy, and this appears to be due to the increasing tendency to establish a continuous Al2O3 layer at the metal surface, although at no temperature in this range is a complete layer established. At the lowest temperature the delta--Ni3Nb lamellae are preferentially oxidized, with fingers of oxide extending into the metal, but at 9000C and above a continuous single-phase delta-free layer is established at the metal surface very early in the oxidation. The oxidation rate of the quaternary alloy increases with increasing temperature. At the lower temperatures a continuous Al2O3 layer is established at the metal surface, but at the highest temperature the aluminum oxidizes internally and a continuous layer is not established, internal oxidation penetrating down the lamellae. It appears that niobium, like chromium, is able to promote the formation of external Al2O3 layers; if this fact is accepted, the beneficial role of chromium in these alloys is difficult to explain