Effect of exposure in steam or argon on the creep properties of Ni-based alloys

Although expensive, Ni-based superalloys are of interest for the ultrasupercritical steam program because of their good creep and oxidation resistance at temperatures above 700°C. As the effect of steam oxidation on the alloy mechanical properties is unknown, creep specimens of alloy CCA617, 740, and 230 were pre-oxidized for 2000 and 4000 h in steam at 800°C before testing in air at the same temperature. Compared with as fabricated material, exposure in steam decreased the creep properties of alloy CCA617, had less of an effect on alloy 740, and did not affect alloy 230. Testing of a specimen repolished after steam exposure as well as microstructural observations indicate that the oxidation affected zone at the specimen surface is not responsible for the properties degradation. Surprisingly, a similar time anneal in an inert environment resulted in a drastic decrease of creep rupture life and an increase in the creep rate and elongation at rupture. Transmission electron microscopy analysis revealed that the mechanical properties decrease for alloy CCA617 is related to the absence of γ′ precipitates within the grains.