Thermal Stability of Nickel-Base Alloys

The thermal stability of three Ni-base samples was assessed at 1850F (1010°C) and 2000F (1093°C) in ambient air as a function of exposure time ranging from 500 to 2000 hrs. Assessments of thermal stability of the samples were made using weight change, oxidation, microstructural evolution, and post-exposure mechanical properties such as Vickers microhardness and compressive yield stress. The three samples included bare Alloy “A” (9Cr-6Al-1.5Hf), Alloy “A” with an overlay coating, and bare Alloy “B” (12Cr-3Al), were not much different in compositions. At 1850F, oxidation as measured by weight change was insignificant up to 2000 h in all the three samples. At 2000F, however, noticeable weight change occurred, increasing linearly with time all in the three samples. The oxidation penetration from surface to matrix for these samples was more intense when exposed to above 1000 hours, forming various oxides, gamma-prime (γ′) depletion zones, and TCP phases. The size and area fraction of γ′ precipitates were determined as a function of temperature and exposure time. Post-exposure mechanical properties were also assessed through Vickers hardness and compressive yield stress. A maximum change in Vickers hardness was about 10% at both temperatures up to 2000 hrs. The change in compressive yield stress was more pronounced than the change in Vickers hardness as a function of thermal exposure and time.