Effect of microstucture and environment on the high-temperature oxidation behavior of alloy 718plus

The effect of microstructure on the oxidation behavior of 718Plus was evaluated in dry air, wet air and steam environments at 650°-700°C. Tests at 800°C were also performed in an attempt to accelerate the testing. Oxidation in wet air simulates turbine combustion environments and causes net mass losses at 650°-700°C because of the volatilization of Cr oxy-hydroxides that is not observed in ambient air testing. At 800°C, mass gains were measured but significant Cr depletion can be measured in the alloy after exposure due to volatilization. Changes in temperature and environment had much larger effects on oxidation performance than the two microstructures evaluated. Advanced characterization techniques were used to compare the differences in the oxide scale formed on alloys with different process conditions. At higher temperatures, the surface oxide was primarily identified as Cr₂O₃ by x-ray diffraction. Cross-section analysis showed an increase in internal oxidation attack with increasing temperature. The internal attack appeared to be associated with rod-shaped Ni₃Nb precipitates (δ-phase). Because of the changes in microstructure at 800°C, the steam and wet air evaluations appeared to be less relevant to lower temperatures. Thus, increasing the exposure temperature to 800°C does not appear to be a good strategy for accelerated testing of 718Plus.