The goal of this research project was to provide fundamental understanding of CaO- and CaSO4-induced degradation observed in aviation gas-turbine engines and to develop a lab-scale test procedure which accurately replicates the degradation observed in field-exposed components. Based on initial assessments, particular attention was paid to characterizing the nature of breakaway internal oxidation caused by CaSO4 deposits and determining how CaSO4-alloy interactions induce subsurface changes in the alloy composition and microstructure which can make the alloy susceptible to internal attack.
Both SEM and TEM analyses were used to characterize the morphology of degradation in field-exposed components. Emphasis was placed on characterizing the composition and phase distribution in the internal oxidation zone (IOZ). Isothermal experiments were conducted to investigate the interactions that take place between CaO or CaSO4 deposits and single crystal superalloys at elevated temperatures. This was achieved by exposing Rene N5 and N500 coupons with CaO or CaSO4 deposits at 900°C or 1150°C in air for various times and characterizing the reaction product. From the results obtained, a novel bi-thermal test procedure was developed which successfully replicated the degradation that occurs in the field-exposed components. It was determined that degradation of the subsurface caused by CaSO4 at 1150°C made the alloys susceptible to internal oxidation when exposed to conditions that better simulate the gas-turbine environment. Following this result, sets of systematic experiments were developed to determine how the environmental variables of atmosphere, thermal profile, and deposit mass influence the oxidation behavior of a subsurface-depleted alloy and how the behavior is linked to the kinetic competition between internal and external oxidation. Finally, it was observed that N5 is more resistant to CaSO4-induced degradation than N500. Because of this, the influence of alloy composition on the degradation resistance of nickel-based alloys was explored by conducting bi-thermal experiments with CaSO4 deposit on a superalloy with intermediate composition to that of N5 and N500 and by conducting oxidation experiments on model Ni-Cr-Al-Re alloys
