Degradation of Aluminide coatings on Platinum-containing model and real superalloys – ALUPLAT

KEYWORDS: High-temperature oxidation, corrosion, tensile and creep of a superalloy and model γ and γ' alloys. Protection by slurry coatings.

Different variants of TROPEA and the reference superalloy (CMSX-4) and model alloys (phases γ and γ′, NiCrAl) were then oxidized, hot-corroded and mechanically tested in order to assess the initiation of the failure based on in situ monitoring by electron microscopy.

We demonstrate that the initiation of failure occurs on the γ' phase under hot corrosion and stress conditions in connection with a high Ta content and its intermetallic character, whereas in oxidation, this phase allows the development of a protective oxide layer. In real alloys, a higher Ta and lower Re content leads to a more marked degradation. Platinum has a negative effect in the presence of SO₂, which it converts into SO₃, thus increasing corrosion. On the other hand, the application of aluminum or aluminum/silicon diffusion coatings greatly reduces oxidation and corrosion of the TROPEA, but their mechanical behavior will have to be evaluated.

It is expected that future aircraft engines using this coated alloy will be more efficient, thus reducing fuel consumption and greenhouse gas emissions.

The oxidation kinetics under air (850-1200°C) of the real materials (TROPEA and CMSX-4) and model materials (phases γ and γ′, thermodynamically designed and cast NiCrAl) were established by thermogravimetry and in situ electron microscopy in order to detect the formation of oxide layers according to the chemistry of the metallurgical phases and the temperatures to which the engine parts are exposed. A transition between the protective layers of Cr2O3 and Al2O3 occurs around 1000°C without platinum intervening in the oxidation mechanisms. In hot corrosion with a Na2SO4 deposit, the platinum in the substrate catalyzes the transformation of SO2 into SO3 and aggravates hot corrosion at 900°C, which is not the case in the absence of SO2. Whether due to oxidation or corrosion, high and low tantalum content of rhenium decreases the strength of superalloys. In situ monitoring by electron microscopy of the reactivity of real and model materials shows that the initiation of degradation depends on the deformation of the substrate and the local chemistry, the γ′ phase being less protective than the γ phase. The intermetallic character of the γ′ phase shows a lower tensile and creep resistance at 950°C than the γ phase while the γ/γ′ combination increases it. Aluminizing (Al and Al/Si) by slurry according to different heat treatments leads to diffusion coatings that protect against oxidation and corrosion on both TROPEA and the CMSX-4 reference, although Pt does not exert any effect.

MAJOR FINDINGS:

The chemical degradation of the variants of TROPEA (new nickel-based superalloy containing platinum) occurs as a result of higher tantalum and lower rhenium contents compared to the reference alloy (CMSX-4). This is also demonstrated on simplified phases γ and γ′. The latter develops a Al2O3 layer that dissolves in hot corrosion and is also brittle because its intermetallic character.

However, Al and Al/Si diffusion coatings limit chemical degradation by the formation of dense layers of Al2O3 and Si also limits acidic hot corrosion, which would allow TROPEA to be used under aggressive atmospheres like those encountered in real engines. It appears that the expected positive effect of platinum is instead negative in the presence of SO2 in the gas atmosphere because hot corrosion is increased while Pt is inert in the absence of SO2 (with or without salt deposit).

These findings majorly contribute to the design of new alloys for high temperatures. “ALUPLAT” has made it possible to establish a solid and synergistic partnership of laboratories at the service of science and industry.

CONCLUSIONS AND PROSPECTS

This study highlights the importance of the chemical composition of nickel-based superalloys for gas turbine blades. Although TROPEA has better mechanical properties than other single-crystal nickel-based superalloys due to its stable microstructure at high temperatures, detailed analysis of its resistance to environmental conditions (oxidation and type I hot corrosion) reveals the significant impact of the type and content of alloying elements in the γ/γ′ microstructure. These results also highlight the complexity of the numerous interactions between materials and the atmosphere and underscore the trade-offs that must be made when developing this type of alloy.

As a key area for study, consideration should also be given to the influence of water vapor produced in a combustion engine, as this can be particularly harmful to the formation of protective layers. Furthermore, Na2SO4 is rarely the only corrosive species, and other sulfates and chlorides may also be present under engine operating conditions, which should be taken into account, as should the thermal cycles experienced in an aircraft engine. Finally, it would be interesting to study the behavior of TROPEA under type II hot corrosion conditions, since the blade root operates at “low temperatures.” In addition, the addition of alloying elements to the γ and γ′ model alloys could provide a better understanding of their role in environmental performance.

Finally, the key point that could not be addressed in ALUPLAT due to lack of material is the mechanical-corrosion coupling in order to elucidate the potential synergistic effects between stress states and the reactivity of this new class of superalloys, with or without coating.