Nickel-base superalloys have been used extensively in high-temperature applications where strength and structural stability are required, most notably in aero gas turbine engines. To increase the efficiency of such engines, a continuous increase in superalloy operating temperatures has been observed. As temperatures continue to increase, multiple aspects of alloy stability become increasingly important. In that regard, the high-temperature performance of superalloys can be generally discussed from two important standpoints, surface stability and structural stability. Historically, structural stability has been the primary concern to alloy designers, such that superalloys that may be exposed to high-temperature applications exceeding 1100°C typically utilize a coating for environmental protection. However, the use of coatings introduces potential deficiencies. For instance, aluminide coatings can lead to extensive instabilities when in contact with newer generation superalloys. Also, a few niche applications exist where the use of a coating is impractical. In such cases, the alloys require both environmental resistance and high-temperature strength.
The primary goal of this study was to develop novel heat-treatable γ-Ni+γ'-Ni 3Al-based alloys having excellent resistance to both high-temperature oxidation and creep. The alloys were developed in a systematic manner using multiple alloying additions, including Pt and Ir, i.e., platinum group metals (PGMs). The microstructures and environmental and thermal stabilities of the alloys studied were fully characterized through a series of experiments, including: oxidation (both isothermal and cyclic); hot corrosion (both Type I and Type II); microstructure analysis (including lattice misfit); and phase equilibria calculations with partitioning coefficient analysis.
Pt modification was found to significantly affect the lattice misfit of an alloy by expanding the γ' lattice parameter through its Ni sublattice site preference. This increased misfit had significant effects on the two-phase microstructure, particularly the γ' precipitate shape. Pt was found to not have strong chemical interactions with the other elements present in the alloys and, hence, did not significantly affect the phase equilibria. However, Pt addition did increase the oxide scale adherence and enhanced the hot corrosion resistance. Moreover, phase stability was increased when alloying with Pt, as deleterious phase precipitation was suppressed. A detrimental effect of Pt was found to be the suppression of the already narrow heat-treatment window of higher order Ni-base superalloys.
The use of Cr was found to have a profound effect on facilitating the exclusive formation of a thermally grown Al2O3 scale. Increased Cr concentrations were needed as strengthening element concentrations increased. Cr was also found to have an effect on the partitioning of other elements within the superalloy, particularly those that already partitioned to the γ-Ni matrix, resulting in preferred precipitate morphology changes.
Results of PGM modification were compared to previous investigations and discussed from a metallurgical point of view.