In the past few decades, there have been attempts to improve physical properties of NiAl based alloys using lamellar and fibrous directionally solidified NiAl-based eutectics (Cr, Mo, etc.) However, since these alloys are meant to be used in turbines they will be in contact with combustion gases, and oxidative corrosion processes are inevitable.
Compared to the number of mechanical properties studies, there are only a few reports on the oxidation behavior of directionally solidified NiAl-based eutectic alloys.
In the present work, oxidation processes are investigated by advanced computational and experimental thermodynamics in the materials systems: NiAl-Cr-O, NiAl-Mo-O, and NiAl-(Cr,Mo)-O. The investigations focus on alloys with equimolar ratio of Ni:Al.
The oxide database for Ni-Al-Cr-Mo-O is compiled from the corresponding thermodynamic literature. It contains all binary oxide systems and most relevant ternary oxide systems with regard to the NiAl-X alloys.
The ionic liquid model is used to describe both the metallic and oxide melts. The spinel phase is modeled as a solution phase using the compound energy formalism with 4 sublattices. It includes the stable oxides Cr3O4, NiCr2O4 and NiAl2O4 as well as the metastable γ-Al2O3. Moreover, the monoxides and the sesquioxides together with metastable oxides like δ-Al2O3 and θ-Al2O3 are included.
In the experiments, we examine the oxidation of pure NiAl as well as the combinations NiAl/Cr and NiAl/Mo, individually. The oxidation of these samples are investigated in a thermobalance at high temperatures under a defined oxygen partial pressure. The oxidation products are examined by metallography, electron microscopy with EDX, and X-ray diffraction. The experimental results are compared with thermodynamic calculations using our oxide database.