Systematic Investigation of Ordered B2 Structure Ru-Al-Ni Alloys Property

Materials used in gas turbines and jet engines require numerous properties such as thermal and chemical stability, oxidation and corrosion resistance, and creep strength. Turbine blades are typically coated with a bond coat layer and a ceramic topcoat on a superalloy substrate. To prevent delamination caused by the mismatch in the coefficient of thermal expansion between the substrate and the coating, as well as the formation of oxides at the interface, platinum (Pt) and other platinum group metals (PGMs) are added to the bond coat materials, typically represented by NiAl. The NiAl layer containing Pt improves the adhesion between the top ceramic coating and the substrate. However, Pt is rapidly sublimated when exposed to a high-temperature oxidizing atmosphere due to the volatility of Pt oxide. Another issue is the manufacturing cost due to the high price of PGMs, especially Pt, rhodium, and iridium.<br/>Ruthenium (Ru) is not only relatively inexpensive among PGMs, but also has a high melting point and a lower vapor pressure of the oxide compared to other noble metal oxides, which prevents volatilization. RuAl has already attracted interest as a bond coat material for Ni-based superalloys because of its strength and plasticity at room temperature and its ability to maintain these properties even at high temperatures. The Ru-Al-Ni ternary alloy, which is Ru added to the well-known bond coat material NiAl, an ordered B2 intermetallic compound, is regarded as a candidate bond coat material. However, the potential of Ru-Al-Ni alloys for the aerospace industry has not been fully explored because systematic characterization of Ru-Al-Ni alloys has not been well conducted.<br/>In this presentation, Ru-Al-Ni alloys with compositions varying to the extent that ordered B2 intermetallic compounds were prepared by the arc melting method. After homogenization heat treatment, the phases were identified by X-ray diffraction (XRD). Vickers hardness at room temperature and high temperature, thermal expansion, and high-temperature oxidation wear tests were conducted.<br/>XRD patterns and microstructural observations by FE-SEM (field emission scanning electron microscopy) and EDX (energy dispersive X-ray spectroscopy) showed that the Al 50 at% alloys were a single ordered B2 structure, while the alloys with Al below 50 at% were a mix of Ni₃Al and a Ru-rich phase. Vickers hardness at room temperature increased with Ni content. Furthermore, Ru-Ni-Al alloys were harder than the NiAl and RuAl binary alloys due to the size effect of Ru and Ni substitution. Particularly in the Ni₃Al phase precipitation region, the hardness exceeded 800 HV because of the B2+γ’ duplex phase.<br/>Linear expansion coefficient measurements from room temperature to 1100°C demonstrated that the coefficient of thermal expansion decreased with increasing Ru concentration. High-temperature oxidation tests at 1100°C for 24 to 120 hours indicated that compositions with higher Ru content and lower Ni content exhibited significantly more depletion over a long period of time. The formation of a dense Al₂O₃ layer was observed on the alloy surface after the oxidation test, and an Al-depleted phase existed in the underlying layer. The Ru-depleted phase was also present on the surface, indicating that the formation of Al₂O₃ and volatilization of Ru oxide were occurring simultaneously.<br/>Ru-Ni-Al alloys exhibit superior high-temperature oxidation properties compatible with the Ni-based superalloy substrate. It is important to obtain systematic property data to utilize these alloys in the aerospace industry as bond coat materials.