Oxidation Behavior of Ru-Containing Ni-Base Single-Crystal Superalloys

Abstract

The 4th and advanced generation Ni-base single crystal superalloys, which contain large amounts of refractory metals for strengthening and platinum group metals, e.g., Ru, for TCP-phase prevention, show excellent high-temperature strengths. However, these alloying elements seem to decrease high-temperature oxidation resistance. In this study, Ni-base superalloys with various amounts of Ta, Re and Ru were examined in isothermal and cyclic exposures at 1373K to investigate the effect on the oxide growth rate and resistance to scale spallation. Structures of the oxide for the alloys were analyzed by XRD, SEM and EDX, and the oxidation kinetics is discussed. Ru and Re were found to degrade the oxidation resistance by the vaporization of their oxide. Ta-rich oxide in the spinel layer affects to stabilize ruthenium and rhenium oxide in the scale and improve the oxidation resistance of Ru-containing Ni-base superalloys.

Intermediate Temperature Internal Oxidation in Fourth Generation Ru-bearing Single-Crystal Nickel-base Superalloys

“…TCPs break up the rafted microstructure [2] and deplete the matrix of solid-solution strengtheners [3]. Although the effect of Ru on ''reverse partitioning'' is the subject of debate, its effect on oxidation performance has been reported only at higher temperatures, typically C1,000°C [4][5][6][7][8][9]. At these temperatures, non-protective oxides form together with sub-surface oxidation.…”

Prediction of Initial Oxidation Behavior of Ni-base Single Crystal Superalloys: A New Oxidation Map and Regression Analysis

“…Alloy E in Figure 2(e) contains a relatively high amount of Co, and Alloy D in Figure 2(d) contains the second highest amount of Co, compared with the other alloys in group B. Co is known as an element that degrades the oxidation resistance of Ni-base superalloys, because CoO grows faster than NiO [34]. Kawagishi also reported that a high amount of Re in substrate degrades the oxidation resistance of Nibase superalloys [2]. High amounts of Re and Ru are generally present in the group B alloy.…”

“…Recent Ni-base superalloys incorporate refractory elements, and these superalloys can consist of more than 10 elements. The addition of refractory elements to Ni-base superalloys is imperative for maintaining excellent creep properties and microstructural stability at elevated temperature; however, the oxidation behavior of these latest Ni-base superalloys is more complex than was found in previous alloys [1][2][3] due to these additional elements.…”

“…Nevertheless, some of these elements degrade the oxidation resistance of Nibase superalloys [2,23]. For example, adding Mo to third generation alloys deteriorates their oxidation resistance because of the formation of Mo oxide, which is more prone to volatilization.…”

“…Cr is generally considered as an important element to enhance oxidation resistance [32], whereas Re and Mo tends to degrade oxidation resistance of Ni-base superalloys. With regard to Ru, the effects are still controversial [1][2][3]21], however, some studies [1,21] reported that Ru-containing Ni-base superalloys show inferior oxidation resistance than the previous generation alloys, although Ru provides excellent microstructure stability that allows further addition of alloying elements for higher strength at an elevated temperature [33]. It would be rationalized that less Cr and high Mo, Re and Ru contents in later generation of Ni-base superalloys could effect for initial oxidation of Ni-base superalloys.…”