Isothermal short‐term oxidation behavior of MAR‐M246 nickel‐based superalloy at 800°C and 1000°C

Abstract

Superalloys are widely employed at high temperatures for structural applications. Hence, knowledge about the oxidation of these materials is essential.However, the literature is scanty when it comes to some families of superalloys. The purpose of this study was therefore to analyze the MAR-M246 polycrystalline alloy in isothermal short-term tests at 800°C and 1000°C for up to 240 h. Thermodynamic simulations were performed to evaluate the material's phase stability as a function of temperature and to assess the expected phases in response to oxygen pressure. The oxidized samples were characterized by SEM-EDS and DRX, which revealed a tendency for scaling of oxidized material, particularly at temperatures of 1000°C. Nevertheless, protective layers of Cr 2 O 3 and Al 2 O 3 oxides were formed, which enabled the formation of fairly thin oxide layers, in addition to NiO and complex oxides.The region of the metallic substrate close to the oxide layer underwent aluminum depletion, causing the gamma-prime phase to disappear, as well as formation of aluminum oxides and titanium nitrides. Last, a good correlation was found between the thermodynamic simulations and the oxides that were formed.

Cyclic Oxidation Behavior of Conventional and Niobium-Modified MAR-M246 Superalloy at 900 and 1000 °C

“…Figure 8a,b show the composition for MC carbides for both MAR-M246(Ta) and MAR-M246(Nb), respectively. These images suggest that the MC carbide is mainly formed by Ta, Nb, Ti, and W. This result converges with the information exhibited in previous studies [14,25]. Comparing these two superalloys, it can be observed that the at% of Ta and Nb in MC carbides are similar in the respective alloys.…”

“…It can be justified by the high content of the primary elements that constitute the alloys (Co, W, and Ni, as can be seen in Table 1) and also by the absence of O. Similar results were found in previous papers [13,25]. Baldan et al [25] also observed spalled areas for the conventional MAR-M246 exposed for isothermal oxidation tests at 1000 • C for 240 h. Additionally, Alkmin et al [13] found spalled areas for MAR-M246(Nb) pseudo-isothermally tested at 1000 • C for 650 h. Figure 5 shows the cross-section SEM/EDS analysis for the two superalloys oxidized for up to 180 cycles at 1000 °C.…”

“…Similar results were found in previous papers [13,25]. Baldan et al [25] also observed spalled areas for the conventional MAR-M246 exposed for isothermal oxidation tests at 1000 • C for 240 h. Additionally, Alkmin et al [13] found spalled areas for MAR-M246(Nb) pseudo-isothermally tested at 1000 • C for 650 h. Figure 5 shows the cross-section SEM/EDS analysis for the two superalloys oxidized for up to 180 cycles at 1000 °C. Both MAR-M246(Ta) and MAR-M246(Nb) had a formation of two well-defined outer and inner oxide layers.…”

“…This region is probably a titanium nitride. Although nitrogen was not identified by EDS analysis, basically due to technical limitation, very similar morphologies were identified in previous studies [13,25], which also studied MAR-M246 superalloy. The region indicated by number 2 is constituted by the most present elements in the superalloy (Ni, Co, and W) and O is not observed; therefore, it is part of the substrate surrounded by oxides.…”