High Temperature Oxidation Behavior of Conventional and Nb-Modified MAR-M246 Ni-Based Superalloy

Abstract

The present investigations focused on the thermal oxidation of two variants of MAR-M246 alloy having the same contents of Ta and Nb in at. pct, considering the effects of total replacement of Ta by Nb. The alloys were produced by investment casting using high purity elements in induction furnace under vacuum atmosphere. The alloys were oxidized pseudo-isothermally at 800°C, 900°C and 1000°C up to 1000 hours under lab air. Protective oxidation products growing on the surface of the oxidized samples were mainly Al 2 O 3 , Cr 2 O 3 . Other less protective oxide such as spinels (NiCr 2 O 4 and CoCr 2 O 4 ) and TiO 2 were also detected as oxidation products. The conventional alloy exhibited slight internal oxidation at 800°C and an enhanced resistance at 900°C and 1000°C. The Nb-modified alloy presented an exacerbated internal oxidation and nitridation at 900°C and 1000°C and an enhanced resistance at 800°C. At 1000°C, Nb-modified alloy was particularly affected by excessive spalling as the main damage mechanisms. From a kinetic point of view, both alloys exhibit the same behavior at 800°C and 900°C, with k p values typical of alumina forming alloys (2 9 10 À14 to 3.6 9 10 À13 g 2 cm À4 s À1 ). However, Ta modified alloys exhibited superior oxidation resistance at 1000°C when compared to the Nb modified alloy due to better adherence of the protective oxide scale.

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

“…Nevertheless, bunsenite NiO and chromia Cr 2 O 3 oxides were detected in the sample oxidized for 240 h. In Figure 9b, which shows the low‐intensity peaks in detail, Al 2 O 3 , TiO 2 and XY 2 O 4 phases (with Cr 2 CoO 4 structure—cubic Fd‐3m) are also detectable, in consonance with the thermodynamic simulations. Alkmin et al [ 16 ] found mainly Al 2 O 3 , Cr 2 O 3 , TiO 2, and spinels such as CoCr 2 O 4 and NiCr 2 O 4 on the MAR‐M246 alloy after pseudo‐isothermal oxidation at 800°C for 1000 h, in good agreement with the results presented here.…”

“…The phases observed were Ni-gamma, Cr 2 O 3 , Al 2 O 3 , NiO, rutile-TiO 2 , XYO 4 (structure from WNiO 4 -monoclinic P2/c) and XY 2 O 4 (again related to (Cr, Co) 3 O 4 structurecubic Fd-3m), in line with the thermodynamic simulations, except for the TiO 2 and XYO 4 phase. Finally, Alkmin et al [16] showed that the main oxides formed on the MAR-M246 alloy after pseudo-isothermal oxidation at 1000°C for 1000 h were Al 2 O 3 , the spinel CoCr 2 O 4, and TiO 2 , also observed here.…”

“…Nevertheless, bunsenite NiO and chromia Cr 2 O 3 oxides were detected in the sample oxidized for 240 h. In Figure 9b, which shows the low-intensity peaks in detail, Al 2 O 3 , TiO 2 and XY 2 O 4 phases (with Cr 2 CoO 4 structure-cubic Fd-3m) are also detectable, in consonance with the thermodynamic simulations. Alkmin et al [16] found mainly Al 2 O 3 , Cr 2 O 3 , TiO 2, and spinels Figure 10a shows the surface of MAR-M246 superalloy after oxidation at 1000°C for 120 h. No scratches from preparation of the sample were detected, indicating that the oxides covering its surface are thick. However, clear regions are visible, probably indicating the occurrence of oxide scaling during cooling of the sample.…”

“…It is worth noting that after pseudoisothermal oxidation tests at 800°C the MAR-M246 superalloy does not suffer severe internal oxidation since the alumina formed seems to be more continuous. [16] The micrograph in Figure 7 shows a cross-section of MAR-M246 superalloy oxidized at 800°C for 120 h, while Table 3 lists the results of the EDS mapping of Regions 1-3 shown in this micrograph. Note the presence of several localized forms of oxidation, which may correspond to the oxidation of carbides.…”

“…These authors stated that the fatigue response depended on internal and external oxidation, as well as the growth of precipitates in front of the crack tip. [ 15 ] Alkmin et al [ 16 ] have studied the MAR‐M246 superalloy pseudo‐isothermally oxidized at 800, 900°C and 1000°C up to 1000 h under lab air. The results have shown that Al 2 O 3 , Cr 2 O 3, and other less protective oxides such as spinels (NiCr 2 O 4 and CoCr 2 O 4 ) and TiO 2 were also detected in the oxide layer.…”