3.1. Mass Variation Kinetics
Figure 1 shows the mass variation per unit area for both superalloys and for both temperatures.
Figure 1a presents the data for specimens exposed to cyclic oxidation tests at 900 °C for up to 180 cycles. This figure reveals that, after two cycles, the conventional MAR-M246(Ta) alloy lost mass while the modified MAR-M246(Nb) presented mass gain. Afterwards, both MAR-M246(Nb) and MAR-M246(Ta) had approximately the same mass gain per unit area between 2 and 10 cycles (0.309 and 0.324 mg/cm
2, respectively). Finally, for up to 180 cycles, the two alloys showed mass gain per unit area stabilization. This suggests that these materials show oxidation resistance in cyclic conditions at 900 °C. Alkmin et al. [
13] observed similar results for up to 200 h of experiments at 900 °C. In their paper, the pair of materials had similar mass gain per unit area for up to 200 h (which is close to 180 cycles), although MAR-M246(Nb) gained a little bit more mass.
Figure 1b presents the mass variation per unit area for samples exposed to cyclic oxidation tests at 1000 °C for up to 180 cycles. In comparison to the experiments at 900 °C, the materials showed an even closer behavior.
Figure 1b indicates a lot of intersections between the standard deviation for these superalloys until cycle 120. After this cycle, a specimen of each alloy was collected for characterization procedures. MAR-M246(Ta) presented higher standard deviations for the 96, 108, and 120 cycles. This suggests that one of the samples suffered mass loss during the experiments. Although mass loss may occur during the process, these two superalloys showed mass gain stabilization; therefore, both materials exhibit good oxidation resistance for cyclic conditions at 1000 °C. On the other hand, Alkmin et al. [
13] obtained different results at 1000 °C, whereby the conventional MAR-M246 also exhibited stabilization of mass gain, while the Nb-modified MAR-M246 presented a higher mass gain up to 200 h, followed by continuous mass loss, due to severe oxide layer spallation. Thus, the results presented here may indicate that, in cyclic conditions, the MAR-M246(Nb) shows higher oxidation resistance than in pseudo-isothermal conditions, at least for up to 200 h (or approximately 180 cycles). Alkmin et al. [
13] showed that MAR-M246(Nb) had a severe mass loss after 200 h, it may be interesting to study long-term cyclic oxidation behavior (more than 180 cycles) to better understand the Nb-modified superalloy under cyclic conditions and verify whether this alloy does not exhibit mass loss after 180 cycles.
The MAR-M246(Nb) (0.84 wt% of Nb) and MAR-M246(Ta) (1.5 wt% of Ta) exhibited a stable mass variation per unit area; thus, these materials had a good oxidation resistance, which may be related to the Nb and Ta content. In general, a small wt% of Nb (less than 2% in wt%) and an increasing wt% of Ta can improve the oxidation resistance. Mallikarjuna et al. [
17] studied Ni-based superalloys exposed to an isothermal oxidation test at 900 °C. Two of those alloys had 1.7 and 6.4 wt% of Ta. Although both showed oxidation resistance and parabolic behavior, the one with higher Ta content exhibited less mass variation per unit area. Smialek and Bonacuse [
18] also observed something similar by studying several Ni-based superalloys exposed to cyclic oxidation testes at 1100 °C. They indicated that, by increasing the wt% of Ta, the oxidation resistance was enhanced. Additionally, these authors showed that by increasing the wt.% of Nb, the mass loss per unit area was higher. Weng et al. [
19] compared three Ni-based superalloy which were exposed to oxidation tests at 800 °C. The only difference between these alloys was the wt% of Nb: 0.0, 2.0, and 2.5%. The alloy that contained 2.0 wt% of Nb showed the highest oxidation resistance. Analyzing this data from the literature, it can be suggested that the low Ta value present in the conventional MAR-M246 does not promote high-temperature oxidation, while low Nb content could improve high-temperature oxidation. Furthermore, no considerable difference was observed in the mass variation curves.
3.2. Topographic and Cross-Section SEM/EDS Analysis
The typical microstructures for both superalloys are similar: carbides are usually found in the interdentritic region, the distance between the dendritic areas are similar, and eutectic pools can be observed, as pointed out by Alkmin et al. [
13]. Since the typical microstructures were presented by our team in previous work [
13], this paper does not aim to deeply discuss this topic, giving more attention to oxide products. In order to gain knowledge of the behavior of the conventional and Nb-modified superalloys, SEM analysis was conducted after cyclic oxidation tests. The analyses were conducted after 180 cycles on the specimen’s surface and cross-section for the two alloys at 900 and 1000 °C. The topographic SEM/EDS analysis for the two superalloys after 180 cycles of oxidation at 900 °C is shown at
Figure 2. Both MAR-M246(Ta) and MAR-246(Nb) exhibit similar characteristics. Region 1 of
Figure 2a,b is mostly constituted by Al and O, possibly an Al-rich oxide. This region is also characterized by a dark grey color. Region 2 of
Figure 2a,b has a totally different morphology in comparison with region 1. This region depicts randomly distributed oxide islands grown across the specimen’s surface. EDS mapping indicates that region 2 is primarily constituted by O, Cr, and Ti.
Figure 3 shows the cross-section SEM/EDS analysis for the two superalloys after 180 cycles at 900 °C. Both superalloys had a formation of two well-defined oxide layers: outer and inner. The outer oxide layer (for both
Figure 3a,b) is mostly constituted by O, Cr, and Ti. This oxide layer was also observed in
Figure 2 in region 2. The inner oxide layer is mainly constituted by Al and O, possible an Al-rich oxide. The formation of Al
2O
3 and Cr
2O
3 can be facilitated by the presence of Ta and Nb. It can explain the dense Al-rich and Cr-rich oxides observed [
17,
20,
21].
Figure 3a indicates a small carbide, and Ti is present in this structure, as also highlighted by Baldan et al. [
12].
Figure 3b shows other carbides in the MAR-M246(Nb) superalloy. These carbides are mainly constituted by Ti and Nb, according to the EDS maps. Tantalum usually acts like a carbide former. Once Nb and Ta have similar functions [
12,
22], it is reasonable to consider that Nb is also a carbide-former. Alkmin et al. [
14] also found out that Nb is an important element in carbide formation. Different Ni-based superalloys also presented Nb as a carbide-former, as reported elsewhere [
10,
11,
23].
It is important to highlight that the superalloy had similar aspects for both top and cross-section analysis at 900 °C for up to 180 cycles. Even after replacing Ta with Nb, the two alloys maintained similar oxide composition and oxide morphology. This indicates that the replacement did not interfere with these aspects of the superalloy. However, the carbide composition for these two superalloys showed different characteristics.
The top SEM/EDS analysis for the two superalloys for up to 180 cycles at 1000 °C is shown in
Figure 4. These analyses are more complex than the analysis shown previously for 900 °C. This indicates the influence of the change in temperature. Both
Figure 4a,b show similar characteristics. Region 1 was constituted of O, Al, and Cr. Region 2 had a higher intensity of Cr and Ti than region 1, although this also indicates the presence of elements such as Ni, Co, and O. Region 3 seemed to be mostly constituted by W and O, a possible W-rich oxide. This oxide was probably a W
20O
58, which usually evaporates in the form of WO
3(g). This evaporation process can reduce the oxide layer stability [
22,
24], therefore, explaining in part, the presence of spalled areas in region 5, as will be soon discussed. Region 4 was mostly constituted by Al and O, possibly an Al-rich oxide such as Al
2O
3. This region is similar to region 1 in
Figure 2 because of its high content of Al and dark grey color. Finally, region 5 is probably a spalled area. 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. Both MAR-M246(Ta) and MAR-M246(Nb) had a formation of two well-defined outer and inner oxide layers. Two different oxide layers were also observed for the tests conducted at 900 °C; however, there is an evident difference in the layers’ morphology. The outer oxide layer (for both
Figure 5a,b) is mostly constituted by O, Co, Cr, Ti, and probably Ni. Possibly, this layer represents region 2, which was analyzed in
Figure 4. The inner oxide layer (for both
Figure 5a,b) is mainly constituted by Al-rich oxides, probably Al
2O
3. This is the layer immediately over the substrate, and it is related to region 4, as shown in
Figure 4.
Figure 5a (MAR-M246(Ta)) indicates two interesting structures: regions 1 and 2. The arrows show region 1, which is a Ti-rich area. 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. This phenomenon is known as incorporation of metal islands into the oxide layer. This process is well described by Das et al. [
26].
3.3. XRD Analysis
Figure 6 reveals the XRD analysis for both MAR-M246(Ta) and MAR-M246(Nb), tested by cyclic oxidation at 900 and 1000 °C up to 180 cycles. The formed oxides are alike for both superalloys and temperatures. The only difference is the absence of the Ni(Co)Al
2O
4 spinel for the MAR-M246(Ta) superalloy at 900 °C.
Table 2 shows all the oxides and spinels identified by the XRD analysis.
Figure 6a,b show the oxide layer evolution for the conventional MAR-M246 for 900 and 1000 °C, respectively.
Figure 6a indicates that the most intense peaks (approximately at 44°, 51°, 75°, and 92°) are related to Ni. This suggests that the oxide layer was thin because of the detection of the substrate, mostly constituted by this element.
Figure 6b reveals that increasing the temperature led to the development of TiO
2, Cr
2O
3, Al
2O
3, and Ni(Co)Cr
2O
4 phases. This can be concluded by analyzing the peaks at approximately 27°, 30°, 36°, and 58°. These peaks are more intense at 1000 °C than at 900 °C. Moreover, at 1000 °C, the peak at 36° (related to Ni(Co)Cr
2O
4) is the most intense peak; in other words, it represents the increase in the oxide layer’s complexity.
Figure 6c,d show the oxide layer evolution for the experimental MAR-M246 for 900 and 1000 °C, respectively. At 900 °C,
Figure 6c, the MAR-M246(Nb) shows similar behavior in comparison to the conventional alloy at this same temperature. The most intense peak (around 44°) is related to Ni. This suggests that the oxide layer was thin, due to Ni detection. Despite this similarity, the Nb-modified superalloy exhibited more intense peaks (approximately at 24°, 27°, 34°, 36°, and 42°) for TiO
2 and Cr
2O
3, in comparison to MAR-M246(Ta) at 900 °C. This suggests that replacing Ta with Nb facilitates the formation of these oxides.
Figure 6d shows the X-ray diffraction analysis for the MAR-M246(Nb) at 1000 °C. The increase in temperature caused a similar impact for both MAR-M246(Ta) and MAR-M246(Nb). At 1000 °C, the most intense peak is related to Ni(Co)Cr
2O
4 and Cr
2O
3 at approximately 36°. This spinel shows more intense peaks (54° and 58°) at 1000 °C than at 900 °C for the experimental superalloy. Finally, replacing Ta with Nb did not lead to considerable changes in the oxides formed, as observed by the DRX analysis at 1000 °C, because
Figure 6b,d are alike.
3.4. Thermodynamic Simulations
In addition to oxidation behavior, the Ta replaced by Nb can affect the phase formation in the substrate. Thus, thermodynamic simulation was performed by JMatPro, and
Figure 7 shows the phase stability between 600 and 1400 °C thermodynamic simulation. As can be seen in
Figure 7, the expected phases are the same and in similar amounts for both MAR-M246(Ta) and MAR-M246(Nb). Similar results were also obtained by Alkmin et al. [
14], who observed similar phases for both alloys using ThermoCalc software for simulation. They also performed a Scheil simulation to evaluate solidification and observed that, in addition to γ and γ’, only boride and MC carbide were expected to form. On the other hand, as observed here, regarding the oxidation test temperatures, the primary carbide, MC, is not stable at 900 and 1000 °C, while M
6C and M
23C
6 are. For the pair of superalloys, the γ’ is mainly formed by Al. Although Ta and Nb are γ’ precipitate formers, they contribute only 0.6 to 1% (at%) of the composition of the phase.
To evaluate the presence of Ta and Nb in carbides, thermodynamic simulation and SEM/EDS analysis were used. According to the simulations for carbide composition (shown on
Figure 8), Nb and Ta were elements present in the primary MC carbides. These structures are not stable at 900 and 1000 °C, as observed in
Figure 7; therefore, carbides constituted by these elements must have been formed during the solidification process, as mentioned before.
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. For the other phases (M
6C and M
23C
6), tantalum and niobium do not play a significant role for temperatures of 900 and 1000 °C.
Figure 9 focuses on showing the differences between the carbides, possibly MC, formed on both superalloys. These analyses show carbides that are observed for specimens exposed to cyclic oxidation tests performed for up to 120 cycles at 900 and 1000 °C. First, it is important to remember that the experimental MAR-M246(Nb) was designed by replacing the number of atoms of Ta by Nb. It is evident the difference in the intensity of Ta and Nb in the formed carbides. It indicates that there are more niobium atoms constituting the carbides than in solid solution in the MAR-M246(Nb) alloy. In order to better understand this phenomenon, regions 1 and 2 (
Figure 9a,b) were analyzed by EDS. Regions 1 and 2 in
Figure 9a indicate 8.1 and 6.8% of Ta in at%, respectively, while in
Figure 9b, the results for regions 1 and 2 are 20.8 and 19.3% of Nb in at%, respectively. Although these values may not describe exactly the real amount of those elements, these atomic percentages are useful for comparing the two superalloys and their carbides. These results diverge from the thermodynamic calculations (
Figure 8). Since the simulations consider a perfect equilibrium through the solidification process, and MC carbides are not stable at 900 and 1000 °C (that is the case for
Figure 9), some differences between calculations and SEM/EDS are expected. Alkmin et al. [
14] also determined that niobium is more present in at% in carbides than tantalum. This difference in carbide formation, however, did not appear to affect oxidation behavior. Future studies should be conducted to evaluate in detail the microstructural aspects of gamma and gamma prime fractions, as Ta and Nb were also present in these main phases.