Dynamic Oxidation Behavior and Exceptional High-Temperature Resistance of Nano Particle-Strengthened Superalloy (MA 754) under Cyclic Temperature Conditions

3.1. Cyclic Oxidation Behavior The mean net mass change of samples after cyclic oxidation test in air containing 6% moisture at 1100 °C versus exposure time is presented in Figure 1. The increase in the weight of the samples is associated with the reaction between the superalloy and oxygen in the surrounding air, resulting in the formation of oxide scales. Conversely, the weight loss of the samples can be attributed to the cracking of the protective oxide film, leading to its spallation from the surface of the superalloy. These cracking and spallation phenomena are the consequences of thermal and oxide growth stresses. The observed fluctuations in the oxidation behavior may be partially attributed to experimental apparatus errors. Similar oxidation behavior was documented by Obigodi-Ndjeng [4] during the investigation of the oxidation behavior of Ni-based superalloy PWA and Ni-Cr-Al ternary alloys. The author concluded that a fluctuating curve might indicate a non-steady oxidation behavior. 3.2. Oxidation Products Figure 2 presents the XRD spectra depicting the spalled oxides of MA 754 superalloy following 100 h of cyclic oxidation at a temperature of 1100 °C. The obtained results reveal that the oxidation products of this Inconel alloy primarily consist of NiO and NiCr₂O₄. A small peak at approximately 25.6° in the XRD pattern, which does not overlap with other phases, suggests the possible presence of α-Al₂O₃. Notably, the prominent peaks corresponding to NiO suggest that this oxide constitutes the predominant component of the oxide scale during the cyclic oxidation process of MA 754, whereas Al₂O₃ exhibits the least formation. Considering that MA 754 superalloy is known as a chromia-forming alloy when exposed to an oxidizing atmosphere [13], the presence of Cr₂O₃ peaks in the XRD pattern is expected. However, the absence of Cr₂O₃ in the oxidation products can be attributed to the following two reasons: Previous investigations have indicated that the protective chromium oxide scale reacts with O₂ at elevated temperatures following reaction 1, forming gaseous CrO₃. The Gibbs free energy (ΔG) for this reaction at 1100 °C is calculated to be approximately +240 kJ·mol⁻¹. This positive value suggests that CrO₃ formation is not spontaneous under standard conditions. However, the process is promoted by high oxygen partial pressure and the presence of water vapor, which was 6% in the test environment. These conditions facilitate chromium volatilization through the formation of CrO₃ and volatile chromium oxyhydroxide species such as CrO₂(OH)₂, thereby contributing to the observed depletion of Cr₂O₃ from the scale [20,21,22,23,24]. Simultaneously, the protective chromium oxide reacts with NiO during high-temperature oxidation, forming non-protective NiCr₂O₄ [5,15]. This reaction can be represented by reaction 2 [5]: The Gibbs free energy equation associated with this reaction is given by: Consequently, the value of ΔG (T) for NiCr₂O₄ at 1100 °C is approximately −873.31 kJ·mol⁻¹. The negative value of ΔG (T) confirms the thermodynamic favorability of NiCr₂O₄ formation. Therefore, from a thermodynamic standpoint, the spontaneous formation of this spinel from its reactants is established. One approach to forecasting the formation of oxides during an alloy’s oxidation involves using an oxide map. Figure 3 depicts the oxide map corresponding to a ternary Ni-Cr-Al system at a temperature of 1100 °C. This diagram encompasses three distinct regions. The position of MA 754 within this superalloy is determined by the atomic percentages of Ni, Cr, and Al in its chemical composition. The star symbol in Figure 3 indicates the determined location of the alloy under investigation, situated next to the NiO/Cr₂O₃ boundary of the ternary diagram within the NiO region. According to Khanna [2], oxides formed in alloys belonging to this region predominantly consist of NiO and spinels, with the possibility of internal oxidation of Cr and Ni. Within this region, the oxidation process is primarily governed by the outward diffusion of Ni ions. Hence, the presence of NiO and NiCr₂O₄, along with the absence of Cr₂O₃, in the XRD pattern of the spalled oxide (Figure 2) can be predicted using the oxide map. However, the small Al₂O₃ peak observed in the XRD pattern (Figure 2), despite the alloy composition placing it in the NiO region of the oxide map (Figure 3), suggests the occurrence of localized or internal oxidation processes. This can be attributed to several factors. First, aluminum, though present in low bulk content (0.5 wt.%), may locally concentrate near grain boundaries or oxide dispersion particles such as Y₂O₃, enabling the nucleation of Al₂O₃ [2,17,25]. Second, aluminum’s strong affinity for oxygen allows for internal oxidation below the surface, leading to subscale Al₂O₃ formation that may only be revealed through spallation or cracking of the outer layers [26,27]. Third, cyclic oxidation in moist air (6% water vapor) can modify oxidation kinetics and favor the transient formation of Al₂O₃ even in low-Al alloys [23,24]. Thus, although not predicted as a major oxide by the equilibrium oxide map, the minor presence of Al₂O₃ is thermokinetically plausible under the current experimental conditions. 3.3. Scale Morphology and Elemental Composition The surface morphology of the MA 754 sample was analyzed using scanning electron microscopy (SEM), as illustrated in Figure 4. The observed image reveals a porous and extensively cracked oxidized surface. The occurrence of cracks serves as an indication of stress concentration within the oxide scale [2]. Based on the findings depicted in Figure 4a, it can be inferred that the MA 754 samples underwent oxide spallation phenomena during the high-temperature oxidation test. Consequently, this particular superalloy mitigates stress concentration by forming cracks in the scale, thereby facilitating scale spallation. A primary factor contributing to oxide spallation is thermal stress, arising from a mismatch between the thermal expansion coefficients of the oxide and the alloy [2]. Given that the oxidation test was conducted under cyclic temperature conditions, the samples experienced repeated cooling and heating cycles. As a result, the cyclic heating conditions not only generated stress within the oxide layer but also led to the accumulation of thermal stresses, thereby promoting scale spallation during the oxidation study [2]. An additional factor contributing to oxide spallation is the generation of stress during the growth of oxide scales [2]. Non-protective oxides, such as NiO and NiCr₂O₄, exhibit a higher growth rate compared to protective oxides like Al₂O₃ [29]. Considering the XRD pattern (Figure 2), which indicates that NiO and NiCr₂O₄ are the primary constituents of the oxidation products of MA 754, it can be inferred that the studied alloy experienced elevated stresses during the growth of oxides. These stresses progressively accumulate during the oxidation of the alloy, ultimately leading to the spallation of the oxide layers. Figure 4b presents a higher magnification of the same region, revealing the detailed flower-like oxide clusters (indicated by yellow arrows). It can be observed that each distinct oxide cluster has a flower-like structure. To provide a more comprehensive understanding of the morphology of oxide clusters, higher-magnification images are presented in Figure 5. These images depict the morphology of spalled oxides and the oxidized sample after a 50 h cyclic oxidation test at 1100 °C. The SEM micrographs reveal a pyramidal-type morphology characterized by flat faces. Notably, each oxide cluster exhibits a flower-like structure composed of numerous oxide pyramids. In addition, the pyramidal oxides observed in Figure 5 are remarkably small, with approximate dimensions below 6 μm. Raynaud et al. [30] and Tawancy [31] previously observed the pyramidal-type morphology of oxides during the oxidation behavior of Ni alloys. In this particular mode of oxide growth, screw dislocations facilitate the creation of new steps, and the growth process occurs through either lattice or grain boundary diffusion [30]. The formation of pyramid-shaped oxides is driven by the depreciation of the system’s surface energy, thereby influencing their growth behavior. The surface morphology of the investigated alloy following 50 h and 100 h of cyclic oxidation at 1100 °C is depicted in Figure 6. After 50 h of exposure to the cyclic oxidation test, the oxides exhibit a pyramidal morphology, as illustrated in Figure 6a. However, with an increase in oxidation time, as shown in Figure 6b, the distinctive geometric shape of the oxides diminishes. Instead, the pyramidal oxides appear to merge, giving rise to a new form. Furthermore, the intersections between the edge planes of each pyramid become flattened, making it difficult to discern clear boundaries between the components of the oxide clusters. This transformation suggests that the pyramidal morphology of the oxides transitions into a nodular shape over time, driven by the reduction of the oxide’s surface energy through the diffusion and movement of its constituent atoms. In order to gain further insights into the nature and composition of pyramidal-type oxides, EDS point analysis was employed. The results of this analysis conducted at points 1 and 2, as depicted in Figure 7a, are presented in Figure 7b,c. These point analyses were performed on the oxidized sample of MA 754 following a 100 h cyclic oxidation test at 1100 °C. The results reveal the presence of Ni, Cr, and O elements within the oxide pyramids. Considering the XRD pattern (Figure 2), which indicates the existence of NiO, NiCr₂O₄ and Al₂O₃ phases, it can be inferred that the pyramidal oxides are associated with the NiO and NiCr₂O₄ phases. Previous studies have reported a similar morphology for NiO and spinel oxides [26,29,30,31,32]. 3.4. Microstructure of Superalloy Figure 8 shows the etched microstructure of the alloy surface before the oxidation test. This image clearly displays the numerous twins of the MA 754 that arise from the fabrication process. The non-etched microstructure of the alloy surface prior to the oxidation study is presented in Figure 9a, revealing the presence of several fine porosities. Microstructures at different depths beneath the oxide layer, obtained after the oxidation test, are shown in Figure 9b–d. By comparing these microstructures, the oxidation process of the MA 754 superalloy can be characterized. The oxidation can be divided into three distinct stages, as follows:

1.

In the initial stage, the grain boundaries undergo degradation due to the attack from the surrounding atmosphere, as depicted in Figure 9b. This degradation leads to the formation of voids along all grain boundaries. The voids exhibit a continuous pattern, revealing the hexagonal arrangement of the grain boundaries. Previous studies have demonstrated that materials tend to experience preferential oxidation along grain boundaries, owing to their higher free energy compared to the grain regions. This preference facilitates faster intergranular transport of the reacting species [25,33,34]. Consequently, the degradation mechanisms of superalloys commence at the grain boundaries.

2.

The second stage of the oxidation process ensues after the formation of voids along all grain boundaries. In this stage, high-energy regions within each grain, such as defects, twins, and porosities, are subject to degradation by the surrounding atmosphere. The presence of distinct voids within the grains, as shown on the left side of Figure 9c, indicates the occurrence of this step.

3.

In the final stage, oxidation progresses in the remaining areas of the grains. Figure 9d illustrates the last step of oxidation, wherein more regions of the grains undergo degradation due to high-temperature oxidation. This process continues until the voids cover the entire surface of the sample.