Oxidation-Induced Decrease in Impact Resistance of TiAl4822 Used in Jet Engine Blades

1. Introduction

TiAl intermetallic compound-based alloys, such as TiAl4822 (Ti-48Al-2Nb-2Cr (at%)), are novel lightweight, heat-resistant materials that have recently found applications in jet engine blades [1,2], passenger car turbocharger turbine wheels [3,4], and engine valves [5,6]. The use of TiAl alloys has helped significantly improve engine efficiency and reduce fuel consumption and CO₂ emissions. As the primary application, over 1 million TiAl4822 cast blades [7,8,9] have been produced to date for use as last-stage turbine blades in turbofan jet engines, such as the LEAP [10].

In the operating environment of jet engine blades, oxidation occurs over long periods at relatively low temperatures (up to 700 °C). Therefore, maintaining strength properties after oxidation is crucial for reliability. The most critical aspect with respect to reliability is impact resistance. For instance, the TNM alloy (Ti-43.5Al-4Nb-1Mo-0.1B (at%) [11,12]), which was the second alloy after TiAl4822 to be put into practical use as a jet engine blade, was withdrawn from service due to frequent impact fractures caused by collisions with foreign particles ejected from inside the engine during operation [13]. The extreme differences between the practical performance of TiAl4822 and TNM alloy are attributed to the difference in engine type (the former is a turbofan, and the latter is a geared turbofan with a higher rotational speed). Nonetheless, maintaining impact resistance after oxidation is essential for TiAl4822 as well. Furthermore, considering maintenance work on the ground, maintaining impact resistance after oxidation is necessary not only under high temperatures during operation but also at room temperature.

However, TiAl alloys, including TiAl4822, have been reported to become brittle, especially at room temperature, because of oxidation [14,15,16,17]. These studies typically use tensile test specimens that have been oxidized for a short time at relatively low temperatures (around 700–900 °C) and attribute embrittlement to oxidation based on the observed decrease in ductility during tensile testing at room temperature. Furthermore, previous studies attribute oxidation embrittlement to oxygen penetration and changes in surface microstructure. The oxidation conditions (temperature and time) considered in previous studies were limited, and the effects of test temperature and test environment were not reported. Furthermore, when considering methods for evaluating embrittlement, it is important to note that the stress loads leading to failure in jet engine blades occur at high speeds rather than low speeds—as evident in the case of TNM alloy—and therefore, impact testing is more practically useful than tensile testing.

Accordingly, this study aims to clarify the practical oxidation-induced embrittlement behavior of TiAl4822 which is currently widely used in jet engine blades. Specifically, Charpy impact tests were conducted—primarily at room temperature—on samples subjected to varying oxidation conditions (temperature and time), as well as different test temperatures and environments, to evaluate embrittlement behavior due to oxidation. We also investigated the possibility that factors other than oxygen penetration and surface microstructural changes—which have been conventionally proposed as causes of oxidation embrittlement—might contribute to this phenomenon.

2. Materials and Methods

2.1. Materials

The current cast TiAl4822 blades use a material with an Al concentration slightly lower than the nominal value, within the permissible composition range [18]. Hence, a cast material with a composition of Ti-47.4Al-2Nb-2Cr (at%) was used in this study. Sponge Ti, Al pellets, Nb flakes, and Cr granules were used as raw materials, and samples were prepared via induction melting. The weight of a single charge was approximately 800 g. The raw materials were melted in a CaO crucible under an Ar atmosphere, which was introduced after evacuation. 0.2 wt% of Ca was added in the form of an Al-10 wt% Ca alloy to deoxidize the material. This addition reduces the oxygen concentration in the cast material to 0.1 wt% or less [19]. After all the raw materials were melted, the molten metal was held at a constant power output for 4 min and then cast into a cast iron mold. The cast material was a flat plate measuring 60 mm × 170 mm × 14 mm in the test specimen sampling section with a riser section on top. The cast material was then subjected to hot isostatic pressing (HIP) treatment at 1200 °C for 4 h at 186 MPa to eliminate internal casting defects.

2.2. Impact Test

Impact resistance was evaluated via the Charpy impact test, the simplest and most practical industrial method for assessing impact resistance. Foreign object damage tests on jet engine blades [20,21] (which typically involve propelling a small steel ball by gas pressure to strike the sample) and Charpy impact tests have been confirmed to yield similar results [22]. Rectangular test specimens measuring approximately 10 mm × 10 mm × 55 mm were fabricated via milling and fine-cutter cutting using a low-load diamond grinding wheel. Because TiAl alloys generally have very low impact resistance, in order to clearly distinguish between the different conditions, the test specimens were not notched, and the tests were conducted using a small hammer with a capacity of 25 J. The test environment was maintained at a temperature of 25 °C and a humidity of approximately 50–70%. Approximately 10 tests were conducted under each condition, with the cut surfaces serving as the front and back of the impact surface. The impact resistance under each condition was compared based on the average absorbed energy.

2.3. Oxidation Conditions and Comparison

Two types of test specimens were used: 1) Charpy test specimens prepared according to the procedure described above and then oxidized (hereinafter referred to as “oxidized specimens”); and 2) specimens made from material with sufficient excess material after HIP treatment, which were heated under conditions identical to those used for the oxidized specimens and then machined from the inside (hereinafter referred to as “heated specimens”). By comparing these two types of specimens, factors such as changes in internal properties due to heating can be eliminated, and the effect of oxidation alone on impact resistance can be evaluated. The effects of oxidation (and heating) temperature were evaluated by holding the samples at temperatures ranging from 300 to 1200 °C (in 100 °C increments) for 1 h at each temperature and subsequently furnace cooling. In addition, to evaluate the effects of oxidation (and heating) time, samples that had been held at 500 and 700 °C for 10 min, 1, 10, 100, 500, and 1,000 h, respectively, and then furnace cooled, were used. The effect of test temperature was also evaluated. Test specimens that had been oxidized (heated) were placed in an electric furnace installed immediately next to the Charpy impact testing machine, heated for approximately 30 min at 50, 100, 150, and 200 °C, then quickly removed and subjected testing. Moreover, the impact resistances of test specimens with the surface oxide layer polished off and specimens coated with machine oil (Shell Tonna S3M 68; the main ingredient is a paraffin-based hydrocarbon) were evaluated.

2.4. Microscopic Observation and X-ray Diffraction

The microstructure of the sample was examined by polishing an area near the center of the wall of the casting and capturing back-scattered electron images in the composition mode at 20 kV using a scanning electron microscope (JSM-6060, JEOL Ltd., Akishima, Japan). Secondary electron images of the surfaces of the oxidized test specimens were acquired using the above-mentioned apparatus at 20 kV. The microstructure of the cross-section near the oxide layer was evaluated via back-scattered electron imaging and elemental distribution mapping at 15 kV using an electron probe microanalyzer (EPMA; JXA-8900R, JEOL Ltd., Akishima, Japan). Because the oxide layer was extremely thin, the oxide products on the oxidized specimens were identified via grazing incident X-ray diffraction (GI-XRD) analysis (SmartLab, Rigaku Ltd., Akishima, Japan) at an incident angle of 0.5°, with an X-ray output of 45 kV and 200 mA, Cu Kα (λ = 1.5418 Å), and a scan speed of 0.4 deg/min.

3. Results and Discussion

3.1. Microstructure of the Base Material

Figure 1 shows the microstructure of the test specimen after HIP treatment. The microstructure was typical of TiAl4822, comprising primarily the γ phase with some lamellar structure mixed in.

3.2. Effect of Oxidation Temperature

Figure 2 shows photographs of the oxidized specimens after 1 h of oxidation at various temperatures. Slight surface color change was observed even at 300 °C, indicating that some oxidation had occurred. Thereafter, up to 800 °C, the surface color gradually darkened and turned black and blue. However, as the traces of the cut remained visible, the oxide layer is believed to be very thin. At 900 °C and above, the color changed to gray to light brown. The disappearance of the cut marks suggests an increase in the thickness of the oxide layer.

Figure 3 shows the relationship between the Charpy absorbed energy at 25 °C and temperature for the (a) oxidized specimen and the (b) heated specimen after 1 h of oxidation (heating). The 25 °C plot corresponds to the as-HIP condition. The absorbed energy for the oxidized specimen was lower than that of the as-HIP specimen at all oxidation temperatures, and this reduction was particularly pronounced at temperatures of 1100 °C and above. In contrast, although the heated specimens exhibited no significant variation with temperature, their absorbed energy was marginally higher than those of the specimens in the as-HIP condition. These results suggest that heating altered their internal properties.

Figure 4 shows the relationship between the oxidation (heating) temperature and the ratio of the average Charpy absorbed energy at 25 °C (oxidized specimen/heated specimen). This comparison allows for excluding the effects of changes in internal properties, enabling evaluation of the effects of oxidation alone. Even for a mild oxidation process such as holding at 300 °C for 1 h, the absorbed energy of the oxidized specimen was approximately 80% of that of the heated specimen. The extent of this decrease remained virtually unchanged up to 900 °C. At temperatures above 1100 °C, the absorbed energy of the oxidized specimen decreased to less than 50 % of that of the heated specimen. These results suggest that the mechanisms via which oxidation decreases the impact resistance differ between temperatures up to 900 °C and temperatures of 1,100 °C or higher.

Figure 5 shows secondary electron images of the surfaces of the oxidized specimens after oxidation for 1 h at 400, 700, 900, and 1100 °C. At 400 °C, surface changes were negligible. At 700 °C, extremely small particles were observed. In contrast, at 900 °C, the entire surface was covered with particulate oxides, and at 1100 °C, coarsening of these particles was observed. Figure 6 shows the GI-XRD patterns and identification results for the oxidized test specimens after 1 h of oxidation at 400, 700, and 900 °C. Table 1 summarizes the phases formed by oxidation, including those at other temperatures, in the order of peak height. Ti₂O was detected after 1 h of oxidation at 400 and 500 °C; TiO₂ was the most abundant at 600 °C and above. Ti-based nitrides were observed at 700 and 800 °C. Al₂O₃ was detected only at 800 °C and above, and hence, it can be concluded that Al₂O₃ is formed during 1-h oxidation only at temperatures above a certain threshold. At 900 °C, in addition to TiO₂ and Al₂O₃, other titanium-based oxides such as Ti₅O₉ and Ti₃O₅ were detected, and at 1100 °C, only TiO₂ and Al₂O₃ were present. However, the purpose of this evaluation was to identify extremely thin oxide layers, and only the surface was evaluated at a very small angle of incidence (0.5°). Therefore, other oxides or nitrides formed beneath the surface layer may not have been detected, particularly in cases where the oxide layer was thick due to high-temperature oxidation.

Figure 7 shows cross-sectional back-scattered electron micrographs of the region near the oxide layer on specimens oxidized for 1 h at 700 and 900 °C. Although the presence of oxides was confirmed via GI-XRD at 700 °C, they were too thin for observation in this cross-sectional microstructure. At 900 °C, a bright layer was observed on the surface, with a gray layer beneath it, which, based on the GI-XRD results, are believed to be TiO₂ and Al₂O₃, respectively. Figure 8 shows the EPMA results for the specimen that was subjected to a 1-h oxidation test at 1100 °C. A thick oxide layer was observed. The outermost layer was TiO₂, and the layer beneath it was Al₂O₃, which is porous. A mixed layer of TiO₂ and Al₂O₃, also porous, was observed beneath the Al₂O₃ layer. In addition, an altered layer with cracks and Al₂O₃ penetrated into the cracks was observed at the interface with the base material. The oxygen concentration in the altered layer was lower than that in the oxide layer, and the Al concentration was lower than that in the base material. Furthermore, Cr and Nb were enriched. As mentioned above, because only the outermost formed layer could be identified via the 0.5° GI-XRD analysis in this study, this phase was not detected. However, based on the degree of enrichment of each element, it is presumed to be the β phase generally formed in TiAl alloys by reducing the Al content and adding a certain amount of β-stabilizing elements, such as Cr. The significantly lower impact resistance of the specimen oxidized for 1 h at 1,100 °C or higher (Figure 4) is attributed to the influence of this altered layer. In other words, as the altered layer was not formed during 1-h oxidation tests at temperatures below 900 °C, the impact resistance was maintained for approximately 80% of that of the heated specimen; however, at temperatures of 1,100 °C or higher, the presence of this altered layer—which contained cracks—is believed to have promoted the initiation and propagation of cracks during the impact test, resulting in a decrease in impact resistance to less than 50%.

3.3. Effect of Oxidation Time.

Figure 9 shows photographs of specimens oxidized at 700 °C for 10 min, 1, 10, 100, 500, and 1,000 h. Surface color changes were observed even after 10 min, confirming oxidation. The surface color changed from blue to gray with an increase in the oxidation time. Traces of the cut remained visible up to 1,000 h, suggesting a thin oxide layer. Figure 10 shows secondary electron images of the surface and back-scattered electron micrographs of a cross-section near the oxide layer of the specimen oxidized at 700 °C for 1,000 h. The surface was covered with particulate oxides believed to be TiO₂. In the cross-section, the outermost layer was TiO₂, and the layer beneath it is believed to be a mixed layer of TiO₂ and Al₂O₃. No altered layer was observed at the interface with the base material.

Figure 11 shows the relationship between time and the ratio of the average absorbed energies (oxidized specimen/heated specimen) after oxidation (heating) for various durations at 500 and 700 °C. Even after mild oxidation—such as a 10-min hold at 500 °C—the absorbed energy of the oxidized specimen at 25 °C decreased to approximately 80% of that of the heated specimen. The extent of this decline remained virtually constant with an increase in the oxidation temperature and oxidation time. In other words, although impact resistance decreases even during very brief oxidation times at relatively low temperatures, the embrittlement behavior due to oxidation remains constant thereafter, regardless of increases in temperature or time.

3.4. Causes of Reduced Impact Resistance at Room Temperature Due to Oxidation

The significant decrease in impact resistance during oxidation at temperatures of 1,100°C or higher is due to an altered layer containing cracks forming at the interface between the oxide layer and the base material. The embrittlement mechanism at these temperatures is believed to differ from that at lower temperatures. This section presents the results of an experimental investigation into the causes of the reduction in impact resistance during oxidation at temperatures below 900 °C, which is of particular importance for jet engine blades.

As mentioned in the introduction, previous studies on oxidation embrittlement in TiAl alloys have used test specimens oxidized for relatively short periods at 700–900 °C. Based on the observed decrease in ductility during tensile testing at room temperature, these studies attributed embrittlement to oxidation. Furthermore, they attribute oxidation embrittlement to oxygen penetration and changes in surface microstructure. In this study, the impact resistance—which is more critical for jet engine blades—was considered, and the oxidation temperature and time were varied. Our results confirmed that even very minimal oxidation—such as exposure to 300 °C for 1 h or 500 °C for 10 min—causes embrittlement (impact resistance decreases to ~80 %). Furthermore, the degree of decline in impact resistance remained virtually unchanged even when the oxidation temperature was increased to 900 °C for a 1-h hold, or when the oxidation time at 500 and 700 °C was increased to 1,000 h. In other words, during oxidation at temperatures below 900 °C, embrittlement acceleration with oxidation progress was minimal. These results suggest that factors other than oxygen penetration or microstructural changes in the surface layer—as claimed in previous studies—may be involved in the reduction in impact resistance. These aspects were experimentally investigated as described below.

3.4.1. Removal of Oxidized Surfaces

For specimens subjected to 1-h oxidation at 700, 900, and 1100 °C, the surface layer was removed, and impact tests were conducted at 25 °C. The surface layer was removed via polishing: the specimens were polished with water-resistant sandpaper until the metal surface was exposed. The average thickness of the removed layer was 8.0, 11.0, and 35.0 μm for specimens oxidized at 700, 900, and 1100 °C, respectively. Compared to previous studies [14,15,16,17] that investigated the oxygen penetration depth, this removal is believed to have eliminated not only the oxidation layer but also the penetration range of oxygen.

Figure 12 shows the absorbed energy at 25 °C for heated specimens, oxidized specimens, and specimens with the oxide layer removed, at 700, 900, and 1100 °C. Approximately 10 specimens were tested under each condition. As mentioned earlier, all oxidized specimens exhibited a decrease in absorbed energy compared to the heated specimens. However, in specimens with the oxide layer removed, impact resistance was restored in all cases, and absorbed energy levels comparable to those of the heated specimens were achieved. The recovery in the 1100 °C oxidation specimen, which exhibited a significant decrease in absorbed energy, is attributed to the removal of the altered layer at the interface between the base material and oxide layer; the altered layer is presumed to have contributed to the decrease in impact resistance due to cracking.

In the analysis described above, since the oxygen penetration zone and the microstructural change layer were also removed, it is believed that this had a significant impact; However, it is important to emphasize that the decrease in the impact resistance of TiAl4822 due to oxidation is attributable to an extremely thin surface layer. Specifically, as was particularly evident in the 700 °C oxidized specimens, a surface alteration layer measuring 8.0 μm or less reduced the impact resistance of TiAl4822 at 25 °C to approximately 80% of its original value. This behavior significantly affects the practical use of TiAl4822 as a jet engine blade, particularly when considering ground maintenance procedures.

3.4.2. Effect of Test Temperature

Figure 13 shows the relationship between the average absorbed energy ratio (oxidized specimen/heated specimen) and test temperature for specimens held at 700 and 1200 °C for 1 h. Approximately 10 specimens were tested in each case. For oxidation (heating) at 1200 °C, this ratio remained largely unchanged up to 200 °C. This is because the decrease in impact resistance during oxidation at 1200 °C is attributed to the influence of the altered layer formed at the interface between the base material and the oxide layer. The adverse effects of this layer at test temperatures up to 200 °C are considered similar to that at 25 °C. In contrast, for the 700 °C oxidation (heating) test specimen, the change in the ratio of absorbed energy is negligible at 50 °C; however, at higher test temperatures, the ratio of absorbed energy gradually increases, reaching nearly the same value at 200 °C. In other words, the decrease in absorbed energy observed for oxidized specimens at 25 °C test disappeared when the specimens were heated to 200 °C. The most likely reason for this is environmental embrittlement due to moisture.

Environmental embrittlement of TiAl alloys has been widely reported [23,24,25,26]. The mechanism is explained as follows: moisture in the environment is adsorbed onto the specimen surface. At active sites, such as newly formed fracture surfaces resulting from deformation or fracture, H⁺ ions are generated by the decomposition reaction M + H₂O → MO + 2H⁺ + 2e⁺ (M: metal element). H⁺ ions then penetrate the interior of the material and accumulate in high-stress regions, such as crack tips, where they weaken the interatomic bonding forces, leading to failure under low stress.

Moisture adhering to the surface (physically adsorbed water, chemically adsorbed water, and surface OH groups, etc.) is known to evaporate when heated to several hundred degrees Celsius for TiAl and Ti alloys [27,28]. In other words, although the impact resistance of the oxidized specimen subjected to 1-h holding at 700 °C decreased in the 25 °C tests due to moisture-induced environmental embrittlement, the impact resistance did not decrease in the 200 °C tests because this moisture had evaporated. Furthermore, in general, moisture tends to more readily adhere to oxidized surfaces than to metal surfaces [29,30], and moisture is particularly prone to adsorption on TiO₂ surfaces [31]. This is believed to be the source of the difference between the heated specimen and the oxidized specimen. Furthermore, because the degree of reduction in impact resistance was similar for oxidation up to 900 °C regardless of the oxidation temperature or duration, it can be inferred that the environmental embrittlement mechanisms under these oxidation conditions were not significantly different.

3.4.3. Prevention of Moisture Adhesion

We evaluated the impact resistance of samples that prevented the adhesion of moisture at their surfaces. Specifically, test specimens oxidized at 900 °C for 1 h were naturally cooled to 25 °C, placed in an electric furnace maintained at 200 °C, and held for approximately 30 min. The specimens were then removed from the electric furnace and quickly immersed in machine oil (which mainly consists of hydrocarbons) to prevent further moisture absorption. The specimens were then cooled back to 25 °C, and Charpy impact tests were conducted. Figure 14 shows the absorbed energy at 25 °C for the heated, oxidized, and oil-immersed specimens following a 1-h hold at 900 °C. As mentioned earlier, the oxidized specimen exhibited a decrease in absorbed energy compared to the heated specimen. However, the oil-immersed specimen exhibited a significant improvement. Considering that the oxygen penetration layer (which previous studies have identified as a cause of embrittlement) was retained in the oil-immersed specimen, it can be concluded that suppressing moisture adsorption was effective to a certain extent.

These results indicate that in TiAl4822 cast material, surface changes (corresponding to less than a few microns) resulting from oxidation decrease the impact resistance at 25 °C to approximately 80% of its original value. This significantly affects its practical use as a jet engine blade, particularly when considering ground maintenance procedures. Furthermore, in addition to traditionally considered factors—such as oxygen penetration—environmental embrittlement caused by moisture adhesion on the test specimen surface (which is more abundant on the oxidized surface than on the metal surface) may have contributed to embrittlement due to oxidation.

4. Conclusion

In this study, regarding the oxidation embrittlement of TiAl4822 for jet engine blades—a concern that has been growing in recent years—we evaluated impact resistance, which is more practically significant than conventional tensile properties, primarily at 25 °C, over a wider range of oxidation temperatures and times than in previous studies. The results revealed the following:

• Even limited oxidation, such as 300 °C for 1 h or 500 °C for 10 min, reduced the absorbed energy by approximately 80%.
• However, the change in the degree of embrittlement remains largely similar under more severe oxidation conditions (e.g., 1h at 900 °C or 1,000 h at 700 °C).
• Oxidation at 1100 °C or higher significantly decreased the impact resistance. This decrease was attributed to the influence of an altered layer containing cracks that was formed at the interface between the base material and the oxide layer.
• TiAl4822 exhibited a marked decrease in impact resistance at 25 °C due to surface changes (on the order of a few micrometers) resulting from limited oxidation. This is considered a major practical challenge for on-ground jet engine blade maintenance.
• No embrittlement occurred at a test temperature of 200 °C. Furthermore, preventing moisture adhesion on the oxidized surface reduced embrittlement.
• Environmental embrittlement via moisture adhesion to the test specimen surface (which increases on the oxidized surface) may contribute to embrittlement due to oxidation, in addition to the previously proposed factors such as oxygen penetration.