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Article

Influence of B on the Practical Properties of TiAl Alloys for Jet Engine Blades and a Comparison of TiAl4822 and XD Alloys

by
Toshimitsu Tetsui
1,* and
Kazuhiro Mizuta
2
1
National Institute for Materials Science, Tsukuba 305-0047, Ibaraki, Japan
2
AeroEdge Co., Ltd., Ashikaga 329-4213, Tochigi, Japan
*
Author to whom correspondence should be addressed.
Metals 2025, 15(10), 1132; https://doi.org/10.3390/met15101132 (registering DOI)
Submission received: 9 September 2025 / Revised: 6 October 2025 / Accepted: 9 October 2025 / Published: 11 October 2025
(This article belongs to the Special Issue Light Alloy and Its Application (3rd Edition))
Browse Figures
Versions Notes

Abstract

B is considered a valuable additive for TiAl alloys, because it is believed to improve their properties by refining their microstructures. However, the effects of B on the practical properties of TiAl alloys for jet engine blades and the optimal addition amount for achieving balanced properties remain unclear. Specifically, there have been very few studies to date in which the practical properties of alloys have been evaluated across a wide range of B addition levels. Therefore, we evaluated various reliability, cost, and performance properties of jet engine blade materials using cast Ti-45,47Al-2Nb-2Mn (the same as XD alloys), with varying B addition levels. The results showed that, in some cases, low B addition levels (0.1–0.2 at.%) could enhance the impact resistance and high-cycle fatigue performance. However, even low B addition levels negatively impacted the machinability, castability, and creep strength. Further, adding 0.4 B or more significantly reduced most practical properties. Compared to XD alloys, TiAl4822 exhibited a superior balance, which is attributed to the higher B content (1 at.%) in XD alloys and the greater effectiveness of Cr relative to Mn in improving the alloy’s high-temperature impact resistance.

1. Introduction

The addition of B to TiAl alloys is believed to reduce their microstructural size and enhance their properties [1,2,3]. Representative B-added alloys include 45XD (Ti-45Al-2Nb-2Mn at.%–0.8 vol.% TiB2; at.% omitted hereafter) [4,5,6,7] and 47XD (Ti-47Al-2Nb-2Mn–0.8 vol.% TiB2). These were among the first practical TiAl alloys developed and have high B contents of approximately 1% [8]. More recently developed practical TiAl alloys [9,10,11] also incorporate B, but at lower levels ranging from 0.08% to 0.2%, which are significantly lower than those in XD alloys.
Currently, TiAl alloys are primarily used in last-stage turbine blades in jet engines [12,13,14]. Replacing conventional Ni-based superalloy blades with TiAl alloys, which have approximately half the density, has improved engine efficiency, providing a successful example of recent material development in jet engine technology. In terms of B-added TiAl alloys, efforts to develop practical applications of 45XD have been reported [15,16]. Additionally, the TNM alloy (Ti-43.5Al-4Nb-1Mo-0.1B) [17], which contains a small amount of B, was used in a PW1100G geared turbofan engine (Pratt & Whitney) [18]. Thus, a wide range of B contents have been added to practical TiAl alloys for jet engine blades. However, because the effects of a wide range of B addition levels on the properties critical for jet engine blades have not been adequately researched, the overall influence of B on the practical properties of TiAl alloys and the optimal B content to balance these properties remain unclear.
The practical properties required for jet engine blades can be categorized as reliability, cost, and performance. The history of the TNM alloy illustrates the crucial role of impact resistance in ensuring reliability. The TNM alloy was first put into practical use in 2017, but its adoption was withdrawn because of frequent impact failures caused by collisions with debris flying originating inside the engine [19], and replaced by conventional Ni-based superalloys, highlighting the critical importance of impact resistance for TiAl alloys used in jet engine blades.
Cost considerations highlight the importance of machinability and castability. Currently, TiAl alloy blades are produced through casting. However, TiAl alloys lack the castability of conventional alloys, such as Ni-based superalloys. Therefore, to create these blades, large parts with substantial excess material, such as oversized blade-like components [20] or ingots [21], are often cast and then machined to the final shape. Therefore, to expand the use of TiAl alloy blades, lowering costs through enhanced machinability and improving castability are essential. In terms of performance, creep strength and high-cycle fatigue properties are important. Although the currently used TiAl alloys have the required material properties for jet engines, enhancements will be needed for future engines that are anticipated to operate at higher temperatures. Furthermore, to commercialize TiAl alloys in jet engine blades, balanced properties are crucial.
In this study, materials with a wide range of B addition levels to Ti-45,47Al-2Nb-2Mn (identical to the XD alloy) were investigated to clarify the influence of B on the practical properties required for TiAl alloys used in jet engine blades, including impact resistance, machinability, castability, creep strength, and high-cycle fatigue properties, which have not been elucidated in past studies. Additionally, we sought to determine the optimal B addition level that achieves a good balance among these properties. Moreover, we compared the practical properties of 45XD and 47XD as jet engine blades with those of TiAl4822 (Ti-48Al-2Nb-2Cr), the only alloy currently used for this application, to clarify the advantages and disadvantages of each alloy and the underlying reasons.

2. Materials and Methods

2.1. Materials

The cast materials were prepared by adding B at concentrations of 0%, 0.1%, 0.2%, 0.4%, 0.6%, 0.8%, and 1.0% to Ti-45,47Al-2Nb-2Mn alloys. Regarding the TiAl4822 used for comparison, a slightly reduced Al content compared to the nominal composition is often used for jet engine blade applications [16]. Therefore, in this study, we prepared a cast material using a Ti-47Al-2Nb-2Cr composition. The raw materials included sponge Ti, Al pellets, Nb flakes, and granular Mn, Cr, and TiB2 powders, with a charge weight of approximately 500 g. These materials were induction-melted in a CaO crucible in an environment replaced with Ar at approximately 80 kPa after vacuuming to approximately 50 Pa using an induction melting furnace (Sanvac SVM-1000R, Sanvac Ltd., Tokyo, Japan). To remove O from the cast material, 0.15 wt.% Ca was added in the form of an Al-10 wt.% Ca alloy, reducing the O concentration in the cast material to 0.1 wt.% or less [22]. After the complete melting of the raw materials, the molten metal was held at a constant output for 4 min before casting into a cast iron mold. The resulting material was produced as a flat plate with a test specimen sampling part measuring 60 mm × 90 mm × 16 mm, with a feeder head above. Finally, the cast material underwent hot isostatic pressing (HIP) at 1200 °C for 4 h at 186 MPa to eliminate internal casting defects.

2.2. Evaluation of Material Properties

2.2.1. Impact Resistance

The Charpy impact test was adopted to assess impact resistance because it is the simplest and most industrially practical method and enables easy testing at high temperatures. This method produces results comparable to foreign object impact tests, where a steel ball collides under gas pressure [23]. The Charpy impact test method is described in detail in [24]. Following HIP, the surface and sides of the cast material were machined to create a 10 mm thick, 55 mm wide plate. This plate was then cut using a diamond grinding wheel to create test specimens with dimensions of approximately 10 mm × 10 mm × 55 mm. Given the low impact resistance of TiAl alloys, unnotched specimens were used to clarify the differences between the alloys. Tests were conducted at three temperatures using a small hammer with a capacity of 30 J: 25 °C, 500 °C (considered to be close to the typical operating temperature of TiAl alloys in jet engines), and 700 °C (assumed to be close to the maximum operating temperature). For the high-temperature Charpy impact test, an electric furnace was installed adjacent to the testing machine. Specimens were preheated in the furnace for approximately 1 h and then quickly transferred to the testing machine. The elapsed time from removal to fracture was 5–10 s. Approximately 10 tests were conducted under each condition, and the impact resistance of each alloy was evaluated based on the average absorbed energy.

2.2.2. Machinability

Machinability was evaluated based on the extent of tool wear, because alloys causing less tool wear enable faster machining and cost savings. The details of the machining tests are provided in [24]. The test specimens were prepared by removing the altered layer that results from HIP treatment. Dry face milling was performed using a seven-blade cutter with a diameter of 100 mm, equipped with square K10 carbide inserts. The insert has outer dimensions of 13.4 mm × 13.4 mm and a thickness of 3.97 mm. The rake and clearance angles are both 45°, and the tip is a 2.2 mm straight edge. The cutter was rotated at 130 rpm, providing insert tips with a peripheral speed of 0.680 m/s. A feed rate of 4.97 mm/s and a single cut depth of 0.2 mm were employed. By repeating this cutting process, approximately 25 cm3 of volume was removed from each alloy per test. Each alloy underwent four identical machining tests, and the tool wear was assessed by measuring the total weight change in the seven inserts before and after each test using a high-resolution analytical balance with a minimum measurement unit of 0.00001 g. The average of the results of the four tests was then analyzed.

2.2.3. Castability

The castability of each alloy was evaluated using a novel method that involves passing molten metal through a ceramic mesh (Al2O3-1 wt.% SiO2). The details of this method are provided in [25]. Figure 1 depicts the testing setup. The metal mold in Figure 1a is divided into upper and lower sections, separated by a ceramic mesh. The lower mold has a hollow cavity 60 mm in diameter. This test was conducted using the melting method described in Section 2.1. Although the temperature of the molten metal could not be measured due to equipment limitations, the quantity of the molten metal and the melting conditions (maintaining a power input of 4.5 kW for 4 min after the raw material had completely melted) were kept constant. Therefore, a relative comparison of the flowability of each alloy was considered feasible.
Figure 1b shows the results after the passage test, with the upper and lower sections displaying the molten metal that did not and did pass through the mesh, respectively. Alloys with a higher passage ratio were considered to demonstrate improved castability. This evaluation result correlates with the molten metal filling performance of actual products, such as turbocharger turbine wheels in passenger vehicles [25]. Each alloy underwent five tests, and the evaluation was based on the average molten metal passage ratio.
To confirm the trends in the results of the above tests, casting tests were conducted on selected alloys using a mold shape that mimics a jet engine blade tip. Molten metal was poured into a metal mold with a cavity shown in Figure 2. The filling behavior of the molten metal in the wedge-shaped section with narrow tips on both sides was then evaluated. The mold was not preheated during this process. A zirconia-based slurry, commonly used as the first layer in ceramic shell molds for the investment casting of TiAl alloys, was applied to the mold surface in contact with the molten metal.

2.2.4. Other Properties

Microstructural characterization of each alloy was conducted using scanning electron microscopy (SEM; JEOL JSM-6060, JEOL Ltd., Akishima, Japan) with backscattered electron imaging, with observations centered on the mid-thickness region of the cast material. The lamellar colony size in each set of five backscattered electron micrographs was measured using the linear intercept method, and the average value was used.
The mechanical properties were evaluated using round-bar specimens with a parallel section diameter of 4.0 mm. Creep tests were performed at 750 °C and 200 MPa using a test specimen with a 4 mm diameter, 20 mm long gauge section with a flange for attaching a strain gauge, and a fixing section consisting of an M8 × P1.25 mm screw. The tests were performed using Ultra-High Temperature Creep Rupture Testing Equipment (Toshin Kogyo Ltd., Tokyo, Japan), and high-cycle fatigue tests were performed at 700 °C with σa values of 200 and 225 MPa (R = 0.05) (σmin/σmax values of 21/421 and 24/474 MPa, respectively) using a test specimen with a 4 mm diameter, 8 mm long gauge section, and a fixing section consisting of an M10 × P1.0 mm screw. Testing was carried out using Servo hydraulic Test Systems (Landmark 370.10, MTS Systems Ltd., Eden Prairie, MN, USA). In addition, tensile tests were conducted at 700 °C using a test specimen featuring a 4 mm diameter, 16 mm long gauge section, and a fixing section consisting of an M8 × P1.25 mm screw. These tests were performed using Universal Testing Systems (5982, Instron Ltd., Norwood, CO, USA). Further, the oxidation behaviors of 45XD, 47XD, and TiAl4822 were evaluated in air at 750, 800, and 850 °C for 1000 h, respectively. The weight gain was measured to evaluate the oxidation resistance. Charpy specimens were used for the oxidation tests, with two samples tested per condition, and the average weight gain was used for evaluation.

3. Results and Discussion

3.1. Effects of B Addition on Practical Properties

3.1.1. Microstructure

Backscattered electron images of the Ti-45,47Al-2Nb-2Mn-0, 0.1, 0.2, and 1.0 B alloys are presented in Figure 3 to show the representative microstructures. These B-added alloys are shown because previous studies have demonstrated that, although changes in the microstructure size are proportional to the amount of B added at low levels, it becomes stabilized beyond a certain concentration [1]. Because the 45Al alloys have single fully lamellar structures, the lamellar colony size is very large in the 0 B addition case. The lamellar colony size decreases with 0.1 B addition and further decreases with 0.2 B addition. Thus, the effect of B addition on the microstructure size is clearly evident. However, the lamellar colony size of the 1.0 B-added alloy is not significantly different from that of the 0.2 B-added alloy. The enlarged image in Figure 3d shows long precipitates, which were identified as Ti-based borides, such as TiB2 and Ti3B4, in previous studies [1,26]. Scholars believe that the amount of borides increases with increasing B content. In contrast, in the 47Al alloys, which have a lamellar colony + γ phase composite structure, the microstructure does not coarsen, even at 0 B, and remains significantly smaller than that of 45Al-0B. Furthermore, owing to the originally small microstructure size, no further reduction in the microstructure size is clearly observed with 0.1, 0.2, or 1.0 B addition.
The relationship between the B content and lamellar colony size in the Ti-45Al alloys is shown in Figure 4. A decrease is observed up to the level of 0.2 B addition, after which the sizes stabilize, suggesting that 0.2 B addition is sufficient to achieve the desired microstructure refinement.

3.1.2. Impact Resistance

The relationship between the absorbed energy in the Charpy impact test and the amount of B addition at 25, 500, and 700 °C for the Ti-45Al-2Nb-2Mn alloys is illustrated in Figure 5. At 25 °C, the absorbed energy is generally low; hence, the difference is not significant. However, a slight decrease in absorbed energy is observed with 0.8 and 1.0 B addition. At 500 °C, a slight increase in absorbed energy is observed with 0.1 B addition. However, a significant decrease in absorbed energy is observed with the addition of 0.2 B and above. Moreover, at 700 °C, the absorbed energy decreases with the addition of 0.1 B and above. Figure 6 shows the microstructure near the fractured zone of the Ti-45Al-2Nb-2Mn-1.0B specimen tested at 500 °C. Numerous cracks are observed at the location where borides are thought to have existed. This finding suggests that the borides act as crack initiation sites, adversely diminishing the impact resistance. We expect that the reason that some cases showed improved impact resistance with the addition of 0.1 B is that the effect of reducing the microstructure size outweighed the adverse effects of boride precipitation.
Figure 7 shows similar results for the Ti-47Al-2Nb-2Mn alloys. A slight increase in absorbed energy occurs with 0.1 B addition at 25 and 700 °C, whereas higher addition levels result in a significant energy loss. Moreover, at 500 °C, a decrease in absorbed energy is observed with 0.1 B addition and above. In other words, as with the Ti-45Al alloys, adding B reduces the impact resistance owing to boride precipitation, except in a few cases, in which the amount added is very low.

3.1.3. Machinability

The relationships between the tool wear weight normalized by the cutting volume and B addition for the Ti-45,47Al-2Nb-2Mn alloys are presented in Figure 8. Overall, the tool wear for the Ti-47Al alloys is lower than that for the Ti-45Al alloys, likely due to the increased ratio of the softer γ phase rather than a fully lamellar structure, as shown in Figure 3. For both sets of alloys, the tool wear increases with increasing B content. Although minor B addition can improve the impact resistance in some cases, it consistently degrades the machinability. Therefore, the reduction in microstructure size helps to improve the impact resistance but has minimal effects on the machinability. In contrast, an increase in the amount of hard borides (TiB2 has a hardness of 32 GPa [27]) due to increased B addition contributes to increased tool wear and reduced machinability.

3.1.4. Castability

The relationship between the average weight ratio of the molten metal that flows through the ceramic mesh and the amount of B added to the Ti-45Al-2Nb-2Mn alloys is presented in Figure 9. As the amount of B increases, the molten metal passage ratio decreases. A prior study [25] indicated that the reduction in melt flowability is possibly due to an increase in the solidus–liquidus range, which was calculated using Thermo-Calc software and the TCTI5 database (Titanium and Ti-Al-Based Alloys Databases, version 5). Watanabe et al. [28] reported that increasing the solidus–liquidus range reduces the fluidity of molten Ti alloys. Although the composition differed slightly from that in this study, previous evaluations have indicated solidus–liquidus ranges of 33 and 47 K for Ti-46.5Al and Ti-46.5Al-0.2B, respectively, suggesting that adding B increases this range. Therefore, the observed decrease in castability in this study with increasing B addition likely results from the expansion of the solidus–liquidus range.
Figure 10 shows the casting test results verified using a turbine blade tip simulation mold for Ti-45Al-2Nb-Mn-0, 0.6, and 1.0 B. The white frames correspond to the wedge-shaped sections on the left and right sides of the mold shown in Figure 2. Owing to the poor castability of the TiAl alloy, the molten metal fails to fill the wedge tip, even at 0 B addition. As the amount of B increases, the unfilled area expands, along with surface irregularities and defects in the central area of the cast material. These tests confirm that the castability of the TiAl alloy decreases with increasing B addition.

3.1.5. Creep Strength

The creep curves at 750 °C and 200 MPa for Ti-45Al-2Nb-2Mn alloys with 0, 0.2, and 1.0 B addition are shown in Figure 11. The alloy with 0 B exhibits the longest creep life, whereas that with 1.0 B addition shows the shortest one. Figure 12 depicts the microstructure near the fracture zone of the alloy with 0 B and 1.0 B addition. In the 0 B alloy, creep voids are concentrated at the interfaces between lamellar colonies, suggesting that the relatively low-strength γ phase at these boundaries deformed preferentially. Conversely, for 1.0 B, creep voids are observed near the precipitated borides, suggesting that localized defects originating from borides progressed and contributed to short-term fracture. Overall, these results suggest that B addition negatively impacts the creep strength, primarily due to boride precipitation surpassing any benefit from decreased microstructural size.

3.1.6. High-Cycle Fatigue Properties

The results of high-cycle fatigue tests conducted at 700 °C, σa = 200 and 225 MPa, and R = 0.05 (σmin/σmax values of 21/421 and 24/474 MPa, respectability) for Ti-45Al-2Nb-2Mn alloys with 0, 0.2, and 1.0 B addition, are presented in Figure 13. The 0.2 B-added alloy exhibits the highest number of cycles to failure, whereas the 0 B-added alloy exhibits the lowest number. Notably, the 0 B alloy fractured before reaching the maximum stress (474 MPa) at σa = 225 MPa.
Figure 14 shows the fractured surfaces and cross-sectional microstructures near the fracture zone of specimens tested at σa = 200 MPa for the 0- and 0.2 B-added alloys. The fracture surface of the 0 B-added alloy is large and smooth. In the cross-section, a linear fracture along the lamellae direction is observed within its large lamellar colony. Conversely, the fracture surface of the 0.2 B-added alloy is small and uneven, and the cross-section is zigzagged due to the reduced lamellar colony size.
A previous study revealed that the high-cycle fatigue strength of TiAl alloys is correlated with their tensile strength [29]. Therefore, tensile tests were conducted on the three alloys at 700 °C, and the results are summarized in Table 1. The 0 B-added alloy demonstrates the lowest tensile strength, whereas the 0.2 B-added alloy has the highest strength. These findings imply that the strength-enhancing effect of the reduced microstructural size by Hall–Petch relation outweighs the adverse effects of B addition on boride precipitation on high-cycle fatigue properties. However, because the number of cycles to failure is lower for the 1.0 B alloy compared to the 0.2 B alloy, we can state that, at similar microstructural sizes (see Figure 4), high-cycle fatigue properties decrease with increased B addition.

3.1.7. Summary of B Effects

Based on the above results, the following conclusions can be drawn regarding the effects of B addition. For Ti-45Al-2Nb-2Mn, which has a single fully lamellar structure and an inherently large microstructure, adding up to 0.2 B significantly reduces the size of the lamellar colonies. However, adding any more than this amount does not result in any further reduction in the microstructure size. By contrast, in Ti-47Al-2Nb-2Mn alloys, which have lamellar colony + γ phase composite structures that are inherently small, the influence of B on the microstructure size is not clear. The impact resistance shows slight improvements for 0.1 B in some cases due to the reduced microstructural size, but higher B levels lead to increased boride precipitation, adversely affecting the properties. Although reducing the microstructural size has a minimal impact on machinability, increasing the precipitation of hard borides directly decreases it. In terms of castability, B addition enlarges the solidus–liquidus range, which decreases the castability, an effect that worsens with increasing B addition. The creep strength decreases with B addition. Only high-cycle fatigue properties benefit from the reduced microstructural size and increased tensile strength caused by 0.2 B addition, outweighing the adverse effects of boride precipitation. Thus, low levels of B addition (0.1 or 0.2) can improve the properties in some cases with microstructures prone to coarsening, but excessive amounts degrade most of the practical properties of the TiAl alloys used in jet engine blades.

3.2. Comparison of Practical Properties of TiAl4822 and XD Alloys as Jet Engine Blades

TiAl alloys for jet engine blades were first commercialized using a cast TiAl4822 alloy. They are currently extensively used in turbo fan engines such as GEnx (GE) [13,14] and LEAP (CFM International) [30,31], with no significant operational issues reported. In contrast, forged TNM alloy, the next to be commercialized, were used in PW1100G geared turbofan engines [18] but were discontinued because of frequent impact failures caused by foreign object damage [19]. The differences in operational environments, particularly the higher rotational speeds of geared turbofans, i.e., higher foreign object impact speeds, are significant, but the degradation of the properties of the TNM alloy after oxidation [32,33,34,35] cannot be overlooked as a contributing factor.
The 45XD alloy, which is cast in a similar way to the TiAl4822 alloy, was considered for practical application in Rolls-Royce engines [15,16], but has not yet been implemented in practice. Therefore, in this study, we compared the practical properties of TiAl4822 and XD alloys as jet engine turbine blade materials to investigate the causes of the practical differences between the two alloys. For TiAl4822, a slightly reduced Al concentration is used in practice [16], Ti-47Al-2Nb-2Cr was used in this study. For 45,47XD alloys under consideration, we used Ti-45,47Al-2Nb-2Mn-1B, having equivalent B contents. Furthermore, it has previously been shown [22,35] that the impact resistance and creep strength of the TNM alloy are inferior to those of TiAl4822; therefore, the TNM alloy was not included in this evaluation.
The practical properties analyzed include those listed in Section 3.1, as well as oxidation resistance. Figure 15 illustrates the weight gains of the three alloys, alongside Ti-47Al-2Nb, after exposure to air at 750, 800, and 850 °C for 1000 h, respectively. Regardless of the test temperature, Ti-47Al-2Nb exhibits the lowest weight gain, whereas Ti-47Al-2Nb-2Cr exhibits the highest. This finding indicates that 2Cr and 2Mn addition lowers the oxidation resistance, with 2Cr having a larger impact. Furthermore, the slightly lower weight gain of 47Al in the Ti-45,47Al-2Nb-2Mn-1B alloy is attributed to its slightly higher Al content.
Table 2 summarizes the properties of the three types of alloys presented in Section 3.1, alongside the newly acquired data for Ti-47Al-2Nb-2Mn-1B (castability, creep strength, high-cycle fatigue properties) and Ti-47Al-2Nb-2Cr (all data) obtained using the same method. It also summarizes the results of the oxidation tests mentioned above. The Charpy absorbed energy, tool wear weight, and mesh passing ratio of the molten metal are the average values. Furthermore, Figure 16 is a radar chart that compares the balance of the practical properties for the three alloys. The Charpy absorbed energy and weight gain due to oxidation represent the results at 500 °C and 800 °C, respectively, which are the midpoints of the three evaluated temperatures.
Ti-45Al-2Nb-2Mn-1B has the lowest overall properties, whereas Ti-47Al-2Nb-2Mn-1B exhibits modest improvements due to its lamellar colony + γ phase composite structure compared to the fully lamellar structure of Ti-45Al-2Nb-2Mn-1B. Ti-47Al-2Nb-2Mn-1B shows slightly better oxidation resistance and high-cycle fatigue properties compared to Ti-47Al-2Nb-2Cr. However, its other properties are significantly poorer, likely due to the negative effects of 1 B addition. As shown in Section 3.1, adding large amounts of B adversely affects most of the properties. The effect of the impact resistance from the added elements 2Mn and 2Cr is also recognized. Although both elements enhance the impact resistance of TiAl alloys [36], the absorbed energy at 500 °C for Ti-47Al-2Nb-2Mn (0% B) is approximately 9.5 J/cm2 (Figure 7), significantly lower than the 19 J/cm2 of Ti-47Al-2Nb-2Cr. Therefore, Cr outperforms Mn in improving the impact resistance, especially at high temperatures.
These results indicate that variations in alloying elements affect the properties of TiAl4822 and XD alloys, which were produced using the same casting method and HIP treatment. In particular, incorporating 1 B into XD alloys significantly compromises their practical properties. In addition, adding 2Cr to TiAl4822 proves to be significantly more advantageous than adding 2Mn to XD alloys to improve high-temperature impact resistance. This property is critically important for ensuring the reliability of TiAl alloys in jet engine blades. In other words, TiAl4822 is a more balanced alloy than XD alloys in terms of reliability (impact resistance), cost (machinability, castability), and performance (creep strength, HCF properties), which is one explanation for the lack of application of XD alloys compared to the widespread use of TiAl4822 in commercial jet engine blades.
Based on the above evaluation, TiAl4822 has been identified as an excellent alloy for fabricating current jet engine blades. However, significant improvements in creep strength, high-cycle fatigue properties, and oxidation resistance are necessary for materials intended for future engines, for which higher operating temperatures are anticipated. This suggests that alloys containing different additives or microstructures will be necessary. In such cases, the addition of trace amounts of B could be an effective approach to enhancing specific properties.

4. Conclusions

We aimed to clarify the effects of B on various practical properties required for TiAl alloys in jet engine blades, utilizing Ti-45, 47Al-2Nb-2Mn and determining the appropriate level of B addition for balanced performance. Additionally, the practical properties of 45XD and 47XD were compared with those of TiAl4822, the only alloy currently used for this application. The results are as follows:
  • In Ti-45Al-2Nb-2Mn alloys, which have an inherently large microstructures owing to their single fully lamellar structures, a reduction in the lamellar colony size was observed with up to 0.2 B addition; however, no further effect was observed at higher concentrations.
  • The impact resistance at room temperature, 500 °C, and 700 °C, alongside the high-cycle fatigue properties at 700 °C, revealed occasional benefits due to the reduced microstructure size and increased tensile strength with the addition of 0.1–0.2 B.
  • However, even low levels of B addition negatively impacted the machinability, castability, and creep strength.
  • Adding 0.4 B or more significantly reduced most practical properties.
  • Although B addition can benefit alloys with coarse microstructures, an addition of 0.1 to 0.2 B is generally adequate.
  • Comparing the TiAl4822 and XD alloys, TiAl4822 provide a more balanced performance than XD alloys due to the adverse effect of high B contents (1.0 at.%) in XD alloys and the superior effectiveness of Cr over Mn in enhancing the high-temperature impact resistance.

Author Contributions

Conceptualization, T.T.; Validation, T.T.; Formal analysis, T.T.; Investigation, T.T.; Resources, T.T.; Data curation, T.T.; Writing—original draft, T.T.; Writing—review & editing, K.M.; Visualization, T.T.; Supervision, K.M.; Project administration, T.T.; Funding acquisition, T.T. and K.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Japan Science and Technology Agency (grant number AS0216001).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

Author Kazuhiro Mizuta was employed by the company AeroEdge. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
HIPHot Isostatic Pressing
SEMScanning Electron Microscopy

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Figure 1. Setup and results of the molten metal mesh passage test. (a) Arrangement of the mesh and metal mold during the casting test. (b) Solidified molten metal examples: those that passed through the mesh vs. those that did not, viewed from the side and above.
Figure 1. Setup and results of the molten metal mesh passage test. (a) Arrangement of the mesh and metal mold during the casting test. (b) Solidified molten metal examples: those that passed through the mesh vs. those that did not, viewed from the side and above.
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Figure 2. Cavity shape of the metal mold used in the casting tests to simulate the tip of a jet engine blade.
Figure 2. Cavity shape of the metal mold used in the casting tests to simulate the tip of a jet engine blade.
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Figure 3. Backscattered electron images showing the microstructure of each alloy: Ti-45Al-2Nb-2Mn- (a) 0 B; (b) 0.1 B; (c) 0.2 B; and (d) 1.0 B; and Ti-47Al-2Nb-2Mn- (e) 0 B; (f) 0.1 B; (g) 0.2 B; and (h) 1.0 B.
Figure 3. Backscattered electron images showing the microstructure of each alloy: Ti-45Al-2Nb-2Mn- (a) 0 B; (b) 0.1 B; (c) 0.2 B; and (d) 1.0 B; and Ti-47Al-2Nb-2Mn- (e) 0 B; (f) 0.1 B; (g) 0.2 B; and (h) 1.0 B.
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Figure 4. Relationship between lamellar colony size and B content in Ti-45Al-2Nb-2Mn alloys.
Figure 4. Relationship between lamellar colony size and B content in Ti-45Al-2Nb-2Mn alloys.
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Figure 5. Relationship between Charpy absorbed energy and B content in Ti-45Al-2Nb-2Mn alloys.
Figure 5. Relationship between Charpy absorbed energy and B content in Ti-45Al-2Nb-2Mn alloys.
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Figure 6. Microstructure near the fractured zone of the 500 °C Charpy test specimen of Ti-45Al-2Nb-2Mn-1.0B.
Figure 6. Microstructure near the fractured zone of the 500 °C Charpy test specimen of Ti-45Al-2Nb-2Mn-1.0B.
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Metals 15 01132 g007
Figure 7. Relationship between Charpy absorbed energy and B content in Ti-47Al-2Nb-2Mn alloys.
Figure 7. Relationship between Charpy absorbed energy and B content in Ti-47Al-2Nb-2Mn alloys.
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Figure 8. Relationship between tool wear weight and B content in (a) Ti-45Al-2Nb-2Mn alloys and (b) Ti-47Al-2Nb-2Mn alloys.
Figure 8. Relationship between tool wear weight and B content in (a) Ti-45Al-2Nb-2Mn alloys and (b) Ti-47Al-2Nb-2Mn alloys.
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Figure 9. Relationship between the ratio of molten metal that flows through a ceramic mesh and B content in Ti-45Al-2Nb-2Mn alloys.
Figure 9. Relationship between the ratio of molten metal that flows through a ceramic mesh and B content in Ti-45Al-2Nb-2Mn alloys.
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Figure 10. Cast material appearance from casting tests using a turbine blade tip casting mold: Ti-45Al-2Nb-2Mn- (a) 0 B; (b) 0.6 B; and (c) 1.0 B.
Figure 10. Cast material appearance from casting tests using a turbine blade tip casting mold: Ti-45Al-2Nb-2Mn- (a) 0 B; (b) 0.6 B; and (c) 1.0 B.
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Figure 11. Creep curves at 750 °C and 200 MPa for Ti-45Al-2Nb-2Mn alloys with 0, 0.2, and 1.0 B addition.
Figure 11. Creep curves at 750 °C and 200 MPa for Ti-45Al-2Nb-2Mn alloys with 0, 0.2, and 1.0 B addition.
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Metals 15 01132 g012
Figure 12. Microstructure near the fracture zone of specimens after a creep test at 750 °C and 200 MPa: Ti-45Al-2Nb-2Mn- (a) 0 B; (b) 1.0 B.
Figure 12. Microstructure near the fracture zone of specimens after a creep test at 750 °C and 200 MPa: Ti-45Al-2Nb-2Mn- (a) 0 B; (b) 1.0 B.
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Figure 13. Results of high-cycle fatigue tests conducted at 700 °C, σa = 200 and 225 MPa, and R = 0.05 for Ti-45Al-2Nb-2Mn alloys with 0, 0.2, and 1.0 B addition.
Figure 13. Results of high-cycle fatigue tests conducted at 700 °C, σa = 200 and 225 MPa, and R = 0.05 for Ti-45Al-2Nb-2Mn alloys with 0, 0.2, and 1.0 B addition.
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Figure 14. Fractured surfaces (a,c) and cross-sectional microstructures near the fracture zone (b,d) of high-cycle test specimens at 700 °C and σa = 200 MPa (R = 0.05) for Ti-45Al-2Nb-2Mn alloys with (a,b) 0 B addition and (c,d) 0.2 B addition.
Figure 14. Fractured surfaces (a,c) and cross-sectional microstructures near the fracture zone (b,d) of high-cycle test specimens at 700 °C and σa = 200 MPa (R = 0.05) for Ti-45Al-2Nb-2Mn alloys with (a,b) 0 B addition and (c,d) 0.2 B addition.
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Figure 15. Weight gain after 1000 h of oxidation testing at each temperature.
Figure 15. Weight gain after 1000 h of oxidation testing at each temperature.
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Figure 16. Comparison of the balance of the practical properties of the three alloy types using a radar chart.
Figure 16. Comparison of the balance of the practical properties of the three alloy types using a radar chart.
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Table 1. Results of tensile test at 700 °C for Ti-45Al-2Nb-2Mn alloys with 0, 0.2, and 1.0 B addition.
Table 1. Results of tensile test at 700 °C for Ti-45Al-2Nb-2Mn alloys with 0, 0.2, and 1.0 B addition.
Composition (at.%)Tensile Test Results at 700 °C
TiAlNbMnB0.2% Yield Strength (MPa)Strength (MPa)Elongation (%)
Bal.45.02.02.00.03724630.7
Bal.45.02.02.00.24085531.3
Bal.45.02.02.01.03915441.2
Table 2. Summary of the practical properties of the three alloy types.
Table 2. Summary of the practical properties of the three alloy types.
PropertiesTi-45Al-2Nb-2Mn-1BTi-47Al-2Nb-2Mn-1BTi-47Al-2Nb-2Cr
Impact resistanceMean Charpy absorbed energy (J/cm2)25 °C3.73.34.8
500 °C5.04.319.0
700 °C4.04.214.6
MachinabilityMean tool weight reduction (mg/cm3)2.91.90.3
CastabilityMean molten metal ratio flowing through ceramic mesh0.690.670.75
Creep strengthCreep life at 750 °C and 200 MPa (h)153413751
High cycle fatigue propertiesCycles to failure at 700 °C, σa = 200 MPa, and R = 0.05 (N)6.5 × 105>1.0 × 1074.1 × 106
Oxidation resistanceWeight gain at 1000 h (mg/cm2)750 °C1.130.991.50
800 °C2.262.122.32
850 °C2.552.373.74
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Tetsui, T.; Mizuta, K. Influence of B on the Practical Properties of TiAl Alloys for Jet Engine Blades and a Comparison of TiAl4822 and XD Alloys. Metals 2025, 15, 1132. https://doi.org/10.3390/met15101132

AMA Style

Tetsui T, Mizuta K. Influence of B on the Practical Properties of TiAl Alloys for Jet Engine Blades and a Comparison of TiAl4822 and XD Alloys. Metals. 2025; 15(10):1132. https://doi.org/10.3390/met15101132

Chicago/Turabian Style

Tetsui, Toshimitsu, and Kazuhiro Mizuta. 2025. "Influence of B on the Practical Properties of TiAl Alloys for Jet Engine Blades and a Comparison of TiAl4822 and XD Alloys" Metals 15, no. 10: 1132. https://doi.org/10.3390/met15101132

APA Style

Tetsui, T., & Mizuta, K. (2025). Influence of B on the Practical Properties of TiAl Alloys for Jet Engine Blades and a Comparison of TiAl4822 and XD Alloys. Metals, 15(10), 1132. https://doi.org/10.3390/met15101132

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