Impact Resistance of Commercially Applied TiAl Alloys and Simple-Composition TiAl Alloys at Various Temperatures

Abstract

For currently used TiAl alloys, the impact resistance is a critically important property that determines their suitability for use, especially in settings when continuous use under harsh conditions is necessary. However, there are almost no examples of the investigation of the impact resistance of these alloys at realistic temperatures. Therefore, in this study, the impact resistance from room temperature to 1000 °C of various cast and forged TiAl alloys proposed to date or still in commercial use, as well as simple composition TiAl alloys and Inconel 713C—a commonly used material—were evaluated using the Charpy impact test, which is the simplest and most realistic way to evaluate industrial impact resistance. It was found that the TiAl alloys underwent brittle fracturing, even at high temperatures, and had significantly lower impact resistances than Inconel 713C. In addition, the impact resistances of all commercial TiAl alloys were inferior to those of the binary alloys, and those of the TiAl4822 and TNM alloy were not significantly different. Crucially, it was found that ternary alloys containing Cr or V had much better impact resistance than the commercial and binary TiAl alloys.

1. Introduction

TiAl4822 (Ti-48Al-2Nb-2Cr (at%); hereinafter, the at% notation is omitted), a commercially used TiAl alloy, is currently used in large quantities for the last-stage turbine blades for jet engines, such as the CFM International LEAP turbofan engine [1,2,3,4]. Previously, the TNM alloy (Ti-43.5Al-4.0Nb-1.0Mo-0.1B) [5,6] was used for the last-stage turbine blades for the Pratt and Whitney PW1100G turbofan engine, but these were suddenly withdrawn from use because of the impact fracture of a considerable number of blades as a result of the high-speed collision of debris scattered from the engine during flight [7]. Therefore, the impact resistance is a crucial property for TiAl alloys and determines whether they can be used for long periods under harsh operating conditions, for example, as turbine blades.

However, to date, there have been few detailed studies of the impact resistance of TiAl alloys. In fact, because TiAl alloys have significantly lower impact resistances than other commonly used materials, such as Ni-based superalloys, there has been little interest in examining the impact resistance differences between TiAl alloys with different compositions. However, as shown by the distinct differences between TiAl4822 and TNM alloy, it is important to assess the impact resistance of each TiAl alloy. Furthermore, considering the environment in which the TiAl alloys are used, that is, as turbine components, high impact resistance is required at temperatures ranging from room temperature to very high temperatures, yet research into the impact resistance of TiAl alloys at different temperatures remains scant. Therefore, in this study, the impact resistances of representative cast and forged TiAl alloys proposed to date or still in commercial use were evaluated. In addition, the impact resistances of simple binary TiAl alloys and ternary TiAl alloys were also evaluated for comparison to examine the effects of the additive elements in the commercial TiAl alloys. Furthermore, to compare the performances of these alloys compared with those commonly used alloys, the impact resistance of the Ni-based superalloy Inconel 713C, which is often used for the last-stage turbine blades for jet engines, was evaluated in the same way.

Foreign object damage (FOD) tests [8] are occasionally used to evaluate the impact resistances of TiAl alloys, but, considering the purpose of this study, a very large number of tests would be required, making this unrealistic in terms of time and cost. Therefore, in this study, the Charpy impact test, which is a simple and practical way to evaluate industrial impact resistance, was used.

2. Materials and Methods

2.1. Cast TiAl Alloys

Four cast commercial TiAl alloys were evaluated: TiAl4822 [1] (Ti-48Al-2Nb-2Cr), 47XD [9,10] (Ti-47Al-2Nb-2Mn-0.8vol% TiB₂), TNB-V2 [11] (Ti-45Al-8Nb-0.2C), and RNT650 [12] (Ti-48.1Al-2.0Nb-0.7Cr-0.3Si). The former two are alloys used for relatively low-temperature applications, such as the last-stage turbine blades for jet engines, and the latter two are used for relatively high-temperature applications such as turbine wheels for passenger-car turbochargers. For comparison, three simple composition TiAl alloys were used: binary Ti-46.5Al and ternary Ti-46.5Al-1.6Cr and Ti-46.5Al-4.0V alloys. In the two ternary alloys, Cr and V, which are additive elements that improve the impact resistance of TiAl alloys [13], were added in optimal amounts from the viewpoint of impact resistance.

For the seven alloys mentioned above, the raw materials were induction-melted in an yttria crucible, and the molten metal was poured into a metal mold to produce the cast materials. Ceramic crucible melting is not normally used for TiAl alloys, but, after examining various crucible materials, it has been found that the increase in oxygen content is relatively small in an yttria crucible, and there is almost no deterioration in the mechanical properties [14,15]. Therefore, to ensure the efficient production of the cast materials, an yttria crucible melting was used.

For melting, the furnace was first purged in vacuo and then filled with Ar gas. Subsequently, about 800 g of raw material per charge was melted. The raw materials used were sponge titanium, Al shot, and various additives: flakes for Nb; granular raw materials for V, Cr, Mn, and Si; and powder for W. C and B were added as powdered TiC and TiB₂, respectively. After the raw materials had melted completely, the molten mixture was maintained in the melted state for approximately 3 min, and then poured into a metal mold divided into two parts, as shown in Figure 1a, to produce a cast material having a flat part with a thickness of 14 mm. Eight cast samples were produced for each alloy. Figure 1b shows examples of the appearances of the cast materials.

When using cast TiAl alloys for actual products, it is common to perform hot isostatic pressing (HIP) to eliminate casting defects. Therefore, in this study, the cast materials were heat-treated at 1200 °C for 4 h, followed by cooling at 10 °C/min, which is equivalent to a general HIP procedure for TiAl alloys [16,17]. Subsequently, the Charpy impact test pieces were processed from the flat part having a thickness of 14 mm.

For the Ni-based superalloy Inconel 713C, a commonly used material that was used for comparison, a cast material of the same shape with the composition of Ni-12.5Cr-2.2Nb-4.2Mo-1.5Fe-6.1Al-0.8Ti-0.12C-0.2Si-0.1Mn-0.012B-0.1Zr (wt%) was prepared by the same method. The molten weight was about 1.5 kg because of the difference in specific gravities, and because Inconel 713C is generally used in the as-cast state, it was evaluated without heat treatment.

2.2. Forged TiAl Alloys

The forged commercial TiAl alloys evaluated in this study were TNM alloy (Ti-43.5Al-4.0Nb-1.0Mo-0.1B) [5,6], which is an isothermally forged alloy, and Ti-42Al-5Mn [18,19,20,21], which is a hot-forged alloy. The latter has a larger amount of the β phase to improve forgeability and is used as a nosecone component in defense applications [21]. As for the simple composition of TiAl alloys that were selected for comparison, binary alloys that do not contain the β phase were not used because they could not be forged; thus, the two ternary alloys, isothermally forged Ti-44.0Al-3.1Cr and hot-forged Ti-42.0Al-3.4Cr, were used. Cr is an additive element that stabilizes the β phase, and in addition, it improves the impact resistance [13]. The composition was set so that the amounts of the β phase at the forging temperature of both alloys would be the same (1150 °C for TNM alloy and 1300 °C for Ti-42Al-5Mn). Specifically, water-cooling tests from 1150 and 1300 °C were carried out for both alloys and the Ti-Al-Cr ternary alloys with various compositions. Then, by measuring the area ratio of the β phase in the water-cooled material, the composition of the ternary alloy was set so that the amount of β phase was the same as that of the two forged commercial alloys at each forging temperature.

The method for melting ingots was the same as that for the above-described cast TiAl alloys, and Mo powder was used as the raw material. The melting weight was about 900 g, and a cylindrical ingot having approximately 60 mm diameter and 80 mm height was produced by pouring the molten metal into a cylindrical two-split metal mold.

For the isothermally forged alloys, the ingots of TNM alloy and Ti-44.0Al-3.1Cr were compressed in the height direction at a strain rate of 10⁻³/s at 1150 °C [22] and formed into discs having a thickness of about 16 mm. For the hot-forged alloys, ingots of Ti-42Al-5Mn and Ti-42.0Al-3.4Cr were removed from a furnace held at 1300 °C, and after about 20 s, they were compressed with a 300 t hydraulic press in the height direction at a strain rate of about 0.4/s and then formed into a disc with a thickness of about 20 mm in one forging operation. Figure 2 shows examples of the appearances of the forged materials. All were well formed without large cracks.

For isothermally forged alloys, three forged samples for each alloy were produced, and a two-step heat treatment consisting of 1230 °C for 1 h, followed by air cooling and then heating at 850 °C for 6 h, followed by furnace cooling, which is the standard heat treatment condition for TNM alloy [23], was carried out. For hot-forged alloys, three forged samples were produced, and heat treatment at 1200 °C for 2 h, followed by furnace cooling, which is the standard heat treatment for Ti-42Al-5Mn [18], was carried out.

2.3. Charpy Impact Tests

For all the cast and forged materials, the front and back surfaces were first machined to remove the oxidized layer and layer with altered microstructure by the heat treatment and to achieve a uniform thickness of 10 mm. After that, a prismatic test piece of about 10 mm × 10 mm × 55 mm was produced by further processing. In the case of the TiAl alloys, if a notch is made in the same way as a normal Charpy test piece, the absorbed energy will decrease significantly, and it is difficult to distinguish the difference between each alloy, so no notch was made. Further, a small hammer with a capacity of 15 J was used for Charpy impact testing of the TiAl alloys because small values of the absorbed energy cannot be measured accurately with a normally sized hammer, such as a capacity of 300 J. On the other hand, Inconel 713C was tested by using a 50 J hammer, but there were samples that did not break with the same specimen size. Therefore, a half-sized test piece without a notch was used, and the cross-sectional area was reduced to 1/2.

The impact resistance of each alloy was compared based on the absorbed energy obtained in the Charpy impact test using the above-mentioned test pieces and hammers. The test temperatures were room temperature, 400, 550, 700, 850, and 1000 °C. High-temperature Charpy impact tests were carried out in the following manner. An electric furnace was installed next to the testing machine, and a test piece held at a predetermined temperature in the furnace for about 1 h was taken out of the furnace and quickly set in the testing machine to perform the Charpy impact test. After taking out the test piece, it took 5 to 10 s to complete the test. As the absorbed energies of the TiAl alloys varied greatly, five samples were tested under the same conditions, and the average value was used to compare the impact resistance of each alloy. This was also applied to the Inconel 713C samples.

3. Results and Discussion

3.1. Microstructure

Figure 3 shows backscattered electron images showing the microstructure of each cast TiAl alloy after heat treatment. Although some of the alloys, such as TNB-V2, showed a very small amount of the β phase, most alloys were mostly composed of the γ phase and lamellar structure.

Figure 4 shows the backscattered electron images showing the microstructure after heat treatment. The microstructures of the two types of isothermally forged materials were similar, formed from lamellar structures, and contain the γ and β phases. The two types of hot-forged materials also had similar structures, consisting of lamellar structures and the γ and β phases, but the amount of β phase was greater than that observed in the isothermally forged materials. The reason for this is that hot forging has a much faster deformation rate than isothermal forging, so the amount of β phase was increased to improve forgeability.

3.2. Impact Resistance of Cast TiAl Alloys

Table 1. Charpy absorbed energies of cast TiAl alloys and Inconel 713C at various temperatures (J/cm²).

Alloys	Data Type	RT	400 °C	550 °C	700 °C	850 °C	1000 °C
	Min.	2.00	1.75	2.36	3.47	4.06	1.31
TiAl4822	Average	2.25	3.14	3.95	4.06	3.83	1.93
	Max.	2.53	4.80	4.94	6.25	5.55	2.26
	Min.	2.17	3.28	3.38	4.03	2.77	1.87
47XD	Average	2.62	3.86	4.53	4.08	3.43	2.15
	Max.	2.82	4.70	5.71	4.13	4.10	2.50
	Min.	1.01	1.97	2.81	2.95	2.70	1.93
TNB-V2	Average	1.61	2.92	3.81	3.67	3.15	2.45
	Max.	2.20	3.45	4.95	4.13	3.64	3.21
	Min.	1.40	2.51	3.64	2.51	2.72	2.02
RNT650	Average	1.71	2.83	3.91	3.34	3.49	2.47
	Max.	2.25	3.12	4.54	3.98	4.36	3.07
	Min.	3.27	4.54	4.54	4.22	6.20	3.36
Ti-46.5Al	Average	3.64	5.55	5.41	5.63	6.85	3.94
	Max.	3.89	6.77	6.39	7.70	7.73	4.53
	Min.	3.04	4.19	6.94	6.19	5.17	3.03
Ti-46.5Al-1.6Cr	Average	4.19	7.01	8.86	8.14	7.27	4.14
	Max.	5.19	10.44	12.35	9.99	9.74	4.46
	Min.	3.82	8.36	6.62	8.04	5.06	5.62
Ti-46.5Al-4.0V	Average	4.68	11.85	11.43	9.53	8.79	7.22
	Max.	5.61	14.03	17.49	10.81	10.99	9.37
	Min.	37.28	29.73	30.21	29.49	11.98	6.23
Inconel713C	Average	50.12	43.27	43.31	34.90	21.09	8.19
	Max.	73.65	64.47	65.48	41.35	37.91	10.51

shows the minimum, average, and maximum absorbed energies of cast TiAl alloys and Inconel 713C at various temperatures. Figure 5 shows a comparison of average absorbed energies of cast TiAl alloys. Figure 6 shows the secondary electron images showing the fractured surface of each TiAl cast alloy tested by Charpy impact tests at 550 and 1000 °C. All samples showed similar brittle fractured surfaces, and dimpling, which is characteristic of ductile fractured surfaces, was not observed at all.

From these results, the trends common to all cast TiAl alloys are as follows. Despite varying with alloy composition, the absorbed energy reaches its maximum in the medium temperature range of 400–850 °C, increasing to about two- to three-times that at room temperature. In contrast, the adsorbed energy drops sharply at 1000 °C. These results are considered to be related to changes in the tensile strength and tensile ductility at each temperature. That is, in general, in the tensile testing of TiAl cast materials, there is almost no decrease in strength compared to that at room temperature, and the elongation increases slightly in the medium temperature range [24,25]. On the other hand, at 950 °C or higher, the strength drops sharply, and the elongation increases significantly [24,25]. Therefore, in each alloy, at the temperature at which the maximum absorbed energy was obtained, it is considered that the elongation increased while the strength was maintained. In contrast, at 1000 °C, although the elongation increased, the strength decreased remarkably, so the absorbed energy decreased rapidly.

However, when compared with Inconel 713C, the absorbed energy of all four cast commercial TiAl alloys was less than 1/20 at room temperature. As the absorbed energy of Inconel 713C decreases as the temperature increases, the difference reduces in the medium temperature range, but the absorbed energy of all four cast commercial TiAl alloys was less than 1/8 that of Inconel 713C at 700 °C or lower, that is, the practical operating temperature for TiAl alloys in jet engines. Considering the above results, as well as the fractured surfaces shown in Figure 6, it can be said that, even at high temperatures, the cast commercial TiAl alloy remains brittle, and its impact resistance is significantly inferior to that of commonly used Inconel 713C.

Next, the impact resistance of each cast TiAl alloy was evaluated based on the binary TiAl alloy. The impact resistance of TiAl4822 is significantly lower than those of the binary alloys, by about two-thirds, regardless of temperature. The 47XD alloy showed similar behavior but had slightly better impact resistance than TiAl4822. The reason for this is assumed to be that these alloys contain the same amount of Nb (2 at%), which lowers the impact resistance of TiAl alloys [13]. Further, the reason why 47XD has slightly better impact resistance is thought to be that the Al concentration is slightly lower. As reported, the impact resistance decreases at Al concentrations of 47.5 at% or greater [13].

Next, the TNB-V2 and RNT650 alloys also have significantly lower impact resistances than the binary TiAl alloys, even lower than those of TiAl4822 and 47XD. The reason for this is thought to be that these alloys contain Si and C, which, like Nb, further reduce the impact resistance [13].

On the other hand, the impact resistances of the Ti-46.5Al-1.6Cr and Ti-46.5Al-4.0V ternary alloys are better than those of the binary alloys and more than twice those of commercial alloys, which is especially noticeable in the latter alloy. In fact, the effect of these elements on improving the impact resistance at room temperature has been confirmed previously [13]. Further, we found that this effect is maintained even at high temperatures.

From the above results, it is questionable whether it is meaningful to add Nb to cast commercial TiAl alloys for low-temperature applications such as last-stage turbine blades for jet engines. More specifically, no special tensile strength or creep strength is currently required for cast TiAl4822 for applications in LEAP engines. In addition, because the alloy is used at low temperatures, it is not necessary to add Nb to improve the oxidation resistance. Furthermore, for turbine blade applications, the entire blade is currently machined from a large cast ingot, so the most important property is machinability, which is directly linked to cost. As it is necessary to soften the material to improve machinability, the cast TiAl4822 material comprising almost a single γ phase is currently used. Thus, in this sense, the addition of Nb, which increases hardness, is also meaningless. Furthermore, because the raw material cost of Nb is significantly higher than those of the other used elements, the omission of Nb could lead to a significant cost reduction.

As alternative alloys for low-temperature applications, the ternary alloys of Ti-Al-Cr or V are sufficient, and Ti-Al-Cr is suitable if the oxidation resistance is of some concern. For example, the Ti-Al-Cr ternary alloy with a slightly adjusted composition has the potential to be superior to current alloys used in last-stage turbine blades for jet engines in terms of impact resistance, machinability, and raw material costs.

3.3. Impact Resistance of Forged TiAl Alloys

Table 2. Charpy absorbed energies of forged TiAl alloys at various temperatures (J/cm²).

Alloys	Data Type	RT	400 °C	550 °C	700 °C	850 °C	1000 °C
	Min.	1.51	3.07	3.07	2.89	4.30	1.96
TNM alloy	Average	1.94	4.25	3.72	3.95	4.85	2.55
	Max.	2.62	4.93	4.37	4.90	5.38	3.13
	Min.	1.78	3.17	4.80	4.49	4.49	2.82
Ti-44.0Al-3.1Cr	Average	3.78	4.08	6.16	5.83	5.80	3.54
	Max.	4.71	4.82	7.79	8.25	7.17	4.38
	Min.	1.48	2.92	2.65	3.69	5.27	2.90
Ti-42.0Al-5.0Mn	Average	2.73	3.24	3.19	4.28	7.97	3.62
	Max.	5.32	3.87	4.30	5.19	11.69	4.81
	Min.	2.10	1.57	1.79	2.63	5.43	3.00
Ti-42.0Al-3.4Cr	Average	2.57	4.57	3.16	4.99	10.02	4.67
	Max.	3.59	9.19	4.00	8.22	14.61	5.84

shows the minimum, average, and maximum absorbed energies of forged TiAl alloys at various temperatures. Figure 7 shows the comparison of average Charpy absorbed energy at each temperature for each forged TiAl alloy, as well as those of the cast Ti-46.5Al and Ti-46.5Al-1.6Cr. Figure 8 shows secondary electron images showing the fractured surface of each TiAl forged alloy tested by Charpy impact tests at 550 and 1000 °C. All samples show completely brittle fractured surfaces, similar to those of the cast TiAl alloys.

The trend in the adsorbed energy at each test temperature is almost the same as those of the cast TiAl alloys. From 400–700 °C, the absorbed energy is two- to three-times higher than that at room temperature, but at 1000 °C, it drops sharply. The reason for this is the temperature-induced changes in tensile strength and ductility, as in the case of the cast TiAl alloys. On the other hand, only two hot-forged alloys showed a significant increase in absorbed energy at 850 °C. The reason for this is thought to be that the effect of the β phase, which is present in large amounts in these alloys, significantly improved the ductility at this temperature while maintaining some strength.

However, compared with those of Inconel 713C (see Table 1), the absorbed energy of two forged commercial TiAl alloys at room temperature is 1/20 or less, and at 700 °C or lower, which is the practical operation temperature for TiAl alloys in jet engines, the adsorbed energy is 1/8 or less. Considering this and the fractured surfaces shown in Figure 8, even at high temperatures, the forged commercial TiAl alloy, as well as the cast commercial TiAl alloys, remain brittle, and their impact resistance values are significantly lower than those of the commonly used Inconel 713C alloy.

Next, the impact resistance of each alloy was compared. The absorbed energy of the TNM alloy is considerably lower than those of the same isothermally forged Ti-44.0Al-3.1Cr and the cast Ti-46.5Al alloys. The reason for this is thought to be that it contains large amounts of Nb and Mo, which lower the impact resistance [13]. On the other hand, the impact resistance is not significantly different from TiAl4822’s shown in Figure 5. In other words, the reason why TiAl4822 could be used and TNM alloy cannot be used in actual jet engines is considered to be the difference in usage environment rather than the difference in impact resistance between the two alloys. As the PW1100G engine, which used the TNM alloy, is a geared turbofan engine, the rotation speed of the turbine is high, and thus, the speed and impact energy when debris collides with the blades are large. Therefore, it can be inferred that the TNM alloy fractured in the actual engine, despite its similar impact resistance to that of TiAl4822.

Comparing the three types of Cr-added ternary alloys—isothermally forged Ti-44.0Al-3.1Cr (area ratio of the β phase: 3.2%), hot-forged Ti-42.0Al-3.4Cr (25.5%), and cast Ti-46.5Al-1.6Cr (0%)—at 700 °C or lower, the absorbed energy decreases in the order of cast alloy, isothermally forged alloy, and hot-forged alloy. In other words, the impact resistance decreases as the amount of β phase increases. Therefore, the β phase is an undesirable phase from the viewpoint of impact resistance at the practical operating temperature of the TiAl alloys.

Nevertheless, the absorbed energies of the two types of hot-forged TiAl alloys below 700 °C are not very different from that of the isothermally forged TNM alloy. The reason for this is that the Cr and Mn contained in hot-forged TiAl alloys are additive elements that improve impact resistance [13], unlike the Nb and Mo contained in TNM, and these offset the adverse effects of the increase in the amount of β phase.

4. Summary

For commercially applied TiAl alloys, impact resistance is a critically important property that determines whether these alloys can be used as products in applications requiring continuous operation under harsh conditions. Therefore, in this study, the impact resistance from room temperature to 1000 °C of various cast and forged commercial TiAl alloys, as well as simple composition TiAl alloys for comparison and the commonly used Inconel 713C alloy, were evaluated using Charpy impact tests. The following results were obtained.

(1)

When compared with Inconel 713C, the absorbed energy of all cast and forged commercial TiAl alloys was less than 1/20 at room temperature and less than 1/8 at temperatures below 700 °C, which is the practical operating temperature for jet engines. That is, the impact resistance of TiAl alloys is significantly inferior to that of commonly used materials, even at high temperatures.

(2)

The impact resistance of all commercial TiAl alloys is inferior to that of binary alloys. The reason for this is thought to be that they contain elements such as Nb that reduce the impact resistance.

(3)

The cast commercial TiAl alloys for high-temperature applications containing Si and C have even lower impact resistances.

(4)

There was no significant difference in the impact resistance between the TiAl4822 and TNM. Therefore, the difference in the success or failure of the two is related to the usage environment.

(5)

Below 700 °C, the impact resistance decreased as the amount of the β phase increased, so the β phase is undesirable from the viewpoint of impact resistance at the practical operating temperatures of TiAl alloys.

(6)

As the ternary alloys with added Cr or V had much higher impact resistances than the commercial and binary TiAl alloys at all temperatures, considering the raw material costs and other factors, they may be more suitable than the current alloys, especially for low-temperature applications.