Titanium alloy

Abstract

The present invention relates to a titanium alloy containing, by mass fraction, - 4.0% to 5.0% aluminum; - 3.50% to 4.50% tin; - 1.0% to 4.0% zirconium; - 2.0% to 5.25% molybdenum; - 1.0% to 2.50% niobium; - 0.10% to 0.25% silicon; - 0.10% to 0.18% oxygen; and the balance being titanium and unavoidable impurities, and the alloy has an Aleq criterion, referred to as the "aluminum equivalent content by mass fraction", of 8.5% or less, where the Aleq criterion is Aleq = [Al] + [Sn] / 3 + [Zr] / 6 + 10*[O] (where [Al], [Sn], [Zr], and [O] are the mass fractions of aluminum, tin, zirconium, and oxygen in the alloy, respectively). as defined herein.

Description

Technical Field

[0001] The present invention relates to the field of metal alloys, and more particularly to alloys used in the aviation industry.

Background Art

[0002] Reducing pollutant emissions is a major challenge for the aviation industry. An approach often proposed to reduce these emissions is to increase the efficiency of the propulsion systems used. However, the efficiency of these systems is limited by their operating temperature, which in turn is limited by the constituent materials of the propulsion systems.

[0003] Furthermore, the constituent materials of the propulsion systems must also have good heat resistance and mechanical properties, particularly mechanical strength, oxidation resistance and fatigue resistance, sufficient for their application to propulsion systems, especially aircraft turbomachinery.

[0004] The use of titanium alloys is known for the manufacture of compressor disks, compressor blades, compressor impellers or turbomachinery nozzles.

[0005] Titanium alloys for disks, blades, impellers or turbomachinery nozzles have seen significant development in their chemical composition, particularly with the aim of improving their mechanical strength at the temperatures at which these alloys are used and their resistance to the environments in which these alloys are used. The complexity of the chemical composition of these alloys can lead to destabilization of their optimal microstructure, so the choice of additive elements and their content is not obvious.

[0006] The main advantages of these materials are that they combine high mechanical strength, a low density that is half that of nickel-based superalloys, and moderate resistance to oxidation and corrosion, all at temperatures below 550°C.

[0007] In this regard, titanium alloys are competitive compared to steels and nickel-based superalloys at temperatures below 550°C. However, the increasing operating temperatures of turbomachinery impose the need for increased heat resistance, especially for commercially available titanium alloys.

[0008] More specifically, the titanium alloys most used in the aerospace industry are so-called "near-α" alloys, which contain a very large fraction of the compact hexagonal α-phase, which generally has good resistance to temperature. For example, the Ti-6Al-2Sn-4Zr-2Mo alloy is representative of this family.

[0009] However, near-α titanium alloys are not competitive for applications at temperatures above 550°C for several reasons.

[0010] First, these alloys are sensitive to so-called "Dwell fatigue". This fatigue can be explained as a type of fatigue similar to creep observed at room temperature, where a holding stage of several minutes under stress is involved.

[0011] Currently, the service life of near-α alloys is limited by their sensitivity to Dwell fatigue. More specifically, Dwell fatigue is not observed at high temperatures, but can occur during engine cooling cycles nevertheless. In addition to their resistance to Dwell fatigue, the desire to increase the operating temperature of aerospace propulsion systems must be accompanied by an improvement in the oxidation resistance and mechanical strength of the materials used.

[0012] More specifically, the increase in operating temperature promotes the deterioration of titanium alloys by corrosion, especially oxidation. Furthermore, the mechanical properties decrease with temperature, and at target temperatures above 550°C, known near-α alloys do not have the resistance required for future applications.

[0013] Therefore, in order to be able to increase the efficiency of aerospace propulsion systems, it is necessary to develop new compositions of titanium alloys. SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

[0014] The present invention aims to exactly meet this need, and for this purpose, proposes an alloy having a composition optimized to provide dwell fatigue resistance, corrosion resistance, and mechanical strength suitable for use in aircraft turbomachinery at operating temperatures up to 650°C.

Means for Solving the Problems

[0015] For this purpose, the present invention, based on the content by mass, - Aluminum: 4.0% to 5.0%; - Tin: 3.50% to 4.50%; - Zirconium: 1.0% to 4.0%; - Molybdenum: 2.0% to 5.25%; - Niobium: 1.0% to 2.50%; - Silicon: 0.10% to 0.25%; - Oxygen: 0.10% to 0.18%; including the balance being titanium and inevitable impurities, represented by Aleq and calculated by the following formula: Aleq = [Al] + [Sn] / 3 + [Zr] / 6 + 10*[O] (wherein, [Al], [Sn], [Zr], and [O] are the mass contents of aluminum, tin, zirconium, and oxygen in the alloy, respectively.) relates to a titanium alloy in which the aluminum equivalent content by mass calculated by the application of the formula is 8.5% or less.

[0016] This alloy is intended for the manufacture of turbomachinery parts such as disks, blades, impellers, or exhaust nozzles.

[0017] Throughout this application, unless otherwise specifically indicated, the contents given for the elements are the contents by mass.

[0018] The inventors have, most fortunately, reached the described alloy composition and found that its behavior can address the above problems. In fact, among numerous properties, the alloy of the present invention has the following properties: - A content density comparable to that of existing titanium alloys; - Corrosion resistance and oxidation resistance suitable for the environment of turbomachinery, and higher than those of currently commercially available alloys at temperatures up to at least 650 °C; - Higher mechanical strength, especially higher resistance to Dwell fatigue than existing alloys; - Appropriate microstructural stability in the use temperature range, especially in the temperature range up to 650 °C; - Low sensitivity to the formation of harmful phases such as the ω phase in the use temperature range, especially in the temperature range up to at least 650 °C.

[0019] The inventors have found, in particular, that iron, chromium and nickel reduce the creep resistance of the alloy. Therefore, in high-temperature applications, it is preferable to avoid the presence of these elements as in the case of the alloys according to the present invention.

[0020] Furthermore, the inventors have found that an aluminum equivalent content rate based on mass of 8.5% or less, or even 8.0% or less, makes it possible to limit the fraction of the α2 phase in the alloy. A large fraction of the α2 phase that may occur in the case of alloys with an aluminum equivalent content rate based on mass greater than that described causes undesirable embrittlement of the alloy. Furthermore, in the case of alloys with a higher aluminum equivalent content rate based on mass, it has been found that the transformation rate of the α phase is too high, resulting in an increase in the sensitivity of the alloy to Dwell fatigue. An increase in the sensitivity of the alloy to Dwell fatigue is precisely what the alloys of the present invention aim to prevent.

[0021] On the one hand, the inventors have succeeded in identifying the importance of the standard of aluminum equivalent content based on mass as an important standard for the existing phenomena. On the other hand, they have succeeded in proposing this optimization by accurately adjusting the content of other elements to meet the technical specifications of alloys that can be used in aircraft turbomachinery with an operating temperature of at least 550 °C.

[0022] In one embodiment, the aluminum equivalent content based on mass can be 6.5% - 8.5%, and further can be 6.5% - 8.0%.

[0023] In one embodiment, the alloy of the present invention has an aluminum content based on mass of 4.0% - 4.8%, and further 4.0% - 4.7%.

[0024] The inventors have found that this additional limitation on the aluminum content makes it possible to avoid the precipitation of an overly large fraction of the α2 phase in the alloy, and improve the mechanical strength of the alloy, particularly by increasing its ductility.

[0025] In one embodiment, the alloy of the present invention has a molybdenum content based on mass of 4.50% - 5.25%.

[0026] Molybdenum stabilizes the β phase of the alloy and contributes to solid-solution strengthening. The β phase contributes to an increase in the ductility of the alloy, and thus to its formability.

[0027] In one embodiment, the silicon content based on mass of the alloy can be 0.1% - 0.15%.

[0028] In fact, silicon contributes to strengthening by solid solution and to the formation of silicides, especially silicides having stoichiometric M3Si and M5Si3. Here, M represents other elements such as titanium, zirconium, molybdenum or niobium. These silicides are beneficial for the creep resistance of the alloy, but if the silicon content is too high, it will conversely cause excessive precipitation of silicides, which in turn harms the ductility of the alloy and can also be the starting point of cracks leading to premature deterioration of the alloy.

[0029] The proposed range of silicon is the one where the optimum is obtained between these two effects.

[0030] In one embodiment, the zirconium content by mass can be 1.0% to 2.0%.

[0031] Zirconium is intended to improve the oxidation resistance of the alloy. However, excessive addition of zirconium stabilizes the α2 phase and, if its fraction is too high, reduces the ductility of the alloy. Therefore, the proposed value is the optimum value found between the two effects.

[0032] Another aspect of the present invention relates to a turbomachine part comprising an alloy as described above.

[0033] In one embodiment, such a part can be a compressor blade, a compressor disk, a compressor impeller, a turbomachine casing or a turbomachine nozzle.

[0034] Another aspect of the present invention relates to a turbomachine comprising one or more turbomachine parts as described above.

Embodiments for Carrying Out the Invention

[0035] Next, the present invention will be described by way of examples having an illustrative purpose for exemplifying specific embodiments of the present invention. A given example should not be construed as limiting the present invention.

[0036] To characterize the properties of the specific alloys of the present invention, the inventors chose to use the results of numerical simulations. More specifically, eleven alloys according to the present invention and three comparative alloys were subject to predictive measurements in order to determine those capabilities that give rise to the capabilities expected of the alloys.

[0037] The composition of the said alloys is shown in Table 1 below. The three comparative examples are near-α titanium alloys frequently used in the aviation industry.

[0038] Comparative alloy 1, comp1, corresponds to the so-called Ti6242S alloy.

[0039] Comparative alloy 2, comp2, corresponds to the so-called Ti6246 alloy.

[0040] Comparative alloy 3, comp3, corresponds to the so-called IMI-834 alloy, which is commercially available, for example, from TIMET under the trade name TIMETAL® 834.

[0041]

Table 1

[0042] To understand the examples that follow and the conclusions that can be drawn from them, it should be noted that the alloys must be evaluated for all of their properties and that no single property should be evaluated in isolation.

[0043] Thus, for example, looking only at density would give the impression that alloy comp3 is the most promising, but this does not take into account that, as is clear from reading Tables 2 and 5, its α2 phase content results in far too low resistance to Dwell fatigue and its α fraction gradient at the β transus is far too high, making this alloy unusable at high temperatures.

[0044] The optimization and selection of a specific alloy are always the result of a compromise between different properties, and it is very important to consider all of the following important parameters in order to understand the specific advantages of the alloys of the present invention for solving technical problems.

[0045] The examples shown below are intended to provide a comparison between various examples according to the present invention, and these examples all show that they have properties superior to those of the comparative alloys, but none of these alloys are suitable for use under the conventional conditions of an aircraft turbomachine having an operating temperature of 550 °C to 650 °C.

[0046] The inventors first determined the density of various alloys.

[0047] The density was weighted by the content of each element using the law of mixtures and reduced by 2.5% overall. Thus, the density ρ of the alloy can be described by the following formula, where w i is the mass % of element i and ρ i is its density.

[0048]

Equation

[0049] This formula gives an error on the order of 1%, which is judged acceptable, for Comparative Examples comp1 to comp3.

[0050] The densities of the examples and comparative examples are shown in Table 2.

[0051]

Table 2

[0052] Various alloys have densities comparable to those of the above alloys, but in many cases they have lower densities.

[0053] A second element for comparison between the alloys according to the present invention and the prior art alloys is kp These are their parabolic oxidation rate constants at 650°C, expressed as follows.

[0054] This constant quantifies the oxidation kinetics (mass gain) of the alloy. The larger this value, the faster the surface oxide is formed, which means that the diffusion of oxygen into the alloy is faster. Therefore, in the target application, it is desirable to make this parameter as low as possible.

[0055] Table 3 shows the parabolic oxidation rate constants of the examples and comparative examples. For the examples and comparative examples according to the present invention, the constant k p was obtained using a regression model based on the collection and use of experimental data.

[0056] [Table 3]

[0057] Table 3 shows that the alloys of the present invention have better oxidation resistance at 650°C than comparative examples comp1 and comp2.

[0058] Furthermore, it should be noted that their values are at least equivalent to those of the third comparative example, which has the best oxidation resistance.

[0059] The alloys according to the present invention were again compared with the comparative examples with respect to their mechanical properties at room temperature and a predetermined temperature.

[0060] For this purpose, Table 4 shows the value obtained by dividing the tensile strength Rm by the density of the alloy. Table 4 also includes the elongation at break A% at 20°C.

[0061] [Table 4]

[0062] The values reported in Table 4 were obtained by a regression model based on the collection and use of experimental data.

[0063] In the above desirable applications, it is preferable that the values of mechanical strength and elongation are as high as possible.

[0064] Table 4 shows that the alloys according to the invention have a mechanical strength of at least the same order of magnitude as that of the prior art alloys at room temperature and at a predetermined temperature.

[0065] Table 4 also shows that the alloys according to the invention allow a compromise of properties that are not accessible with the prior art alloys. For example, even if none of the alloys according to the invention have a mechanical strength at 650 °C higher than that of alloy comp3, almost all of them have a high elongation at break.

[0066] Finally, the specific thermodynamic properties of the alloys of the invention that make it possible to ensure good heat resistance were also evaluated by numerical simulation.

[0067] This simulation was carried out by thermodynamic equilibrium calculations performed by the CALPHAD method using the commercially available thermodynamic database TCTI3 (Thermo-Calc Software AB, Sweden).

[0068] The various thermodynamic properties thus evaluated are shown in Table 5 below.

[0069]

Table 5

[0070] In Table 5, the symbol "-" means that the obtained value is not significant and the numerical result can be considered as 0.

[0071] The β transus temperature characterizes the stability range of the β phase. The lower the β transus temperature, the more stable the β domain.

[0072] The column of Δη represents the absolute value of the difference between the α-phase fraction at equilibrium at 700 °C and the α-phase fraction at equilibrium at 650 °C. This index indicating the magnitude of the change in the alloy's composition between these two temperatures demonstrates the stability of the alloy at these temperatures close to the intended use temperature. The aim is to maintain a small variation Δη, and it should be noted that all the alloys of the present invention have a Δη value smaller than that of Comparative Example Comp2.

[0073] Table 5 also includes a column showing the α2-phase content at equilibrium at 650 °C.

[0074] Even if it is desirable that the content of the α2-phase is zero, a low content is, however, not prohibited. This is because this phase has been observed to contribute to strengthening by precipitation.

[0075] Table 5 also shows the silicide content. The presence of silicides in the alloy ensures a certain strengthening by precipitation, and such strengthening is desirable and is further observed for all the alloys according to the present invention.

[0076] Finally, Table 5 describes the α-fraction gradient at the β-transus. This value is an indicator of the transformation kinetics of the β-phase during cooling. It has been found that too high a value (in absolute value) is related to alloys in which the α-precipitates have a morphology that increases the alloy's susceptibility to Dwell fatigue.

[0077] Furthermore, alloys comp1 and comp3 are known to be sensitive to this type of fatigue, and the value of the α-gradient at the β-transus is relatively high (in absolute value).

[0078] Conversely, alloy comp2 is known to be resistant to Dwell fatigue. The values for the alloys according to the present invention are relatively close to the gradient value of alloy comp2 and are significantly lower (in absolute value) than those of alloy comp1 or comp3 in any case. Therefore, the alloys according to the present invention are expected to have good resistance to Dwell fatigue.

[0079] From the above-described examples, in particular, it can be seen that the alloys comp1 and comp2 of the comparative examples have very high parabolic oxidation rate constants compared to the other alloys considered, but the comparative alloys comp3 and comp1 do not have satisfactory sensitivity to Dwell fatigue.

[0080] On the other hand, the alloys of the present invention can have acceptable behavior for each of the above-mentioned important variables, in particular, - Their oxidation behavior is acceptable compared to the oxidation behavior of the prior art alloys comp1 and comp2, as is evident from the fact that the parabolic oxidation rate constant k p is much smaller than the oxidation rate constant k p of the alloys comp1 and comp2; - Their mechanical behavior is defined by a constant Rm / ρ that is substantially close to the constant Rm / ρ of the prior art alloys, in particular those prior art alloys where the elongation at break is often higher than the elongation at break of the alloys of the comparative examples; - They have high resistance to Dwell fatigue, as evidenced by the value of the α fraction gradient in the β transus compared to the prior art alloys comp3 (and comp1).

[0081] As a result, the alloys of the present invention are better candidates for high-temperature applications than the prior art alloys, since they provide a better compromise in which at least one property allows their use at high temperatures than the prior art alloys.

[0082] Throughout this application, unless otherwise specifically mentioned, all value ranges should be understood to include the upper and lower limit values.

Claims

1. Based on mass, - 4.0% to 5.0% aluminum; - 3.50% to 4.50% tin; - 1.0% to 4.0% zirconium; - 2.0% to 5.25% molybdenum; - 1.0% to 2.50% niobium; - 0.10% to 0.25% silicon; - 0.10% to 0.18% oxygen; It contains [a certain substance], and the remainder consists of titanium and unavoidable impurities. It is represented as Aleq, and the following formula: Aleq=[Al]+[Sn] / 3+[Zr] / 6+10*[O] (In the formula, [Al], [Sn], [Zr], and [O] are the mass content of aluminum, tin, zirconium, and oxygen in the alloy, respectively.) A titanium alloy having a mass-based aluminum equivalent content of 8.5% or less, calculated by the application of [specific formula / method].

2. The titanium alloy according to claim 1, wherein the aluminum equivalent content by mass is 6.5% to 8.0%.

3. The titanium alloy according to claim 1, wherein the aluminum equivalent content by mass is 4.0% to 4.8%.

4. The titanium alloy according to claim 1, wherein the molybdenum content by mass is 4.50% to 5.25%.

5. The titanium alloy according to claim 1, wherein the silicon content by mass is 0.1% to 0.15%.

6. The titanium alloy according to claim 1, wherein the zirconium content by mass is 1.0% to 2.0%.

7. A turbomachine component comprising the alloy described in any one of claims 1 to 6.

8. The turbomachinery component according to claim 7, wherein the component is selected from a compressor blade, a compressor disc, a compressor impeller, a turbomachinery casing, or a turbomachinery nozzle.

9. A turbomachinery comprising the component described in claim 7.