Titanium alloy

Abstract

The invention relates to a titanium alloy comprising, as contents by weight: - 4.0% to 5.0 % of aluminium; - 3.50% to 4.50% of tin; - 1.0% to 4.0% of zirconium, - 2.0% to 5.25% of molybdenum; - 1.0% to 2.50% of niobium; - 0.10% to 0.25% of silicon; - 0.10% to 0.18% of oxygen; the balance being constituted of titanium and unavoidable impurities, the alloy further verifying that the criterion Aleq known as "equivalent aluminium content by weight" is less than or equal to 8.5%, the criterion Aleq being defined as Aleq =[Al]+[Sn] / 3+[Zr] / 6+10*[O] where [Al], [Sn], [Zr] and [O] are the weight contents respectively of aluminium, tin, zirconium and oxygen.

Description

[0001] Description

[0002] Title of the invention: Titanium alloy

[0003] Technical Field

[0004] The invention relates to the field of metal alloys and more specifically to alloys used in the aeronautical field.

[0005] Prior art

[0006] Reducing pollutant emissions is a major challenge for the aeronautics industry. One commonly cited approach to reducing these emissions is increasing the efficiency of the propulsion systems used. However, the efficiency of these systems is limited by their operating temperature, which is itself limited by the materials used in the propulsion systems.

[0007] Furthermore, the materials constituting propulsion systems must have, in addition to good temperature resistance, sufficient mechanical characteristics for application in propulsion systems, and in particular aeronautical turbomachines, particularly in terms of mechanical strength, resistance to oxidation and resistance to fatigue.

[0008] The use of titanium alloys is known for the manufacture of compressor discs, compressor blades, compressor impellers or turbomachine nozzles.

[0009] Over time, titanium alloys for turbomachine discs, blades, impellers or nozzles have undergone significant changes in chemical composition, in particular with the aim of improving their mechanical resistance to temperature and their resistance to the environment in which these alloys are used. The complexity of the chemistry of these alloys can lead to a destabilization of their optimal microstructure, so that the choice of addition elements and their contents is not trivial.

[0010] The main advantages of these materials are that they combine high mechanical strength, a density half that of nickel-based superalloys, and reasonable resistance to oxidation and corrosion, all at temperatures below 550°C.

[0011] As such, titanium alloys are competitive compared to steels or nickel-based superalloys at temperatures below 550°C. However, an increase in the operating temperatures of turbomachines requires an increase in their temperature resistance, particularly compared to commercial titanium alloys.

[0012] More specifically, the most widely used titanium alloys in the aeronautical field are so-called "quasi-o" alloys comprising a very large fraction of the hexagonal compact phase a and these generally have good temperature resistance. For example, the Ti-6AI-2Sn-4Zr-2Mo alloy is a representative of this family.

[0013] However, quasi-o titanium alloys are not competitive for applications at temperatures above 550 °C, for several reasons.

[0014] First of all, these alloys are sensitive to so-called "dwell fatigue". This fatigue can be described as a type of fatigue similar to creep observed at room temperature, involving a holding phase of several minutes under stress.

[0015] The sensitivity of quasi-o alloys to dwell fatigue currently limits their service life. Indeed, although dwell fatigue is not observed at high temperatures, it can nevertheless occur during engine cooling cycles. In addition to their resistance to dwell fatigue, the desire to increase the operating temperature of aeronautical propulsion systems must be accompanied by an increase in the oxidation resistance and mechanical strength of the materials used.

[0016] Indeed, increasing the operating temperature promotes the deterioration of titanium alloys through corrosion, and in particular oxidation. Furthermore, the mechanical characteristics decrease with temperature and the known quasi-o alloys do not exhibit, at target temperatures above 550 °C, the resistances required by future applications.

[0017] Therefore, in order to increase the efficiency of aeronautical propulsion systems, it is necessary to develop new titanium alloy compositions.

[0018] Statement of the invention

[0019] The invention aims precisely to meet this need, and for this purpose proposes alloys with compositions optimized to present resistance to dwell fatigue, resistance to corrosion and mechanical resistance compatible with use in an aeronautical turbomachine at operating temperatures of up to 650°C.

[0020] The invention therefore relates to a titanium alloy comprising, in mass contents:

[0021] - 4.0% to 5.0% aluminum;

[0022] - 3.50% to 4.50% tin;

[0023] - 1.0% to 4.0% zirconium,

[0024] - 2.0% to 5.25% molybdenum;

[0025] - 1.0% to 2.50% niobium;

[0026] - 0.10% to 0.25% silicon;

[0027] - 0.10% to 0.18% oxygen; the remainder being made up of titanium and unavoidable impurities, the alloy having an equivalent mass content of aluminium, noted Aleq, less than or equal to 8.5% and calculated by applying the following formula: Aleq = [Al] + [Sn] / 3 + [Zr] / 6 + 10 * [O] where [Al], [Sn], [Zr] and [O] are the mass contents in the alloy respectively of aluminium, tin, zirconium and oxygen.

[0028] This alloy is intended for the manufacture of turbomachine components such as discs, blades, impellers or exhaust nozzles.

[0029] Throughout the application, and unless otherwise indicated, the contents given for an element are mass contents. It is to the credit of the inventors to have arrived at the alloy compositions described for which behaviors are observed making it possible to respond to the problem. Indeed, and among other characteristics, the alloys of the invention have:

[0030] - a contained density, of the order of that of existing titanium alloys;

[0031] - corrosion and oxidation resistance compatible with turbomachine environments, and this for temperatures up to at least 650°C, and at least higher than that of current commercial alloys;

[0032] - mechanical resistance, in particular resistance to dwell fatigue, superior to that of existing alloys;

[0033] - adequate microstructural stability over the intended temperature range of use, in particular up to 650°C;

[0034] - low sensitivity to the formation of deleterious phases such as the œ phase over the intended operating temperature range, in particular for temperatures up to at least 650°C.

[0035] The inventors have notably noted that iron, chromium and nickel reduce the creep resistance of the alloy. Thus, it is preferable for high temperature applications to avoid the presence of these elements as is the case in the alloys according to the invention.

[0036] Furthermore, the inventors have found that the equivalent aluminum mass content less than or equal to 8.5%, or even less than or equal to 8.0%, makes it possible to limit the fraction of the o2 phase in the alloy. The significant fraction of o2 phase that can appear for alloys whose equivalent aluminum mass contents are higher than those described is responsible for undesirable embrittlement of the alloy. Furthermore, for alloys whose equivalent aluminum mass content is higher, it has been observed that the transformation kinetics of the a phase will be too high, leading to increased sensitivity of the alloy to dwell fatigue, which the alloys of the invention are precisely intended to avoid.The inventors have succeeded on the one hand in identifying the importance of the criterion of the equivalent mass content of aluminium as an important criterion for the phenomena present, and on the other hand in proposing an optimisation of this, by precisely adjusting the content of the other elements to satisfy the technical specifications of an alloy usable in an aeronautical turbomachine whose operating temperature would be at least 550°C.

[0037] In one embodiment, the equivalent aluminum mass content may be between 6.5% and 8.5%, or even between 6.5% and 8.0%.

[0038] In one embodiment, the alloy of the invention comprises an aluminum mass content of between 4.0% and 4.8%, or even between 4.0% and 4.7%.

[0039] The inventors found that this additional limitation of the aluminium content makes it possible to avoid the precipitation of too large a fraction of the o2 phase of the alloy, which improves the mechanical strength of the alloy, in particular by increasing its ductility.

[0040] In one embodiment, the alloy of the invention comprises a molybdenum mass content of between 4.50% and 5.25%.

[0041] Molybdenum stabilizes the P phase of the alloy and contributes to solid solution strengthening. The P phase contributes to increasing the ductility of the alloy and therefore its formability.

[0042] In one embodiment, the mass content of silicon in the alloy may be between 0.1% and 0.15%.

[0043] Indeed, silicon contributes to solid solution strengthening and the formation of silicides, notably silicides of stoichiometries MsSi and M5Si3 where M represents another element, for example titanium, zirconium, molybdenum, niobium. These silicides are beneficial for the creep resistance of alloys, but too high a silicon content can, on the contrary, lead to excessive precipitation of silicides which then harm the ductility of the alloy, and can become crack initiation points leading to premature failure of the alloy.

[0044] The proposed silicon ranges are those for which an optimum between these two effects could be obtained. In one embodiment, the mass content of zirconium may be between 1.0% and 2.0%.

[0045] Zirconium tends to improve the alloy's resistance to oxidation. However, excessive additions of zirconium stabilize the O2 phase, a too large fraction of which reduces the ductility of the alloy, and the proposed values ​​are optimums found between the two effects.

[0046] According to another of its aspects, the invention relates to a turbomachine part comprising an alloy as just described.

[0047] In one embodiment, such a part may be a compressor blade, a compressor disc, a compressor impeller, a turbomachine housing, or a turbomachine nozzle.

[0048] According to another of its aspects, the invention relates to a turbomachine comprising one or more turbomachine parts as they have just been described.

[0049] Description of the embodiments

[0050] The invention is now described by means of examples, presented for descriptive purposes to illustrate certain embodiments of the invention. The examples given should not be construed as limiting the invention.

[0051] To characterize the properties of certain particular alloys of the invention, the inventors chose to use results of numerical simulations. More specifically, 11 alloys according to the invention and three comparative alloys were subjected to predictive measurements, in order to determine their capacity to produce the capacities expected for an alloy.

[0052] The composition of the alloys in question is given in Table 1 below. The three comparative examples are quasi-o titanium alloys frequently used in the aeronautical field.

[0053] Comparative alloy 1, compl corresponds to the so-called Ti6242S alloy.

[0054] Comparative alloy 2, comp2 corresponds to the so-called Ti6246 alloy. Comparative alloy 3, comp3 corresponds to the so-called IMI-834 alloy, for example commercially available under the commercial reference TIMETAL® 834 from TIMET.

[0055] [Table 1]

[0056] To understand the examples that follow, and the conclusions that can be drawn, it should be noted that an alloy must be evaluated for all of its properties, and not for one property taken in isolation.

[0057] Thus, for example, if we only look at the density we will have the impression that the comp3 alloy is the most promising, but this would be without taking into account that this alloy cannot be used at high temperature because of its too low resistance to dwell fatigue visible in its O2 phase content and its too high slope of fraction a at the P transus, as will appear when reading tables 2 and 5.

[0058] The optimization and choice of a particular alloy is always the result of a compromise between the different characteristics, and it is very important to look at all the important parameters which will be presented below to understand the particular interest of the alloys of the invention for the resolution of the technical problem.

[0059] The following examples aim to provide comparisons between the different examples according to the invention, and to show that the latter have overall better properties than the comparative alloys, none of which is suitable for use in the conventional conditions of an aeronautical turbomachine having an operating temperature between 550°C and 650°C.

[0060] The inventors first determined the density of the different alloys.

[0061] The density was determined by a law of mixtures weighting the density of each element by its mass content, the whole being reduced by 2.5%. Thus, the density of an alloy p can be written according to the formula below in which Wi is the mass percentage of element i and Pi its density.

[0062] [Math. 1] - 0.975

[0063] This formula gives errors of the order of 1% for the comparative examples compl to comp3, which are considered acceptable. The densities of the examples and comparative examples are given in Table 2.

[0064] [Table 2]

[0065] The different alloys have densities comparable to those of the alloys and often even lower.

[0066] A second element of comparison of the alloys according to the invention with the alloys of the prior art is their parabolic oxidation constant at 650°C noted k p .

[0067] This constant quantifies the oxidation kinetics of an alloy (mass gain). The higher it is, the faster the surface oxide forms, or equivalently, the faster the oxygen diffuses within the alloy. This parameter is therefore desired to be as low as possible for the intended applications. Table 3 lists the parabolic oxidation constants for the examples and comparative examples. For the examples according to the invention and the comparative examples, the constants k p are obtained by a regression model based on the collection and exploitation of experimental data.

[0068] [Table 3]

[0069] Table 3 illustrates that the alloys of the invention exhibit better oxidation resistance at 650°C than the comparative examples compl and comp2. Furthermore, it can be noted that the values ​​are at least comparable to the third comparative example which is the one exhibiting the best oxidation resistance. The alloys according to the invention are further compared to the comparative examples for their mechanical characteristics at room temperature and at temperature.

[0070] For this, the tensile strength values ​​Rm divided by the density of the alloy are reported in Table 4. Table 4 also includes the elongation at break A% at 20 °C.

[0071] [Table 4]

[0072] The values ​​reported in Table 4 are obtained by a regression model based on the collection and exploitation of experimental data.

[0073] For the desired applications, it is preferable that the mechanical strength values, as well as the elongation, are as high as possible.

[0074] Table 4 illustrates that the alloys according to the invention have mechanical strengths at room temperature and at temperature at least of the same order as those of the alloys of the prior art.

[0075] Table 4 further illustrates that the alloys according to the invention allow for compromises in characteristics that are not accessible for the alloys of the prior art. For example, even if none of them has a mechanical strength at 650 °C higher than that of the comp3 alloy, almost all of them had a higher elongation at break. Finally, certain thermodynamic characteristics of the alloys of the invention, which ensure their good temperature resistance, were also evaluated by numerical simulation.

[0076] The simulations are made by thermodynamic equilibrium calculations operated by the CALPHAD method using the commercial thermodynamic database TCTI3 (Thermo-Cale Software AB, Sweden).

[0077] The different thermodynamic characteristics thus evaluated are presented in Table 5 below.

[0078] [Table 5]

[0079] In Table 5, the "-" sign means that the value obtained was not significant, and the numerical result can be assimilated to 0.

[0080] The transus temperature P characterizes the stability domain of phase 0. The lower the transus temperature P, the more stable the P domain.

[0081] The Ao column represents the absolute difference between the equilibrium a-phase fraction at 700 °C and the equilibrium a-phase fraction at 650 °C. This indicator of the extent of the change in the constitution of an alloy between these two temperatures demonstrates the stability of the alloy at temperatures close to the intended service temperatures. The objective is to maintain a low Ao variation, and it can be noted that all the alloys of the invention have an Ao value lower than that of the comparative example Comp2.

[0082] Table 5 also includes a column showing the O2 phase content at equilibrium at 650 °C.

[0083] Although a zero O2 phase content is desirable, a low content is not prohibited, as it has been observed that this phase contributes to precipitation strengthening.

[0084] Table 5 also includes the silicide content. The presence of silicides in the alloy ensures a certain strengthening by precipitation which is desirable, and which is also observed for all the alloys according to the invention.

[0085] Finally, Table 5 describes the slope of fraction a at transus 0. This value is an indicator of the kinetics of transformation of the P phase upon cooling. It has been observed that a value that is too high (in absolute value) is associated with an alloy in which the precipitates a have a morphology that increases the sensitivity of the alloy to dwell fatigue.

[0086] Moreover, the compl and comp3 alloys are known to be sensitive to this fatigue mode and have a relatively high slope value a at the P transus (in absolute value).

[0087] Conversely, it is known that the comp2 alloy is much less subject to dwell fatigue. Since the values ​​of the alloys according to the invention are relatively close to the slope value of the comp2 alloy, and in any case much lower (in absolute value) than the values ​​of the comp or comp3 alloys, it is expected that the alloys according to the invention have good resistance to dwell fatigue.

[0088] From the examples just given, we see in particular that the alloys of the comparative examples compl and comp2 present very high parabolic oxidation constants compared to the rest of the alloys considered, while the comparative alloys comp3 and compl are not satisfactory for their sensitivity to dwell fatigue.

[0089] On the other hand, the alloys of the invention allow acceptable behavior for each of the important quantities described above, and in particular: - their oxidation behavior is more acceptable than that of the compl and comp2 alloys of the prior art, as evidenced by the parabolic oxidation constants kp which are much lower than those of the compl and comp2 alloys;

[0090] - their mechanical behavior is defined by the Rm / p constants which are substantially close to those of alloys of the prior art, with in particular an elongation at break often higher than those of the alloys of the comparative examples;

[0091] - increased resistance to dwell fatigue compared to the prior art comp3 (and compl) alloy, as evidenced by the value of the fraction slope a at the transus p.

[0092] Therefore, the alloys of the invention are better candidates for higher temperature applications than the prior art alloys because they offer better compromises than the prior art alloys, for which at least one characteristic does not allow their use at higher temperatures. Throughout the application and unless otherwise stated, all value ranges must be understood to include the limits included.

Claims

Demands

1. Titanium alloy comprising, by mass content: - 4.0% to 5.0% aluminium; - 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; the remainder being titanium and unavoidable impurities, the alloy having a mass content of equivalent aluminum, noted Aleq, less than or equal to 8.5% and calculated by application of the following formula: Aleq = [Al]+[Sn] / 3+[Zr] / 6+10*[O] where [Al], [Sn], [Zr] and [O] are the mass contents in the alloy respectively of aluminum, tin, zirconium and oxygen.

2. Titanium alloy according to claim 1, wherein the mass content of equivalent aluminum is between 6.5% and 8.0%.

3. Titanium alloy according to claim 1 or 2, wherein the aluminum mass content is between 4.0% and 4.8%.

4. Titanium alloy according to any one of claims 1 to 3, in which the mass content of molybdenum is between 4.50% and 5.25%.

5. Titanium alloy according to any one of claims 1 to 4, wherein the silicon mass content is between 0.1% and 0.15%.

6. Titanium alloy according to any one of claims 1 to 5, in which the mass content of zirconium is between 1.0% and 2.0%.

7. Turbomachine part comprising an alloy according to any one of claims 1 to 6.

8. Turbomachine part according to claim 7, which part being selected from a compressor blade, a compressor disc, a compressor impeller, a turbomachine casing or a turbomachine nozzle.

9. Turbomachine comprising a part according to one of claims 7 or 8.

Images