Titanium alloy

Abstract

The invention relates to a titanium alloy comprising, in weight content: 4.0% to 5.0% of aluminum; 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 remainder consisting of titanium and unavoidable impurities; the equivalent aluminum weight content of the alloy being denoted Aleq, less than or equal to 8.5%, and defined using the following formula: Aleq = [AI] + [Sn] / 3 + [Zr] / 6 + 10*[O], where [Al], [Sn], [Zr] and [O] are the weight contents in the alloy of aluminum, tin, zirconium and oxygen, respectively.

Description

Technical Field

[0001] This invention relates to the field of metal alloys, and more specifically to alloys for use in the aerospace industry. Existing technology

[0002] Reducing pollution emissions is a major challenge for the aviation industry. A frequently proposed method for reducing these emissions is to improve the efficiency of the propulsion systems used. However, the effectiveness of these systems is limited by their operating temperature, which in turn is limited by the materials that constitute the propulsion system.

[0003] In addition to good temperature resistance, the materials that make up the propulsion system must also have sufficient mechanical properties for the propulsion system, especially for aero-turbine engines, particularly in terms of mechanical strength, oxidation resistance and fatigue resistance.

[0004] As is well known, titanium alloys are used to manufacture compressor discs, compressor blades, compressor impellers, or turbine engine nozzles.

[0005] Over time, the chemical composition of titanium alloys used in discs, blades, impellers, or turbine engine nozzles has changed significantly, with the aim of improving their mechanical strength at temperatures and their tolerance to the environments in which these alloys are used. The complexity of these alloy chemical compositions can lead to instability in their optimal microstructure, making the selection of added elements and their amounts far from insignificant.

[0006] The main advantage of these materials is that they combine high mechanical strength and density with a half-nickel-based superalloy, along with reasonable resistance to oxidation and corrosion, all at temperatures below 550°C.

[0007] Therefore, at temperatures below 550°C, titanium alloys are competitive with steel or nickel-based superalloys. However, the increasing operating temperatures of turbine engines necessitate improvements in their temperature resistance, particularly for commercial titanium alloys.

[0008] More precisely, the most commonly used titanium alloys in the aerospace industry are so-called "quasi-alpha" alloys, which contain a large portion of dense hexagonal alpha phase, and the latter typically exhibits good temperature resistance. For example, the Ti-6Al-2Sn-4Zr-2Mo alloy is a representative of this group.

[0009] However, for a variety of reasons, quasi-α titanium alloys are not competitive in applications above 550°C.

[0010] First, these alloys are sensitive to a phenomenon known as “dwell fatigue.” This fatigue can be described as a near-creep fatigue observed at ambient temperature, involving a phase where stress is maintained for several minutes.

[0011] Currently, quasi-alpha alloys have limited service life due to their susceptibility to "dwelling" fatigue. More specifically, although dwelling fatigue has not been observed at high temperatures, it can still occur during engine cooling cycles. In addition to resisting "dwelling" fatigue, the desire to increase the operating temperature of aerospace propulsion systems also necessitates improving the oxidation resistance and mechanical strength of the materials used.

[0012] More specifically, increasing the operating temperature promotes corrosion damage to titanium alloys, particularly oxidation. Furthermore, mechanical properties decrease with increasing temperature, and at target temperatures above 550°C, quasi-alpha alloys are known to lack the resistance required for future applications.

[0013] Therefore, in order to improve the efficiency of aerospace propulsion systems, it is necessary to develop new titanium alloy compositions. Summary of the Invention

[0014] The purpose of this invention is to meet this need, and for this purpose, to propose an alloy with optimized composition to provide residence fatigue resistance, corrosion resistance and mechanical strength compatible with use in aero-turbine engines at operating temperatures up to 650°C.

[0015] For this purpose, the present invention relates to a titanium alloy comprising, by weight:

[0016] -4.0% to 5.0% aluminum;

[0017] -3.50% to 4.50% tin;

[0018] Zirconium content ranging from -1.0% to 4.0%,

[0019] Molybdenum content ranges from -2.0% to 5.25%.

[0020] -1.0% to 2.50% niobium;

[0021] -0.10% to 0.25% silicon;

[0022] -0.10% to 0.18% oxygen;

[0023] The remainder consists of titanium and unavoidable impurities;

[0024] The equivalent aluminum weight content of the alloy is denoted as Aleq, which is less than or equal to 8.5%, and is calculated using the following formula: Aleq=[Al]+[Sn] / 3+[Zr] / 6+10*[O], where [Al], [Sn], [Zr] and [O] are the weight contents of aluminum, tin, zirconium and oxygen in the alloy, respectively.

[0025] This alloy is used to manufacture turbine engine components such as disks, blades, impellers, or exhaust nozzles.

[0026] Throughout the application, unless otherwise stated, the elemental contents given are by weight.

[0027] The inventors have an advantage in obtaining the described alloy composition, and the observed behavior of this alloy composition enables them to address the problem. In fact, among other properties, the alloy of the present invention also possesses:

[0028] - The density it contains is comparable to that of existing titanium alloys;

[0029] - Corrosion and oxidation resistant, compatible with turbine engine environments, temperatures up to 650°C, and at least higher than the temperature of currently commercial alloys;

[0030] - Mechanical strength, especially resistance to residence fatigue, is greater than that of existing alloys;

[0031] - It exhibits sufficient microstructure stability within the expected operating temperature range, especially up to 650°C;

[0032] - It is less sensitive to the formation of harmful phases (e.g., ω-phase) within the expected operating temperature range, especially at temperatures up to at least 650°C.

[0033] The inventors have noted in particular that iron, chromium, and nickel reduce the creep resistance of the alloy. Therefore, for high-temperature applications, it is preferable to avoid the presence of these elements, as is the case in the alloy according to the invention.

[0034] Furthermore, the inventors discovered that an equivalent aluminum weight content of less than or equal to 8.5%, or even less than or equal to 8.0%, allows for the limitation of the α2 phase fraction in the alloy. For alloys with an equivalent aluminum weight content higher than those described, a large fraction of α2 phase may occur, leading to undesirable embrittlement. Moreover, for alloys with a large equivalent aluminum weight content, excessively high α-phase transformation kinetics have been observed, resulting in increased susceptibility to residence fatigue, which the alloy of this invention aims to avoid.

[0035] The inventors successfully established the importance of the metric of aluminum weight content as a crucial standard for the existence of this phenomenon. Furthermore, they proposed optimizing this by precisely adjusting the content of other elements to meet the technical specifications of alloys used in aero-turbine engines, with an operating temperature of at least 550°C.

[0036] In one embodiment, the weight content of equivalent aluminum can be between 6.5% and 8.5%, or even between 6.5% and 8.0%.

[0037] In one embodiment, the alloy of the present invention contains 4.0% to 4.8%, or even 4.0% to 4.7% by weight of aluminum.

[0038] The inventors have noted that this additional restriction on aluminum content allows for the avoidance of precipitation of too large a fraction of the α2 phase, resulting in α2 alloys, which improves the mechanical strength of the alloys, and in particular, enhances their ductility.

[0039] In one embodiment, the alloy of the present invention contains between 4.50% and 5.25% molybdenum by weight.

[0040] The β phase in molybdenum-stabilized alloys contributes to solid solution strengthening. The β phase also improves the alloy's ductility, thereby enhancing its formability.

[0041] In one embodiment, the silicon weight content of the alloy can be between 0.1% and 0.15%.

[0042] In fact, silicon contributes to solid solution strengthening and the formation of silicides, especially silicides with stoichiometry of M3Si and M5Si3, where M represents another element, such as titanium, zirconium, molybdenum, or niobium. These silicides are beneficial to the creep resistance of alloys, but excessive silicon content can lead to excessive precipitation of silicides, thereby impairing the ductility of the alloy and potentially becoming crack initiation points, resulting in premature alloy degradation.

[0043] The proposed silicon range represents the optimal value between these two effects.

[0044] In one embodiment, the zirconium weight content can be between 1.0% and 2.0%.

[0045] Zirconium often improves the oxidation resistance of alloys. However, excessive addition of zirconium can stabilize the α2 phase, and an excessive fraction of α2 phase can reduce the ductility of the alloy. The recommended value is the optimal balance between these two effects.

[0046] According to another aspect, the present invention relates to a turbine engine component comprising the alloy just described.

[0047] In one embodiment, such a component may be a compressor blade, a compressor disc, a compressor impeller, a turbine engine housing, or a turbine engine nozzle.

[0048] According to another aspect, the present invention relates to a turbine engine comprising one or more turbine engine components as just described. Detailed Implementation

[0049] The invention will now be described by way of examples, which are for descriptive purposes and illustrate certain implementations of the invention. The examples given should not be construed as limiting the invention.

[0050] To characterize the properties of certain specific alloys of the present invention, the inventors chose to use the results of numerical simulations. More precisely, 11 alloys and three comparative alloys according to the present invention are the subjects of predictive measurements in order to determine their ability to produce the expected alloy capacity.

[0051] The compositions of the alloys under discussion are given in Table 1 below. Three comparative examples are quasi-α titanium alloys commonly used in the aerospace field.

[0052] Comparative alloy 1 (Comparative 1) corresponds to the so-called Ti6242S alloy.

[0053] Comparative alloy 2 (Comparative 2) corresponds to the so-called Ti6246 alloy.

[0054] Comparison 3 (Comparison 3), corresponding to the so-called IMI-834 alloy, can be found, for example, from TIMET with the commercial reference number Obtained through 834 commercial purchase.

[0055] [Table 1]

[0056]

[0057]

[0058] To understand the examples below and the conclusions that can be drawn, it should be noted that all properties of the alloy must be evaluated, rather than considering one property in isolation.

[0059] Therefore, for example, if we only look at density, we would consider Contrast 3 alloy to be the most promising, but this does not take into account that the alloy cannot be used at high temperatures because its α2 phase content and its fractional slope during β transformation are too high, and its residence fatigue resistance is too low, as shown in the readings in Tables 2 and 5.

[0060] The optimization and selection of a particular alloy is always the result of trade-offs between different properties, and it is important to review all the important parameters presented below to understand the alloys of particular interest in this invention in order to solve the technical problems.

[0061] The following examples are intended to present a comparison between different embodiments of the invention and to show that these embodiments generally have better performance than the comparative alloys, but none of them are suitable for use in the conventional conditions of aerospace turbines operating at temperatures between 550°C and 650°C.

[0062] The inventors first determined the density of the different alloys.

[0063] Density is determined by the law of mixtures, where the density of each element is weighted according to its weight content, and the overall density is reduced by 2.5%. Therefore, the density ρ of the alloy can be written according to the following formula, where Wi ρ is the mass percentage of element i. i It is its density.

[0064] [Mathematics 1]

[0065]

[0066] For comparative examples 1 to 3, the formula gives an error on the order of 1%, which is considered acceptable.

[0067] The densities of the examples and comparative examples are shown in Table 2.

[0068] [Table 2]

[0069] Reference number <![CDATA[ρ(g.cm -3 )]]> Ex1 4.64 Ex2 4.65 Ex3 4.65 Ex4 4.68 Ex5 4.60 Ex6 4.66 Ex7 4.64 Ex8 4.70 Ex9 4.69 Ex10 4.77 Ex11 4.82 Comparison 1 4.53 Comparison 2 4.75 Comparison 3 4.51

[0070] The density of different alloys is comparable to, or even lower than, that of those alloys.

[0071] The second element in comparing the alloys according to the present invention with those of the prior art is their parabolic oxidation constant at 650°C, denoted as k. p .

[0072] This constant quantifies the oxidation kinetics (mass gain) of the alloy. A higher value indicates a faster rate of surface oxide formation, or equivalently, a faster rate of oxygen diffusion within the alloy. Therefore, for the target application, it is desirable to keep this parameter as low as possible.

[0073] Table 3 lists the parabolic oxidation constants for the embodiments and comparative examples. For the embodiments and comparative examples according to the present invention, the constant k... p Obtained through the collection of experimental data and the use of regression models.

[0074] [Table 3]

[0075] Reference number <![CDATA[kp at 650 °C (×10 13 g 2 / cm 4 / s)]]> Ex1 1.68 Ex2 1.68 Ex3 1.53 Ex4 1.68 Ex5 2.26 Ex6 1.34 Ex7 1.14 Ex8 1.60 Ex9 1.32 Ex10 1.49 Ex11 1.49 Comparison 1 3.56 Comparison 2 4.35 Comparison 3 1.52

[0076] Table 3 shows that the alloy of the present invention has better oxidation resistance than comparative examples 1 and 2 at 650°C.

[0077] In addition, it should be noted that these values ​​are at least comparable to the third pair, which is the pair that exhibits the best antioxidant properties.

[0078] The mechanical properties of the alloy according to the invention will be further compared with those of the comparative example at room temperature and high temperature.

[0079] Therefore, the tensile strength value Rm divided by the alloy density is listed in Table 4. Table 4 also includes the elongation at break A% at 20°C.

[0080] [Table 4]

[0081]

[0082] The values ​​reported in Table 4 were obtained through the collection of experimental data and the regression model used.

[0083] For the desired application, the mechanical strength and elongation should be as high as possible.

[0084] Table 4 illustrates the mechanical strength of the alloys according to the present invention at room temperature and at temperatures at least on the same order of magnitude as those of alloys in the prior art.

[0085] Table 4 further illustrates that the alloys according to the present invention can achieve properties that are unattainable by alloys of the prior art. For example, even though their mechanical strength at 650°C is not higher than that of the alloy in Comparative 3, almost all of the alloys have higher elongation at break.

[0086] Finally, certain thermodynamic properties of the alloys of this invention were evaluated through digital simulation, which ensured their good temperature resistance.

[0087] The simulation was performed using thermodynamic equilibrium calculations via the CALPHAD method of the commercial thermodynamic database TCTI3 (Thermo-Cale Software AB, Sweden).

[0088] The different thermodynamic properties evaluated are listed in Table 5 below.

[0089] [Table 5]

[0090]

[0091] In Table 5, the symbol "-" indicates that the obtained value is not significant, and the numerical result can be taken as 0.

[0092] The β transition temperature characterizes the stability domain of the β phase. The lower the β transition temperature, the more stable the β domain.

[0093] The Δα column represents the absolute difference between the fraction of α phase at equilibrium at 700°C and the fraction of α phase at equilibrium at 650°C. This index of the degree to which the alloy structure changes between these two temperatures demonstrates the stability of the alloy at these temperatures close to its intended operating temperature. The aim is to maintain a low change in Δα, and it can be noted that all alloys of this invention have lower Δα values ​​than Comparative Example 2.

[0094] Table 5 also includes a column indicating the content of the α2 phase at equilibrium at 650°C.

[0095] If it is desired that the α2 phase content is zero, then a low content is not prohibited, since this phase has been observed to contribute to enhancement through precipitation.

[0096] Table 5 also includes the silicide content. The presence of silicides in the alloy ensures a certain degree of strengthening through precipitation, which is desirable and has been observed for all alloys according to the invention.

[0097] Finally, Table 5 describes the slope of the α fraction at the β transformation. This value is an indicator of the β phase transformation kinetics during cooling. It has been observed that excessively large values ​​(absolute values) are associated with alloys in which the α precipitates have a morphology that increases the alloy's susceptibility to residence fatigue.

[0098] Furthermore, it is known that alloys 1 and 3 are sensitive to this fatigue mode and have relatively high α-slope values ​​(absolute values) at the β transition.

[0099] Conversely, it is well known that alloy 2 is less susceptible to residence fatigue. Since the slope values ​​of the alloy according to the invention are relatively close to those of alloy 2, and in all cases much lower (in absolute value) than those of alloys 1 or 3, the alloy according to the invention is expected to have good resistance to residence fatigue.

[0100] From the examples just given, we can see in particular that the alloys of Comparative Examples 1 and 2 exhibit very high parabolic oxidation constants compared to the other alloys considered, while Comparative Alloys 3 and 1 do not give satisfactory results and remain sensitive to fatigue.

[0101] On the other hand, the alloy of the present invention enables acceptable behavior for each of the above-mentioned important variables, in particular:

[0102] - Their oxidation behavior is more acceptable than that of the prior art's Comparative 1 and Comparative 2 alloys, as evidenced by the fact that their parabolic oxidation constant kp is much lower than that of the Comparative 1 and Comparative 2 alloys;

[0103] - Their mechanical properties are defined by the constant Rm / p, which are essentially close to the mechanical properties of alloys in the prior art, especially the elongation at break, which is generally higher than that of the comparative alloys.

[0104] - Compared with the prior art alloys 3 (and 1), the resistance to residence fatigue is increased, as demonstrated by the slope value of the α fraction at the β transition.

[0105] Therefore, the alloys of the present invention are better candidates for higher temperature applications compared to alloys of the prior art, because they offer a better trade-off than alloys of the prior art, for which at least one property does not allow them to be used at higher temperatures.

[0106] Throughout the application, unless otherwise stated, it must be understood that all value ranges include the range boundaries.

Claims

1. A titanium alloy comprising, in weight content: - 4.0% to 5.0% of aluminum; - 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 remainder consisting of titanium and unavoidable impurities; the equivalent aluminum weight content of the alloy, noted Aleq, is less than or equal to 8.0% and is calculated using the following formula: Aleq = [AI] + [Sn] / 3 + [Zr] / 6 + 10*[O], where [AI], [Sn], [Zr] and [O] are respectively the weight content of aluminum, tin, zirconium and oxygen in the alloy.

2. The titanium alloy of claim 1, wherein, the equivalent aluminum weight content is between 6.5% and 8.0%.

3. The titanium alloy of claim 1 or 2, wherein, the aluminum weight content is between 4.0% and 4.8%.

4. The titanium alloy of any of claims 1-3, wherein, the molybdenum weight content is between 4.50% and 5.25%.

5. The titanium alloy of any of claims 1-4, wherein, the silicon weight content is between 0.1% and 0.15%.

6. The titanium alloy of any of claims 1-5, wherein, the zirconium weight content is between 1.0% and 2.0%.

7. A turbine engine part comprising the alloy according to any one of claims 1 to 6.

8. The turbine engine part according to claim 7, which part is chosen from a compressor blade, a compressor disc, a compressor wheel, a turbine engine casing or a turbine engine nozzle.

9. A turbine engine comprising the part of claim 7 or 8.

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