NICKEL-BASED SUPERALLOY, SINGLE-CRYSTALLINE GUIDE VANE AND TURBINE ENGINE

Description

Technical field

[0001] This presentation concerns nickel-based superalloys for gas turbines, in particular for the fixed blades, also called distributors or rectifiers, or mobile blades of a gas turbine, for example in the field of aeronautics. Prior art

[0002] It is known, for example from documents WO 2020 / 025880, EP 1927669 and FR 3073527, to use nickel-based superalloys for the manufacture of fixed or moving monocrystalline blades of gas turbines for aircraft or helicopter engines.

[0003] The main advantages of these materials are that they combine high creep resistance at high temperatures with resistance to oxidation and corrosion.

[0004] Over time, nickel-based superalloys for single-crystal blades have undergone significant changes in chemical composition, particularly with the aim of improving their high-temperature creep properties while maintaining resistance to the highly aggressive environment in which these superalloys are used.

[0005] Furthermore, metallic coatings suitable for these alloys have been developed to increase their resistance to the aggressive environment in which these alloys are used, including oxidation resistance and corrosion resistance. In addition, a low thermal conductivity ceramic coating, which acts as a thermal barrier, can be added to reduce the temperature at the metal surface.

[0006] Typically, a complete protection system has at least two layers.

[0007] The first layer, also called the undercoat or bonding layer, is deposited directly onto the nickel-based superalloy part to be protected, also called the substrate, such as a blade. The deposition step is followed by a step of diffusing the undercoat into the superalloy. Deposition and diffusion can also be carried out in a single step.

[0008] Materials generally used to make this undercoat include aluminoforming metal alloys of the MCrAlY type (M = Ni (nickel) or Co (cobalt)) or a mixture of Ni and Co, Cr = chromium, AI = aluminum and Y = yttrium, or nickel aluminide type alloys (NixAly), some also containing platinum (NixAlyPtz).

[0009] The second layer, generally referred to as a thermal barrier coating or “TBC”, is a ceramic coating comprising, for example, yttria-containing zirconia, also referred to as “YSZ” or “YPSZ” and having a porous structure. This layer can be deposited by various processes, such as electron beam evaporation (“EB-PVD” or “Electron Beam Physical Vapor Deposition”), thermal spraying (“APS” or “SPS” or “Suspension Plasma Spraying”), or any other process that provides a porous ceramic coating with low thermal conductivity.

[0010] Due to the use of these materials at high temperatures, for example from 650 °C to 1100 °C, inter-diffusion phenomena occur at the microscopic scale between the nickel-based superalloy of the substrate and the metallic alloy of the underlayer. These inter-diffusion phenomena, associated with the oxidation of the underlayer, modify in particular the chemical composition, the microstructure and consequently the mechanical properties of the underlayer from the manufacturing of the coating, then during the use of the blade in the turbine. These inter-diffusion phenomena also modify the chemical composition, the microstructure and consequently the mechanical properties of the superalloy of the substrate under the coating. In superalloys heavily loaded with refractory elements, particularly rhenium, a secondary reaction zone (SRZ) can thus form in the superalloy under the underlayer over a depth of several tens, or even hundreds, of micrometers.The mechanical characteristics of this ZRS are significantly inferior to those of the substrate superalloy. The formation of ZRS is undesirable because it leads to a significant reduction in the mechanical strength of the superalloy.

[0011] These changes in the bonding layer, combined with the stress fields related to the growth of the alumina layer that forms in service on the surface of this bonding layer, also called "TGO" in accordance with the English acronym for "Therally Grown Oxide", and the differences in thermal expansion coefficients between the different layers, generate decohesions in the interfacial zone between the underlayer and the ceramic coating, which can lead to partial or total spalling of the ceramic coating. The metal part (superalloy substrate and metal underlayer) is then exposed and directly exposed to the combustion gases, which increases the risk of damage to the blade and therefore to the gas turbine.

[0012] Furthermore, the complexity of the chemistry of these alloys can lead to destabilization of their optimal microstructure with the appearance of particles of undesirable phases when parts formed from these alloys are maintained at high temperature. This destabilization has negative consequences on the mechanical properties of these alloys. These undesirable phases of complex crystalline structure and brittle nature are called topologically close-packed phases ("TCP") or "TCP" phases according to the English acronym for "Topologically Close-Packed".

[0013] In addition, casting defects are likely to form in parts, such as blades, during their manufacture by directional solidification. These defects are generally parasitic grains of the “Freckle” type, the presence of which can cause premature failure of the part in service. The presence of these defects, linked to the chemical composition of the superalloy, generally leads to the rejection of the part, which results in an increase in the production cost. Statement of the invention

[0014] The present presentation aims to propose nickel-based superalloy compositions for the manufacture of single-crystal components, exhibiting increased performance in terms of service life and mechanical strength and allowing to reduce the production costs of the part (reduction of the scrap rate) compared to existing alloys. These superalloys exhibit a high-temperature creep resistance superior to that of existing alloys while demonstrating good microstructural stability in the volume of the superalloy (low sensitivity to PTC formation), good microstructural stability under the thermal barrier coating sub-layer (low sensitivity to ZRS formation), good resistance to oxidation and corrosion while avoiding the formation of parasitic grains of the "Freckle" type.

[0015] For this purpose, the present disclosure relates to a superalloy as defined in claim 1 in the appendix.

[0016] This superalloy is intended for the manufacture of single-crystal gas turbine components, such as fixed or moving blades.

[0017] Thanks to this composition of the nickel (Ni)-based superalloy, creep resistance is improved compared to existing superalloys, particularly at temperatures up to 1100 °C and thermal barrier adhesion is enhanced compared to that observed on existing superalloys.

[0018] This alloy therefore has improved high-temperature creep resistance. As the alloy has a long service life, it also has improved corrosion and oxidation resistance. This alloy can also have improved thermal fatigue resistance.

[0019] These superalloys have a density less than or equal to 7.95 g / cm 3 (grams per cubic centimeter).

[0020] A single-crystal nickel-based superalloy part is obtained by a directed solidification process under thermal gradient in a lost-wax casting process. The single-crystal nickel-based superalloy comprises an austenitic matrix of face-centered cubic structure, a nickel-based solid solution, called gamma phase ("γ"). This matrix contains precipitates of hardening gamma prime phase ("γ'") of ordered cubic structure L1 2 of type Ni 3 Al. The assembly (matrix and precipitates) is therefore described as a γ / γ' superalloy.

[0021] Furthermore, this composition of the nickel-based superalloy allows the implementation of a heat treatment which redissolves the γ' phase precipitates and the γ / γ' eutectic phases which form during the solidification of the superalloy. It is thus possible to obtain a single-crystal nickel-based superalloy containing γ' precipitates of controlled size, preferably between 300 and 500 nanometers (nm), and containing a small proportion of γ / γ' eutectic phases.

[0022] The heat treatment also makes it possible to control the molar fraction of γ' phase precipitates present in the nickel-based single-crystal superalloy. The molar percentage of γ' phase precipitates may be greater than or equal to 50%, preferably greater than or equal to 60%, even more preferably equal to 70%.

[0023] Furthermore, a high fraction of γ' phase precipitates hinders the movement of dislocations and promotes the hot creep resistance of the alloy. On the other hand, at lower temperatures (<950 °C), diffusion phenomena are less and the majority of damage occurs by shearing of the γ' phase precipitates. Thus, at lower temperatures, the intrinsic strength of the γ' phase precipitates is a determining factor for the static or creep mechanical resistance of the alloys. The chemistry of the alloys of the invention has therefore been adjusted to ensure high creep mechanical resistance from 650 to 1100 °C.

[0024] The major addition elements are cobalt (Co), chromium (Cr), molybdenum (Mo), tungsten (W), aluminum (Al), titanium (Ti) and tantalum (Ta).

[0025] The minor addition elements are hafnium (Hf) and silicon (Si), for which the maximum mass content is as defined in claim 1.

[0026] Examples of unavoidable impurities include sulfur (S), carbon (C), boron (B), yttrium (Y), lanthanum (La), and cerium (Ce). Unavoidable impurities are defined as elements that are not intentionally added to the composition and are introduced with other elements. For example, the superalloy may contain 0.005% carbon by mass.

[0027] The addition of tungsten, chromium, cobalt or molybdenum mainly serves to strengthen the γ austenitic matrix of face-centered cubic (FCC) crystal structure by solid solution hardening.

[0028] The addition of aluminum (Al), titanium (Ti) or tantalum (Ta) promotes the precipitation of the hardening phase γ'-Ni 3 (Al, Ti, Ta).

[0029] The simultaneous addition of silicon and hafnium improves the hot oxidation resistance of nickel-based superalloys by increasing the adhesion of the alumina layer (Al 2 O 3 ) that forms on the surface of the superalloy at high temperature. This alumina layer forms a passivation layer on the surface of the nickel-based superalloy and a barrier to the diffusion of oxygen from the outside to the inside of the nickel-based superalloy. However, hafnium can be added without also adding silicon or conversely silicon can be added without also adding hafnium and still improve the hot oxidation resistance of the superalloy.

[0030] Furthermore, the addition of chromium or aluminum improves the oxidation and high-temperature corrosion resistance of the superalloy. In particular, chromium is essential for increasing the hot corrosion resistance of nickel-based superalloys. However, too high a chromium content tends to reduce the solvus temperature of the γ' phase of the nickel-based superalloy, i.e., the temperature above which the γ' phase is completely dissolved in the γ matrix, which is undesirable. Therefore, the chromium content is between 12.5 and 15.5% by mass in order to maintain a high solvus temperature of the γ' phase of the nickel-based superalloy, for example, greater than or equal to 1200 °C, but also to avoid the formation of topologically compact phases in the γ matrix highly saturated with alloying elements such as molybdenum or tungsten.

[0031] The addition of cobalt, which is an element close to nickel and partially substitutes for nickel, forms a solid solution with nickel in the γ matrix. Cobalt helps strengthen the γ matrix, reduce the sensitivity to PTC precipitation and ZRS formation in the superalloy under the protective coating. However, too high a cobalt content tends to reduce the solvus temperature of the γ' phase of the nickel-based superalloy, which is undesirable.

[0032] Also, the chromium and cobalt content is optimized to obtain adequate solvus temperatures with the intended applications both for the desired mechanical properties and for the heat treatment capacity of the superalloy with a heat treatment window compatible with industrial needs, i.e. a difference between the solvus temperature and the solidus temperature of the superalloy that is sufficiently wide.

[0033] The addition of refractory elements, such as molybdenum, tungsten or tantalum, makes it possible to slow down the mechanisms controlling the creep of nickel-based superalloys and which depend on the diffusion of chemical elements in the superalloy.

[0034] A very low sulfur content in a nickel-based superalloy increases the resistance to oxidation and hot corrosion, as well as the resistance to spalling of the thermal barrier. Thus, a low sulfur content, less than 2 ppm by mass (parts per million by mass), or ideally less than 0.5 ppm by mass, optimizes these properties. Such a mass sulfur content can be achieved by producing a low-sulfur master cast or by a desulfurization process carried out after casting. In particular, it is possible to maintain a low sulfur level by adapting the superalloy production process.

[0035] Nickel-based superalloys are superalloys whose mass percentage is predominantly nickel. It is understood that nickel is therefore the element whose mass percentage in the alloy is the highest.

[0036] The superalloy may comprise (outside the present invention), in mass percentages, 5.25 to 6.25% of aluminum, 0.50 to 2.25% of tantalum, 2.0 to 3.5% of titanium, 0 to 7.0% of cobalt, 12.5 to 15.5% of chromium, 0.50 to 2.5% of molybdenum, 0 to 1.5% of tungsten, 0.05 to 0.15% of hafnium, 0 to 0.15% of silicon, preferably 0.05 to 0.15% of silicon, the remainder being constituted by nickel and unavoidable impurities.

[0037] The superalloy comprises, in mass percentages, 5.25 to 6.25% of aluminum, 0.50 to 2.0% of tantalum, 2.5 to 3.5% of titanium, 0 to 7.0% of cobalt, 12.5 to 15.5% of chromium, 0.50 to 2.5% of molybdenum, 0.05 to 0.15% of hafnium, 0 to 0.15% of silicon, preferably 0.05 to 0.15% of silicon, the remainder being nickel and unavoidable impurities.

[0038] The superalloy may comprise, in mass percentages, 5.5% aluminum, 1.0% tantalum, 3.0% titanium, 14.0% chromium, 2.0% molybdenum, 0.10% hafnium, 0.10% silicon, the remainder being nickel and unavoidable impurities.

[0039] The superalloy may comprise, in mass percentages, 5.75% aluminum, 1.5% tantalum, 3.0% titanium, 4.0% cobalt, 14.0% chromium, 1.5% molybdenum, 0.10% hafnium, 0.10% silicon, the remainder being nickel and unavoidable impurities.

[0040] The superalloy may comprise, in mass percentages, 6.0% aluminum, 1.0% tantalum, 3.0% titanium, 6.0% cobalt, 14.0% chromium, 1.0% molybdenum, 0.10% hafnium, 0.10% silicon, the remainder being nickel and unavoidable impurities.

[0041] The superalloy may comprise, in mass percentages, 5.5% aluminum, 1.5% tantalum, 3.0% titanium, 15.0% chromium, 1.0% molybdenum, 0.10% hafnium, 0.10% silicon, the remainder being nickel and unavoidable impurities.

[0042] The superalloy may comprise, in mass percentages, 5.5% aluminum, 1.0% tantalum, 3.0% titanium, 13.0% chromium, 2.0% molybdenum, 0.10% hafnium, 0.10% silicon, the remainder being nickel and unavoidable impurities.

[0043] The superalloy may comprise, in mass percentages, 5.75% aluminum, 1.5% tantalum, 3.0% titanium, 4.0% cobalt, 13.0% chromium, 1.5% molybdenum, 1.0% tungsten, 0.10% hafnium, 0.10% silicon, the remainder being nickel and unavoidable impurities.

[0044] The superalloy may comprise, in mass percentages, 5.5% aluminum, 1.75% tantalum, 2.5% titanium, 15.0% chromium, 1.0% molybdenum, 0.50% tungsten, 0.10% hafnium, 0.10% silicon, the remainder being nickel and unavoidable impurities.

[0045] The superalloy may comprise, in mass percentages, 5.5% aluminum, 1.5% tantalum, 3.0% titanium, 15.0% chromium, 1.0% molybdenum, 0.50% tungsten, 0.10% hafnium, 0.10% silicon, the remainder being nickel and unavoidable impurities.

[0046] This disclosure also relates to a single-crystal blade for a turbomachine comprising a superalloy as defined previously.

[0047] This blade therefore has improved high temperature creep resistance. This blade therefore has improved oxidation and corrosion resistance.

[0048] In some embodiments, the blade may include a protective coating comprising a metallic undercoat deposited over the superalloy and a ceramic thermal barrier deposited over the metallic undercoat.

[0049] Due to the composition of the nickel-based superalloy, the formation of a secondary reaction zone in the superalloy resulting from inter-diffusion phenomena between the superalloy and the sub-layer is avoided, or limited.

[0050] In some embodiments, the metallic underlayer may be an MCrAlY type alloy or a nickel aluminide type alloy.

[0051] In some embodiments, the ceramic thermal barrier may be a yttria-based zirconia material or any other low thermal conductivity ceramic (zirconia-based) coating.

[0052] In some embodiments, the blade may have a structure oriented along a crystallographic direction. <001> .

[0053] This orientation generally gives the blade the optimal mechanical properties.

[0054] This disclosure also relates to a turbomachine comprising a blade as defined previously. Brief description of the drawings

[0055] Other characteristics and advantages of the subject of the present disclosure will emerge from the following description of embodiments, given as non-limiting examples, with reference to the appended figures. Fig. 1 ] There figure 1 is a schematic longitudinal sectional view of a turbomachine. Detailed description

[0056] Nickel-based superalloys are intended for the manufacture of single-crystal blades using a directed solidification process in a thermal gradient. The use of a single-crystal seed or a grain selector at the beginning of solidification makes it possible to obtain this single-crystal structure. The structure is oriented, for example, along a crystallographic direction. <001> which is the orientation that generally gives the optimal mechanical properties to superalloys.

[0057] As-solidified single-crystal nickel-based superalloys have a dendritic structure and consist of γ' Ni 3 (Al, Ti, Ta) precipitates dispersed in a γ matrix of face-centered cubic structure, a nickel-based solid solution. These γ' phase precipitates are heterogeneously distributed in the volume of the single crystal due to chemical segregations resulting from the solidification process. Furthermore, γ / γ' eutectic phases are present in the inter-dendritic regions and constitute preferential sites for crack initiation. These γ / γ' eutectic phases form at the end of solidification. In addition, the γ / γ' eutectic phases are formed at the expense of fine precipitates (sub-micrometer size) of the hardening γ' phase. These γ' phase precipitates constitute the main source of hardening of nickel-based superalloys.Also, the presence of residual γ / γ' eutectic phases does not allow the hot creep resistance of the nickel-based superalloy to be optimized.

[0058] It has indeed been shown that the mechanical properties of superalloys, in particular creep resistance, were optimal when the precipitation of γ' precipitates was ordered, i.e. the γ' phase precipitates were aligned regularly, with a size ranging from 300 to 500 nm, and when all the γ / γ' eutectic phases were put back into solution.

[0059] As-solidified nickel-based superalloys are therefore heat-treated to obtain the desired distribution of the different phases. The first heat treatment is a microstructure homogenization treatment which aims to dissolve the γ' phase precipitates and eliminate the γ / γ' eutectic phases or significantly reduce their molar fraction. This treatment is carried out at a temperature above the solvus temperature of the γ' phase and below the incipient melting temperature of the superalloy (T solidus ). Quenching is then carried out at the end of this first heat treatment to obtain a fine and homogeneous dispersion of the γ' precipitates. Tempering heat treatments are then carried out in two stages, at temperatures below the solvus temperature of the γ' phase.In a first step, to increase the γ' precipitates and obtain the desired size, then in a second step, to increase the molar fraction of this phase up to approximately 70% at room temperature.

[0060] There figure 1 represents, in section along a vertical plane passing through its main axis A, a double-flow turbojet 10. The double-flow turbojet 10 comprises, from upstream to downstream according to the circulation of the air flow, a fan 12, a low-pressure compressor 14, a high-pressure compressor 16, a combustion chamber 18, a high-pressure turbine 20, and a low-pressure turbine 22.

[0061] The high-pressure turbine 20 comprises a plurality of moving blades 20A rotating with the rotor and rectifiers 20B (fixed blades) mounted on the stator. The stator of the turbine 20 comprises a plurality of stator rings 24 arranged opposite the moving blades 20A of the turbine 20.

[0062] These properties make these superalloys interesting candidates for the manufacture of single-crystal parts intended for the hot parts of turbojets.

[0063] It is therefore possible to manufacture a moving blade 20A or a rectifier 20B for a turbomachine comprising a superalloy as defined previously.

[0064] It is also possible to manufacture a moving blade 20A or a rectifier 20B for a turbomachine comprising a superalloy as defined previously coated with a protective coating comprising a metallic underlayer.

[0065] A turbomachine may in particular be a turbojet such as a double-flow turbojet 10. The turbomachine may also be a single-flow turbojet, a turboprop or a turboshaft engine. Examples

[0066] Eight nickel-based single-crystal superalloys of the present disclosure (Ex 1 to Ex 8) were studied and compared with two commercial single-crystal superalloys (reference alloys). The two commercial single-crystal superalloys are: RR2000 ®< (CEx 1) and Inconel 738 ®< (CEx 2). The chemical composition of each of the single-crystal superalloys is given in Table 1, with composition CEx 1 further comprising 1.0% by mass of vanadium (V) and composition CEx 2 further comprising 0.90% by mass of niobium (Nb) and 0.17% by mass of carbon (C). All these superalloys are nickel-based superalloys, i.e. the remainder of the compositions presented to 100% is nickel and unavoidable impurities. [Table 1] Al Your You Co Cr Mo W Hf If Ex 1 5,5 1,0 3,0 0 14,0 2,0 0 0,10 0,10 Ex 2 5,75 1,5 3,0 4,0 14,0 1,5 0 0,10 0,10 Ex 3 6,0 1,0 3,0 6,0 14,0 1,0 0 0,10 0,10 Ex 4 5,5 1,5 3,0 0 15,0 1,0 0 0,10 0,10 Ex 5 5,5 1,0 3,0 0 13,0 2,0 0 0,10 0,10 Ex 6 5,75 1,5 3,0 4,0 13,0 1,5 1,0 0,10 0,10 Ex 7 5,5 1,75 2,5 0, 15,0 1,0 0,50 0,10 0,10 Ex 8 5,5 1,5 3,0 0 15,0 1,0 0,50 0,10 0,10 CEx 1 5,5 0 4,0 15,0 10,0 3,0 0 0 0 CEx 2 3,4 1,75 3,4 8,5 16,0 1,75 2,6 0 0 Density

[0067] The room temperature density of each superalloy was estimated using a modified version of Hull's formula (FC Hull, Metal Progress, November 1969, pp. 139-140). This empirical equation was proposed by Hull. The empirical equation is based on the law of mixtures and includes correction terms derived from a linear regression analysis of experimental data (chemical compositions and measured densities) for 235 superalloys and stainless steels.

[0068] This Hull formula has been modified, in particular to take into account elements such as rhenium, and this, from 272 nickel-based, cobalt-based and iron-based superalloys. The modified Hull formula is as follows: D = 100 / ∑ % X / D X + ∑ A x x % X where DX are the densities of the elements Cr, Ni, ..., X and D is the density of the superalloy, the densities being expressed in g / cm 3< , where A x is a coefficient expressed in g / cm 3< of the elements Cr, Ni, ..., X and are as follows: A Ni = -0.0011; A Al = 0.0622; A Ta = 0.0121; A Ti = 0.0317; A Co = -0.0001; A Cr = -0.0034; A Mo = 0.0033; AW = 0.0033; A Re = 0.0036; A Hf = 0.0156. where %X are the contents, expressed as mass percentages, of the elements of the superalloy Cr, Ni, ..., X.

[0069] The calculated densities for alloys Ex 1 to Ex 8 are greater than or equal to 7.80 and less than 7.95 g / cm 3< (see Table 2).

[0070] Density is of prime importance for rotating component applications such as turbine blades. Increasing the density of the superalloy blade material requires strengthening the disc that supports it, and therefore adds another weight premium. Commercial alloys with similar densities, such as CEx 1 and CEx 2, do not meet current superalloy development standards for blades. CEx 1 and CEx 2 are developed for conventional foundries. Sensitivity to ZRS formation

[0071] To estimate the sensitivity of nickel-based superalloys containing rhenium to ZRS formation, Walston (US 5,270,123) established the following equation: ZRS % 1 / 2 = 13 , 88 % Re + 4 , 10 % W − 7 , 07 % Cr − 2 , 94 % Mo − 0 , 33 % Co + 12 , 13 where ZRS(%) is the linear percentage of ZRS in the superalloy under the coating and the concentrations of the alloying elements are in atomic percentages.

[0072] This equation (2) was obtained by multiple linear regression analysis from observations made after aging for 400 hours at 1093°C (degree centigrade) of samples of various nickel-based superalloys of the René N6 ®< alloy family under a NiPtAl coating.

[0073] The higher the value of the parameter [ZRS(%)] 1 / 2<, the more sensitive the superalloy is to the formation of ZRS. In particular, negative values ​​are representative of a low sensitivity to this defect.

[0074] Thus, as can be seen in Table 2, for superalloys Ex 1 to Ex 8, the values ​​of the parameter [ZRS(%)] 1 / 2< are all significantly negative and these superalloys therefore have a low sensitivity to the formation of ZRS under a NitPtAl coating, a coating which is often present for turbine blade applications (rotating blade and / or nozzle).

[0075] No-Freckles Parameter (NFP) NFP = % Ta + 1 , 5 % Hf + 0 , 5 % Mo − 0 , 5 % % Ti / % W + 1 , 2 % Re where %Cr, %Ni, ...%X are the contents, expressed as mass percentages, of the elements of the superalloy Cr, Ni, ..., X.

[0076] The NFP parameter quantifies the sensitivity to the formation of parasitic grains of the “Freckles” type during the directional solidification of the part (document US 5,888,451). To avoid the formation of “Freckles” type defects, the NFP parameter must be greater than or equal to 0.7. A low sensitivity to this type of defect is an important parameter because it implies a low rate of rejects linked to this defect during the manufacture of parts.

[0077] As can be seen in Table 2, superalloys Ex 1 to Ex 8 and CEX 1 and CEx 2 have an NFP parameter greater than or equal to 0.7. Superalloys Ex 1 to Ex 5 and CEx 1 have an infinite value, these compositions containing neither rhenium nor tungsten. Prime Gamma Resistance (PGR)

[0078] The intrinsic mechanical strength of the γ' phase increases with the content of elements that replace aluminum in the Ni 3 Al compound, such as titanium, tantalum, and part of the tungsten. The γ' phase compound can therefore be written as Ni 3 (Al, Ti, Ta, W). The RGP parameter allows the level of hardening of the γ' phase to be estimated: RGP = C Ti + C Ta + C W / 2 / C Al where C Ti , C Ta , CW and C Al are the concentrations, expressed in atomic percentage, of the elements Ti, Ta, W and AI respectively in the superalloy.

[0079] A higher RGP parameter is favorable to a better mechanical resistance of the superalloy. It can be seen in Table 2 that the RGP parameter calculated for superalloys Ex 1 to Ex 8 is higher than 0.30 but is lower than those of CEx 1 and CEx 2. This difference is mainly due to the reduction of the titanium content, the excessive addition of which is considered deleterious on the corrosion content. The values ​​achievable for superalloys Ex 1 to Ex 8 take into account a compromise between mechanical resistance and environmental resistance.

[0080] It should be noted that compared to CEx 1, superalloys Ex 1 to Ex 8 include tantalum, or even tungsten, which contribute to the strengthening of the γ' phase and therefore to the compensation, at least partial, of the reduction in the titanium content.

[0081] It should be noted that CEx 2 has a value of the RGP parameter approximately double that of superalloys Ex 1 to Ex 8, this is notably due to the fact that CEx 2 includes less γ' phase in order to ensure its flowability and subsequent processing. Furthermore, this value is also due to the lower aluminum content of CEx 2 compared to superalloys Ex 1 to Ex 8. Cost of superalloys

[0082] The cost per kilogram of superalloys Ex 1 to Ex 8 and CEx 1 and CEx 2 is calculated based on the composition of the superalloy and the costs of each compound (updated April 2020). This cost is given for information purposes only.

[0083] Superalloys Ex 1 to Ex 8 have a cost of approximately $60 / kg which is of the same order of magnitude as the cost of CEx 1 and CEx 2 alloys.

[0084] Table 2 shows different parameters for superalloys Ex 1 to Ex 8 and CEx 1 and CEx 2. [Table 2] Density (g / cm 3 < ) [ZRS(%)] 1 / 2< NFP RGP cost ($ / kg) Ex 1 7,87 -95 - 0,33 58 Ex 2 7,87 -95 - 0,33 61 Ex 3 7,82 -94 - 0,31 62 Ex 4 7,86 -100 - 0,35 61 Ex 5 7,89 -88 - 0,33 55 Ex 6 7,93 -87 0,90 0,35 58 Ex 7 7,89 -100 2,30 0,31 62 Ex 8 7,85 -100 1,30 0,35 62 CEx 1 7,82 -72 - 0,41 53 CEx 2 8,17 -114 1,46 0,70 70 Temperature of solvus of the γ' phase

[0085] The CALPHAD method was used to calculate the equilibrium γ' phase solvus temperature of superalloys Ex 1 to Ex 8 and CEx 1 and CEx 2.

[0086] As can be seen in Table 3, superalloys Ex 1 to Ex 2 have a solvus temperature γ' above 1200 °C. Heat Treatment Interval (TTH)

[0087] The CALPHAD method was used to calculate the heat treatment range of superalloys Ex 1 to Ex 8 and CEx 1 and CEx 2.

[0088] The manufacturability of the alloys of the invention was also estimated from the possibility of industrially resolving the γ' phase precipitates to optimize the mechanical properties of the alloys. The heat treatment interval was estimated from the calculation of the solidus temperature and the solvus temperature of the γ' phase precipitates of the alloys. Superalloys Ex 1 to Ex 8 have wide heat treatment windows, above 60 °C, which is compatible with industrial furnaces. Mole fraction of γ' phase

[0089] The CALPHAD method was used to calculate the mole fraction (in mole percentage) of γ' phase at equilibrium in superalloys of superalloys Ex 1 to Ex 8 and CEx 1 and CEx 2 at 750 °C and 1100 °C. Mole fraction of σ-type PTC

[0090] The CALPHAD method was used to calculate the mole fraction (in mole percentage) of σ phase at equilibrium in superalloys Ex 1 to Ex 8 and CEx 1 and CEx 2 at 750 °C (see Table 3).

[0091] The calculated σ-phase mole fractions are relatively low, reflecting low sensitivity to PTC precipitation.

[0092] It should be noted that the total amount of PTC phase includes the content of the chromium-rich BCC / B2 phase, the potentially deleterious nature of which with respect to mechanical properties can approach that of the topologically compact phases. [Table 3] Transformation temperature (°C) Mole fraction of γ' phase (mol%) Mole fraction of σ-type PTC (in mol %) Solvus TTH 750 °C 1100 °C 750 °C Ex 1 1223 72 66 36 4,6 Ex 2 1223 68 68 38 6,1 Ex 3 1215 78 69 38 5,9 Ex 4 1229 63 66 37 4,1 Ex 5 1228 73 66 37 3,1 Ex 6 1228 66 69 40 5,3 Ex 7 1227 73 65 34 3,4 Ex 8 1229 61 67 37 4,5 CEx 1 1208 92 69 40 4,8 CEx 2 1136 161 48 12 0,2

[0093] Superalloys Ex 1 to Ex 8 have γ' solvus temperatures higher than those of the reference alloys, 7 to 21 °C compared to CEx 1 and almost 80 °C compared to CEx 2. The γ' precipitate fractions of superalloys Ex 1 to Ex 8 are similar to those of CEx 1 and much higher than those of CEx 2 (approximately +37% at 750 °C and +200% at 1100 °C).

[0094] The content of PTC-type embrittling phases in superalloys Ex 1, Ex 4, Ex 5, Ex 7, Ex 8 is lower than that of CEx 1, and similar to that of CEx 1 for Ex 6. It is higher than that of CEx 1 for superalloys Ex 2 and Ex 3, while remaining contained.

[0095] Furthermore, the density of superalloys Ex 1 to Ex 8 is of the same order of magnitude as that of CEx 1. Given that the range of variation in the density of nickel-based superalloys for single-crystal casting can reach more than 9 g / cm 3< , this similarity indicates a significant reduction that can have significant beneficial effects for rotating parts.

[0096] According to these predictions, the superalloys of the invention have a chemical composition and a microstructure which makes it possible to envisage mechanical strength superior to that of the reference alloys CEx 1 and CEx 2 while having a density lower than that of the first.

[0097] The superalloys of the invention have been designed to maintain high corrosion resistance (~900 °C) and oxidation resistance (~1100 °C) at high temperatures. The flow circulating through the turbines of turbojet engines is loaded with products which are generally a result of the fuel combustion reaction, but which also include water, sand, and salts contained in the incoming air ingested by the turbomachine. The fuel also contains impurities and sulfur products (always existing regardless of the cleanliness of the fuel). Thus, on the one hand the alloys oxidize under the operating conditions imposed by the engines (temperature, pressure) by reactions with the various gases contained (O 2 (g), CO x , NO x , H 2 O, etc.) in the engine environment.On the other hand, they can undergo accelerated corrosion phenomena (called hot corrosion) by reaction with alkali sulfates M 2 SO 4 (M = Na, K, Ca) liquid at around 900°C which can be present in the deposits which form on the surface of the parts. For better resistance to these two phenomena, oxidation and corrosion, we seek to form protective oxides of the alumina type (Al 2 O 3 ) for oxidation and chromia (Cr 2 O 3 ) for corrosion. Thus, the corrosion and oxidation properties of the alloys of the invention were estimated from the chromium and aluminum content of the alloys.

[0098] The alloys of the invention have chromium contents higher than that of CEx 1 and lower than that of CEx 2. The aluminium contents of the alloys of the invention are higher than or equal to those of the reference alloys, in particular that of CEx 2. The oxidation and corrosion resistance of these alloys is assumed to be similar to or higher than that of the reference alloys CEx 1 and CEx 2.

[0099] According to the various criteria taken into account, the example alloys of the invention thus present a strong potential for high temperature applications, notably for the manufacture of turbine blades, combining low density, high mechanical strength, low sensitivity to the formation of defects (PTC, ZRS, casting defects), while retaining high resistance to oxidation and corrosion.

[0100] Although the present disclosure has been described with reference to a specific exemplary embodiment, it is obvious that various modifications and changes may be made to these examples without departing from the general scope of the invention as defined by the claims. Furthermore, individual features of the various embodiments recited may be combined in additional embodiments. Therefore, the description and drawings should be considered in an illustrative rather than restrictive sense.

Claims

1. A nickel-based superalloy comprising, in weight percentages: 5.25 to 6.25% aluminium, 0.50 to 2.0% tantalum, 2.5 to 3.5% titanium, 0 to 7.0% cobalt, 12.5 to 15.5% chromium, 0.50 to 2.5% molybdenum, 0 to 2.0% tungsten, 0.05 to 0.15% hafnium, 0 to 0.15% silicon, 0 to 2 ppm sulfur, the remainder consisting of nickel and unavoidable impurities.

2. The superalloy according to claim 1, comprising in weight percentages: 5.5% aluminium, 1.0% tantalum, 3.0% titanium, 14.0% chromium, 2.0% molybdenum, 0.10% hafnium, 0.10% silicon, 0 to 2 ppm sulfur, the remainder consisting of nickel and unavoidable impurities.

3. The superalloy according to claim 1, comprising in weight percentages: 5.75% aluminium, 1.5% tantalum, 3.0% titanium, 4.0% cobalt, 14.0% chromium, 1.5% molybdenum, 0.10% hafnium, 0.10% silicon, 0 to 2 ppm sulfur, the remainder consisting of nickel and unavoidable impurities.

4. The superalloy according to claim 1, comprising in weight percentages: 6.0% aluminium, 1.0% tantalum, 3.0% titanium, 6.0% cobalt, 14.0% chromium, 1.0% molybdenum, 0.10% hafnium, 0.10% silicon, 0 to 2 ppm sulfur, the remainder consisting of nickel and unavoidable impurities.

5. The superalloy according to claim 1, comprising in weight percentages: 5.5% aluminium, 1.5% tantalum, 3.0% titanium, 15.0% chromium, 1.0% molybdenum, 0.10% hafnium, 0.10% silicon, 0 to 2 ppm sulfur, the remainder consisting of nickel and unavoidable impurities.

6. The superalloy according to claim 1, comprising in weight percentages: 5.5% aluminium, 1.0% tantalum, 3.0% titanium, 13.0% chromium, 2.0% molybdenum, 0.10% hafnium, 0.10% silicon, 0 to 2 ppm sulfur, the remainder consisting of nickel and unavoidable impurities.

7. The superalloy according to claim 1, comprising in weight percentages: 5.75% aluminium, 1.5% tantalum, 3.0% titanium, 4.0% cobalt, 13.0% chromium, 1.5% molybdenum, 1.0% tungsten, 0.10% hafnium, 0.10% silicon, 0 to 2 ppm sulfur, the remainder consisting of nickel and unavoidable impurities.

8. The superalloy according to claim 1, comprising in weight percentages: 5.5% aluminium, 1.75% tantalum, 2.5% titanium, 15.0% chromium, 1.0% molybdenum, 0.50% tungsten, 0.10% hafnium, 0.10% silicon, 0 to 2 ppm sulfur, the remainder consisting of nickel and unavoidable impurities.

9. The superalloy according to claim 1, comprising in weight percentages: 5.5% aluminium, 1.5% tantalum, 3.0% titanium, 15.0% chromium, 1.0% molybdenum, 0.50% tungsten, 0.10% hafnium, 0.10% silicon, 0 to 2 ppm sulfur, the remainder consisting of nickel and unavoidable impurities.

10. A single-crystal blade (20A, 20B) for a turbomachine, comprising a superalloy according to any one of claims 1 to 9.

11. The blade (20A, 20B) according to claim 10, comprising a protective coating comprising a metal sublayer deposited on the superalloy and a ceramic thermal barrier deposited on the metal sublayer.

12. The blade (20A, 20B) according to claim 10 or 11, having a structure oriented in a <001> crystallographic direction.

13. A turbomachine comprising a blade (20A, 20B) according to any one of claims 10 to 12.