TURBINE COMPONENT MADE OF RHENIUM AND / OR RUTHENIUM-CONTAINING SUPERALLOY AND ASSOCIATED MANUFACTURING PROCESS

Description

FIELD OF THE INVENTION

[0001] The invention relates to a turbine part, such as a turbine blade or a distributor vane for example, used in aeronautics. STATE OF THE ART

[0002] In a turbojet engine, the exhaust gases generated by the combustion chamber can reach high temperatures, above 1200°C, or even 1600°C. The parts of the turbojet engine, in contact with these exhaust gases, such as the turbine blades for example, must therefore be able to maintain their mechanical properties at these high temperatures.

[0003] For this purpose, it is known to manufacture certain parts of the turbojet engine in "superalloy". Superalloys constitute a family of high-strength metal alloys that can work at temperatures relatively close to their melting points (typically 0.7 to 0.8 times their melting temperatures).

[0004] It is known to introduce rhenium and / or ruthenium into a superalloy to increase its mechanical resistance capacity, in particular creep resistance, at high temperature. In particular, the introduction of rhenium and / or ruthenium makes it possible to increase the operating temperature of these superalloys by approximately 100°C compared to the first polycrystalline superalloys.

[0005] However, increasing the average mass fraction of rhenium and / or ruthenium in the superalloy requires a reduction in the average mass fraction of chromium in the superalloy, in order to maintain a stable allotropic structure of the superalloy, in particular a γ-γ' phase. However, the chromium in the superalloy promotes the formation of Cr 2 O 3 oxide, which has the same crystallographic structure as α-Al 2 O 3 and thus allows the nucleation of a layer of α-Al 2 O 3 . This stable layer of α-Al 2 O 3 helps to protect the superalloy against oxidation. Increasing the average mass fraction of rhenium and / or ruthenium therefore results in a lower resistance to oxidation of the superalloy compared to a superalloy without rhenium and / or ruthenium.

[0006] In order to strengthen the thermal resistance of these superalloys and protect them against oxidation and corrosion, it is also known to cover them with a thermal barrier.

[0007] THE figures 1 to 3 schematically illustrate a section of a prior art turbine part 1, for example a turbine blade 7 or a distributor vane. The part 1 comprises a substrate 2 made of a monocrystalline metal superalloy covered with a coating 10, for example an environmental barrier comprising a thermal barrier.

[0008] The environmental barrier typically comprises an underlayer, preferably a metallic underlayer 3, a protective layer and a thermally insulating layer. The underlayer 3 covers the metallic superalloy substrate 2. The underlayer 3 is itself covered with the protective layer, formed by oxidation of the metallic underlayer 3. The protective layer makes it possible to protect the superalloy substrate 2 from corrosion and / or oxidation. The thermally insulating layer covers the protective layer. The thermally insulating layer may be made of ceramic, for example yttria-containing zirconia.

[0009] The sub-layer 3 is typically made from simple nickel aluminide β-NiAl or platinum-modified β-NiAlPt. The average atomic fraction of aluminium (between 35% and 45%) of the sub-layer 3 is sufficient to form exclusively a protective layer of aluminium oxide (Al 2 O 3 ) making it possible to protect the superalloy substrate 2 against oxidation and corrosion. Such sub-layers are for example described in document WO 2007 / 008227 A2.

[0010] However, when the part is subjected to high temperatures, the difference in nickel, and especially aluminum, concentrations between the superalloy substrate 2 and the metallic sub-layer 3 causes diffusion of the different elements, in particular nickel in the substrate towards the metallic sub-layer, and aluminum in the metallic sub-layer towards the superalloy. This phenomenon is called "inter-diffusion".

[0011] Interdiffusion can lead to the formation of primary and secondary reaction zones (called "SRZ" or Secondary Reaction Zone in English) in a part of the substrate 2 in contact with the sub-layer 3.

[0012] There figure 2is a microphotograph of the section of a sub-layer 3 covering a substrate 2 of a part 1. The microphotograph is taken before the part is subjected to a series of thermal cycles to simulate the temperature conditions of the part 1 during its use. The substrate 2 is rich in rhenium, that is to say that the average mass fraction of rhenium is greater than or equal to 0.04. It is known to use rhenium in the composition of superalloys to increase the creep resistance of superalloy parts. Typically, the substrate 2 has a γ-γ' phase, and in particular a γ-Ni phase. The sub-layer 3 is of the β-NiAlPt type. The substrate 2 has a primary inter-diffusion zone 5, in the part of the substrate directly covered by the sub-layer 3. The substrate 2 also has a secondary inter-diffusion zone 6, directly covered by the primary inter-diffusion zone 5.The scale bar corresponds to a length equal to 20 µm.

[0013] There figure 3 is a microphotograph of the section of sub-layer 3 covering substrate 2 of part 1. The microphotograph shows sub-layer 3 and substrate 2 after subjecting them to the series of thermal cycles described above. Sub-layer 3 covers substrate 2. Substrate 2 has a primary inter-diffusion zone 5 and a secondary inter-diffusion zone 6. The scale bar corresponds to a length equal to 20 µm.

[0014] Inter-diffusion phenomena lead to premature depletion of the aluminum sub-layer, which promotes phase transformations in the sub-layer (B-NiAl → γ'-Ni 3 Al, martensitic transformation). These transformations modify the allotropic structure of the sub-layer 3 and / or the inter-diffusion zones, and generate cracks 8 there, promoting flaking (or rumplingin English) of the protective layer of aluminum oxide.

[0015] Thus, inter-diffusions between the superalloy substrate 2 and the sub-layer 3 can have harmful consequences on the life of the superalloy part. STATEMENT OF THE INVENTION

[0016] An aim of the invention is to propose a solution for effectively protecting a superalloy turbine part from oxidation and corrosion while increasing its service life, during use, compared to known parts.

[0017] Another object of the invention is to limit or prevent the formation of secondary reaction zones while allowing an aluminum oxide to be formed during use of the part.

[0018] Finally, another aim of the invention is to at least partially prevent the formation of cracks in the substrate of a part subjected to high temperature conditions, for example above 1000°C, as well as the flaking of the protective layer of aluminum oxide.

[0019] These aims are achieved within the scope of the present invention by means of a method for manufacturing a turbine part, comprising a substrate made of a monocrystalline nickel-based superalloy, comprising chromium and at least one element chosen from rhenium and ruthenium, having a γ-γ' phase, an average mass fraction of rhenium and ruthenium greater than or equal to 4% and an average mass fraction of chromium less than or equal to 5% and preferably less than or equal to 3%, a sub-layer covering at least part of a surface of the substrate, the sub-layer (4) having a γ-γ' phase and an average atomic fraction: in chromium between 5% and 10%, in aluminum between 10% and 20%, in platinum between 15% and 25%, the method comprising at least the steps of: a) depositing an enrichment layer on the substrate, the enrichment layer having at least an average atomic fraction of platinum greater than 90% and an average atomic fraction of chromium between 3% and 10%, b) heat treating the assembly formed by the substrate and the enrichment layer, at a temperature between 1000°C and 1200°C, for more than one hour, preferably for more than 2 hours, so that the enrichment layer diffuses at least partially into the substrate.

[0020] The invention is advantageously supplemented by the following characteristics, taken individually or in any of their technically possible combinations: the heat treatment of the assembly formed by the substrate and the enrichment layer is carried out at a temperature between 1050°C and 1150°C for substantially 4 hours, during step a) of depositing an enrichment layer, at least one layer of chromium and one layer of platinum are deposited separately, the chromium layer(s) having a total thickness between 200 nm and 2 µm and the platinum layer(s) having a total thickness between 3 µm and 10 µm, during step a) of depositing an enrichment layer, chromium and platinum are deposited simultaneously, the deposition of the enrichment layer is carried out by a method chosen from physical vapor deposition, thermal projection, electron gun evaporation, pulsed laser ablation and cathode sputtering, the sub-layer exclusively has a γ-γ' phase, the sub-layer has an average atomic fraction in silicon less than 2%,the underlayer has a thickness of between 5 µm and 50 µm, and preferably between 5 µm and 15 µm, the method comprises a step of forming a protective layer of aluminum oxide covering the underlayer, the method comprises a step of forming a thermally insulating layer of ceramic covering the protective layer of aluminum oxide. DESCRIPTION OF FIGURES

[0021] Other characteristics, aims and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and which must be read in conjunction with the appended drawings in which: [ Fig. 1 ] There figure 1 , already commented on, schematically illustrates a section of a turbine part conforming to the state of the art, for example a turbine blade or a distributor vane. Fig. 2 ] There figure 2is a scanning electron microscopy photograph of the microstructure of a substrate and undercoat of the turbine part, before the part has been subjected to a series of thermal cycles. Fig. 3 ] There figure 3 is a scanning electron microscopy photograph of the microstructure of a substrate and an underlayer of the turbine part, after the part has been subjected to a series of thermal cycles. Fig. 4 ] There figure 4 schematically illustrates a method of manufacturing a part comprising a substrate and an underlayer, in accordance with one embodiment of the invention. Fig. 5 ] There Figure 5 is a scanning electron microscopy photograph of a substrate and undercoat of the part, before the part has been subjected to a series of thermal cycles. Fig. 6 ] There figure 6is a scanning electron microscopy photograph of a substrate and undercoat of the part, before the part has been subjected to a series of thermal cycles.

[0022] Throughout the figures, similar elements have identical references. DEFINITIONS

[0023] The term "superalloy" refers to an alloy that exhibits, at high temperature and high pressure, very good resistance to oxidation, corrosion, creep and cyclic stresses (particularly mechanical or thermal). Superalloys find particular application in the manufacture of parts used in aeronautics, for example turbine blades, because they constitute a family of high-strength alloys that can work at temperatures relatively close to their melting points (typically 0.7 to 0.8 times their melting temperatures).

[0024] A superalloy may have a two-phase microstructure comprising a first phase (called the "γ phase") forming a matrix, and a second phase (called the "γ' phase") forming precipitates that harden in the matrix. The coexistence of these two phases is referred to as the γ-γ' phase.

[0025] The "base" of the superalloy refers to the main metallic component of the matrix. In most cases, superalloys include an iron, cobalt, or nickel base, but sometimes also a titanium or aluminum base. The base of the superalloy is preferably a nickel base.

[0026] Nickel-based superalloys have the advantage of offering a good compromise between resistance to oxidation, resistance to fracture at high temperatures and weight, which justifies their use in the hottest parts of turbojets.

[0027] Nickel-based superalloys consist of a γ phase (or matrix) of the austenitic face-centered cubic γ-Ni type, possibly containing α-substitution solid solution additives (Co, Cr, W, Mo), and a γ' phase (or precipitates) of the γ'-Ni 3 X type, with X = Al, Ti or Ta. The γ' phase has an ordered L12 structure, derived from the face-centered cubic structure, consistent with the matrix, i.e. having an atomic mesh very close to it.

[0028] Due to its ordered nature, the γ' phase has the remarkable property of having a mechanical resistance which increases with temperature up to approximately 800°C. The very strong coherence between the γ and γ' phases gives nickel-based superalloys very high mechanical resistance when hot, which itself depends on the γ / γ' ratio and the size of the hardening precipitates.

[0029] A superalloy is, in all the embodiments of the invention, rich in rhenium and / or ruthenium, that is to say that the average mass fraction of rhenium and ruthenium in the superalloy is greater than or equal to 4%, making it possible to increase the creep resistance of the superalloy parts compared to the superalloy parts without rhenium. A superalloy is also, in all the embodiments of the invention, poor in chromium on average, that is to say that the average mass fraction in the whole of the superalloy in chromium is less than 0.05, preferably less than 0.03. Indeed, the depletion of chromium during an enrichment in rhenium and / or ruthenium of the superalloy makes it possible to maintain a stable allotropic structure of the superalloy, in particular a γ-γ' phase.

[0030] The term "atomic fraction" refers to the mole fraction, that is, the ratio of the amount of matter of an element or group of elements to the total amount.

[0031] The term "mass fraction" refers to the ratio of the mass of an element or group of elements to the total mass. DETAILED DESCRIPTION OF THE INVENTION

[0032] There figure 4 illustrates a method of manufacturing a part 1, comprising a substrate 2 and an underlayer 4. The substrate 2 used is of the CMSX-4 plus type (registered trademark) and has the chemical composition, in average atomic fraction, described in table 1. [Tables 1] Cr Co Mo Your W Cb D Al You Hf Neither 3,5 10 0,6 8 6 0 4,8 5,7 0,85 0,1 Stay

[0033] In a first step 401 of the method, an enrichment layer 11 is deposited on the substrate 2. The enrichment layer 11 has at least one average atomic fraction of platinum greater than 90% and an average atomic fraction of chromium of between 3% and 10%. The enrichment layer 11 comprises at least chromium and platinum, and preferably chromium, platinum, hafnium and silicon.

[0034] Preferably, the enrichment layer 11 does not comprise nickel. The different elements of the enrichment layer 11 may be alloyed.

[0035] The different elements of the enrichment layer 11 can be deposited simultaneously. The enrichment layer 11 can also comprise several superimposed layers: each element can be deposited separately. In particular, at least one layer of platinum and at least one layer of chromium can be deposited separately. In this case, the chromium layer(s) have a total thickness of between 200 nm and 2 µm and the platinum layer(s) have a total thickness of between 3 µm and 10 µm. Thus, the quantity of metals diffused during the method according to one embodiment of the invention is optimized.

[0036] The deposition of the layer(s) forming the enrichment layer 11 can be carried out under vacuum, for example by vapor phase (PVD process, acronym for the English term “ Physical Vapor Deposition»). Various PVD methods can be used for the fabrication of the enrichment layer 11, such as sputtering, electron gun evaporation, laser ablation and electron beam assisted physical vapor deposition. The enrichment layer 11 can also be deposited by thermal spraying.

[0037] During a second step 402 of the method, the assembly formed by the substrate 2 and the enrichment layer 11 is heat-treated, so that the enrichment layer 11 diffuses at least partially into the substrate 2. Thus, a sub-layer 4 is formed on the surface of the substrate 2. The heat treatment is preferably carried out for more than one hour at a temperature between 1000°C and 1200°C, preferably for more than two hours at a temperature between 1000°C and 1200°C, and even more preferably substantially four hours at a temperature between 1050°C and 1150°C.

[0038] Generally, a sufficient quantity of platinum and chromium is deposited during step 401 so that, after the heat treatment step 402, the average atomic fraction of platinum in the sub-layer 4 is between 15% and 25%, and so that the average atomic fraction of chromium in the sub-layer 4 is greater than 5% and preferably between 5% and 20%. The quantity of platinum and chromium deposited in the enrichment layer 11 is therefore higher the lower the atomic molar fraction of chromium and platinum in the substrate 2, which is typically the case for a substrate 2 enriched in rhenium and / or ruthenium.

[0039] The thickness of the enrichment layer 11 is preferably between 100 nm and 20 µm.

[0040] There Figure 5is a scanning electron microscopy photograph of the microstructure of a substrate 2 and a sub-layer 4 of a part 1. The sub-layer 4 is manufactured by the process illustrated in the figure 4 , in which an enrichment layer 11 comprising only chromium and platinum is deposited, during step 401 of the method. The scale bar of the Figure 5corresponds to a length equal to 20 µm. Sub-layer 4 generally has a γ-γ' phase and an average atomic fraction of chromium greater than 5%, preferably between 5% and 20%, of aluminum between 10% and 20%, of platinum between 15% and 25%. In particular, sub-layer 4 has an average atomic fraction of chromium substantially equal to 5.8%, an average atomic fraction of aluminum substantially equal to 11%, an average atomic fraction of platinum substantially equal to 21%, an average atomic fraction of hafnium less than 0.5% and an average atomic fraction of silicon less than 1%.

[0041] The sub-layer 4 preferably has exclusively a γ-γ' phase. Indeed, the introduction of elements into the substrate 2 by the enrichment method described above makes it possible not to cause a phase transition of the substrate 2, and thus to avoid mechanical stresses in the substrate 2 which could cause the appearance of cracks 8. A substantially horizontal line divides the sub-layer 4 into two superimposed parts: this line corresponds to the limit between the substrate 2 and the enrichment layer 11, before the heat treatment step 402 during the manufacture of a part 1.

[0042] The thickness of the sub-layer 4 is typically between 1 µm and 100 µm, and preferably between 5 µm and 50 µm.

[0043] In particular, the average atomic fraction of chromium in sub-layer 4 helps to promote the formation of α-Al 2 O 3 when the part is used under working conditions.

[0044] In reference to the figure 6 , sub-layer 4 helps prevent cracks from forming during prolonged heat treatment, representative of the working conditions of a turbine. The scale bar corresponds to a length equal to 20 µm. The figure 6 is a scanning electron microscopy photograph of a part 1 comprising the substrate 2 and the underlayer 4, after the prolonged heat treatment. During the prolonged heat treatment, the part 1 is placed in air for 100 hours at 1050°C and then for 10 hours at 1150°C. No crack 8 is detectable in the substrate 2 after the prolonged heat treatment.

Claims

1. Process for manufacturing a turbine part (1) for aeronautics, comprising : - a substrate (2) made of single-crystal nickel-based superalloy, comprising chromium and at least one element chosen from rhenium and ruthenium, having a γ-γ' phase, an average mass fraction of rhenium and ruthenium greater than or equal to 4% and an average mass fraction of chromium less than or equal to 5% and preferably less than or equal to 3%, - an underlayer (4) covering at least part of a surface of the substrate (2), the sublayer (4) having a γ-γ' phase and an average atomic fraction: - of chromium of between 5% and 10%, - of aluminium of between 10% and 20%, - of platinum of between 15% and 25%, this process comprising at least the steps of : a) deposition of an enrichment layer (11) on the substrate (2), the enrichment layer (11) having at least an average platinum atomic fraction of more than 90% and an average chromium atomic fraction of between 3% and 10%, b) heat treatment of the assembly formed by the substrate (2) and the enrichment layer (11), at a temperature of between 1000°C and 1200°C, for more than one hour, preferably for more than 2 hours, so that the enrichment layer (11) diffuses at least partially into the substrate (2).

2. Process according to claim 1, in which the heat treatment of the assembly formed by the substrate (2) and the enrichment layer (11) is carried out at a temperature of between 1050°C and 1150°C for substantially 4 hours.

3. Process according to claim 1 or 2, in which, during step a) of depositing an enrichment layer, at least one chromium layer and one platinum layer are deposited separately, the chromium layer or layers having a total thickness of between 200 nm and 2 µm and the platinum layer or layers having a total thickness of between 3 µm and 10 µm.

4. Process according to claim 1 or 2, in which, during step a) of depositing an enrichment layer, chromium and platinum are deposited simultaneously.

5. Process according to one of the preceding claims, in which the deposition of the enrichment layer (11) is carried out by a method chosen from physical vapour deposition, thermal spraying, electron gun evaporation, pulsed laser ablation and cathode sputtering.

6. Process according to any of the preceding claims, wherein the sublayer (4) has exclusively a γ-γ' phase.

7. Process according to one of the preceding claims, in which the underlayer (4) has an average silicon atomic fraction of less than 2%.

8. Process according to one of the preceding claims, in which the underlayer (4) has a thickness of between 5 µm and 50 µm, and preferably of between 5 µm and 15 µm.

9. Process according to one of the preceding claims, characterised in that it comprises a step consisting of forming a protective layer of aluminium oxide covering the undercoat (4).

10. A process as claimed in claim 9, characterised in that it comprises a step of forming a thermally insulating ceramic layer overlying the protective aluminium oxide layer.