SUPERALALLY TURBINE PART CONTAINING RHENIUM AND ASSOCIATED MANUFACTURING PROCESS

FIELD OF THE INVENTION

The invention relates to a turbine part, such as a turbine blade or a nozzle blade for example, used in aeronautics.

STATE OF THE ART

In a turbojet, the exhaust gases generated by the combustion chamber can reach high temperatures, greater than 1200 ° C, or even 1600 ° C. The parts of the turbojet, in contact with these exhaust gases, such as turbine blades for example, must thus be able to retain their mechanical properties at these high temperatures.

To this end, it is known practice to manufacture certain parts of the turbojet engine in “superalloy”. Superalloys are 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).

In order to reinforce the thermal resistance of these superalloys and to protect them against oxidation and corrosion, it is known practice to cover them with a coating which acts as a thermal barrier.

FIG. 1 schematically illustrates a section of a turbine part 1, for example a turbine blade 6 or a distributor fin. Part 1 comprises a substrate 2 in monocrystalline metal superalloy covered with a thermal barrier 10.

Thermal barrier 10 typically comprises a metallic underlayer, a protective layer and a thermally insulating layer. The metallic underlayer covers the metallic superalloy substrate. The metal underlayer is itself covered with the layer

protective, formed by oxidation of the metal underlayer. The protective layer protects the superalloy substrate from corrosion and / or oxidation. The thermally insulating layer covers the protective layer. The thermally insulating layer can be ceramic, for example yttriated zirconia.

The metallic sublayer provides a bond between the surface of the superalloy substrate and the protective layer: the metallic sublayer is sometimes called a "bonded sublayer".

An undercoat can be made from 6-NiAl simple nickel aluminide or 6-NiAlPt modified platinum. The average aluminum mass fraction (between 0.35 and 0.45) of the sublayer is sufficient to exclusively form a protective layer of aluminum oxide (Al2O3) making it possible to protect the superalloy substrate against oxidation and corrosion.

However, when the part is subjected to high temperatures, the difference in the concentrations of nickel, and especially of aluminum, between the superalloy substrate and the metallic sublayer causes the various elements to diffuse, in particular the nickel from the substrate to the surface. metal underlayer, and aluminum from the metal underlayer to the superalloy. This phenomenon is called "inter-diffusion " .

Inter-diffusion can result in the formation of primary and secondary reaction zones (called “SRZ” or Secondary Reaction Zone) in a part of the substrate in contact with the sublayer.

Figure 2 is a photomicrograph of the section of an underlayer 3 covering a substrate 2. The photomicrograph is performed before the part is subjected to a series of thermal cycles to simulate the working conditions in temperature of the part. 1. Substrate 2 is rich in rhenium, that is to say that the average mass fraction of rhenium is greater than 0.04. It is known to use rhenium in the composition of superalloys to increase the creep resistance of superalloy parts. It is also known to use superalloys having a low average chromium mass fraction, that is to say less than 0.08, to increase the resistance to oxidation and corrosion of the structure when the substrate is rich in rhenium. Typically, the substrate 2 has a γ-γ 'Ni phase. The sub-layer 3 is of the 6-NiAlPt type. The substrate has a primary inter-diffusion zone 5, in the part of the substrate 2 directly covered by the sublayer 3. The substrate 2 also has a secondary inter-diffusion zone 6, directly covered by the zone of primary inter-diffusion 5. The thickness of the secondary inter-diffusion zone illustrated in FIG. 2 is substantially 35 μm and more generally between 20 and 50 μm.

Figure 3 is a photomicrograph of the section of the sublayer 3 covering the substrate 2. The photomicrograph shows the sublayer 3 and the substrate 2 after having subjected them to the series of thermal cycles described above. The sub-layer 3 covers the substrate 2. The substrate 2 has a primary inter-diffusion zone 5 and a secondary inter-diffusion zone 6. Locally, the thickness of the secondary inter-diffusion zone can be. greater than 100 pm and can reach 150 μιτι, as shown by the white segment in Figure 3.

The combination of a superalloy comprising rhenium, poor in chromium, with an underlayer of β-Ni ' AlPt type results in the formation of secondary reaction zones. The formation of secondary reaction zones strongly degrades the mechanical properties (creep, fatigue) of the superalloy by causing the appearance of cracks 8 and / or high mechanical stresses in the substrate 2 by subjecting a part 1 to high temperature conditions, for example greater than 1000 ° C.

Thus, the inter-diffusions between the superalloy substrate and the sublayer can have harmful consequences on the life of the superalloy part.

SUMMARY OF THE INVENTION

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

This aim is achieved in the context of the present invention thanks to a turbine part comprising a substrate in monocrystalline nickel-based superalloy, comprising rhenium, having a γ-γ 'Ni phase, and an average chromium mass fraction of less than 0, 08, a nickel-based metallic superalloy sublayer covering the substrate, characterized in that the metallic superalloy sublayer comprises at least aluminum, nickel, chromium, silicon, Thafnium and is present predominantly by volume a Y '-NISAL phase

Since the metal sublayer has an allotropic structure close to the structure of the substrate, the formation of secondary reaction zones is prevented and / or limited. Thus, the formation of cracks in the substrate of a part subjected to high temperature conditions, for example greater than 1000 T as well as the chipping of the protective layer of aluminum oxide are limited or prevented.

In addition, as the metallic sublayer comprises aluminum, while having mainly by mass a y'-Ni 3 Al phase , the metallic sublayer can be oxidized to form a protective aluminum layer for a longer time. , under working conditions, than by using the known metal sublayers.

The turbine part can also have the following characteristics:

the sub-layer also has a γ-Ni phase;

- the average rhenium mass fraction of the substrate is greater than - the average platinum mass fraction of the sublayer is between 0 and 0.05;

the average mass fraction of aluminum in the sublayer is between 0.06 and 0.25;

- the average mass fraction of chromium in the sublayer is between 0.07 and 0.20;

- the average mass fraction in hafnium of the sublayer is less than 5%;

- The average silicon mass fraction of the sublayer is less than 5%;

the sub-layer further comprises at least one element chosen from cobalt, molybdenum, tungsten, titanium and tantalum;

- a protective layer of aluminum oxide covers the sub-layer;

- a thermally insulating ceramic layer covers the protective layer;

- the thickness of the sublayer is between 5 μιτι and 50 μητι.

The invention further relates to a method of manufacturing a turbine part comprising a vacuum deposition step of a sublayer of a nickel-based superalloy having predominantly by volume a y'-Ni 3 Al phase , as well as optionally a γ-Ni phase, on a nickel-based superalloy substrate comprising rhenium and exhibiting a γ-γ 'Ni phase.

The deposition can be implemented by a method chosen from physical vapor deposition, thermal spraying (for example by an HVOF system, acronym for Hi $> h Velocity Oxy-Fuel), Joule evaporation, laser ablation pulsed and sputtering.

The undercoat can be deposited by co-spraying and / or co-evaporating targets of different metallic materials.

PRESENTATION OF THE DRAWINGS

Other characteristics and advantages will be felt from the following description, which is purely illustrative and not limiting, and should be read with reference to the appended figures, among which:

- Figure 1 schematically illustrates the section of a turbine part, for example a turbine blade or a distributor fin;

FIG. 2 is a photomicrograph of the section of an underlayer covering a substrate;

- Figure 3 is a photomicrograph of the section of an underlayer 3 covering a substrate;

- Figure 4 schematically illustrates a thermal barrier section covering the substrate of a turbine part according to one embodiment of the invention.

DEFINITIONS

The term “superalloy” denotes a complex alloy exhibiting, at high temperature and at high pressure, very good resistance to oxidation, corrosion, creep and cyclic stresses (in particular 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 resistance alloys which can work at temperatures relatively close to their melting points (typically 0 , 7 to 0.8 times their melting temperatures).

A superalloy can have a two-phase microstructure comprising a first phase (called “γ phase”) forming a matrix, and a second phase (called “γ phase”) forming precipitates hardening in the matrix.

The "base" of the superalloy refers to the main metal component of the matrix. In the majority of cases, the superalloys include an iron, cobalt or nickel base, but also sometimes a titanium or aluminum base.

“Nickel-based superalloys” have the advantage of offering a good compromise between resistance to oxidation, resistance to rupture at high temperature and weight, which justifies their use in the hottest parts of turbojets.

Nickel-based superalloys consist of a γ phase (or matrix) of the cubic austenitic type with γ-Ni face centered, optionally containing additives in solid solution of substitution a (Co, Cr, W, Mo), and a γ 'phase (or precipitates) of the γ'-Νι * 3Χ 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, that is to say having an atomic mesh very close to it.

By virtue of 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 confers a very high mechanical resistance to hot nickel-based superalloys, which itself depends on the γ / γ' ratio and on the size of the hardening precipitates.

A superalloy is, in all the embodiments of the invention, rich in rhenium, that is to say that the average mass fraction of rhenium of the superalloy is greater than 0.04, making it possible to increase the strength. the creep of superalloy parts compared to superalloy parts without rhenium. A superalloy is also, in all the embodiments of the invention, poor in chromium, that is to say that the average chromium mass fraction is less than 0.08, preferably less than 0.05, to increase the resistance to oxidation of the structure during the presence of rhenium in the superalloy.

Nickel-based superalloys thus generally exhibit high mechanical strength up to 70CTC, then a mechanical strength which decreases sharply above 800 ° C.

The term "mass fraction" refers to the ratio of the mass of an element or a group of elements to the total mass.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 4 schematically illustrates a thermal barrier section 10 covering the substrate 2 with a turbine part 1 according to one embodiment of the invention.

The elements illustrated in FIG. 4 can be independently representative of the elements of a turbine blade 6 such as that shown in FIG. 1, of a distributor fin, or of any other element, part or part of a turbine. .

The substrate 2 is formed from a nickel-based superalloy. The average mass fraction of rhenium substrate 2 is greater than 0.04 and preferably between 0.045 and 0.055. Preferably, the average chromium mass fraction of the substrate is low, that is to say less than 0.08 and preferably less than 0.05.

The thermal barrier 10 comprises a metallic sub-layer 3, a protective layer 4 and a thermally insulating layer 9.

The substrate 2 is covered by the metallic sublayer 3. The metal sub-layer 3 is covered by the protective layer 4. The protective layer 4 is covered by the thermally insulating layer 9.

The deposition of a metallic sub-layer 3 having an allotropic structure close to the structure of the substrate 2 makes it possible to prevent the formation of secondary reaction zones. In particular, the sub-layer 3 deposited has a γ phase and a γ 'phase, like the substrate.

The sublayer 3 has an alumino-forming composition, allowing the part to resist oxidation and corrosion. In particular, the sub-layer 3 mainly has in volume a γ'-Νί 3 en phase. Preferably, the sublayer 3 also has a γ-Νι ' phase. The sublayer 3 thus has both a structure close to the structure of the substrate 2, while comprising a reserve of aluminum allowing it to form a protective layer 4 of aluminum oxide by oxidation, for a longer time, compared with a sub-layer having a majority γ-Ni phase in which the mass fraction of aluminum is smaller. Preferably, the average mass fraction of the aluminum sub-layer 3 is between 0.06 and 0.25 and preferably between 0.06 and 0.12.

Table 1, below, shows examples of the compositions of the sublayer 3 in a nickel-based superalloy. The different compositions are designated by the letters from A to C. The mass fractions, in percent, of the sublayer 3 having a y phase, and the volume fraction of the sublayer 3 having a γ 'phase, are also described. for an underlayer 3 having undergone a heat treatment at 1000 ° C.

Table 1

Composition A corresponds to a sub-layer 3 of NiCrAlHfSiPt type and has a predominant γ'-Νι ' 3ΑΙ phase and a γ-Ni phase. Composition B corresponds to a sub-layer 3 of NiCrAlHfSi type and has a majority phase γ'-Νι ' 3ΑΙ and preferably a γ-Ni phase. For a sublayer 3 having undergone a heat treatment at 1100T, the mass fraction of the sublayer 3 exhibiting a γ phase is 40% m and the mass fraction of the sublayer 3 exhibiting a γ 'phase is 60 % m. Composition C corresponds to a sub-layer 3 of NiCrAlHfSi type and has a major γ'-Νι ' 3ΑΙ phase and a γ-Ni phase.

In general, the sublayer 3 preferably has an average mass fraction of platinum of less than 0.02 and / or an average mass fraction of chromium between 0.07 and 0.17. Thus, the resistance to oxidation of the room is increased.

The sublayer 3 can be deposited under vacuum, for example in the vapor phase (PVD process, acronym of the English term Physical Vapor Déposition). Different PVD methods can be used for the fabrication of the sublayer 3 such as sputtering, joule evaporation, laser ablation and electron beam assisted physical vapor deposition. The sublayer 3 can also be deposited by thermal spraying.

Thus, the sublayer 3 can be deposited on the substrate 2 without using a method of forming an sublayer by diffusion of chemical elements in the substrate 2, such as platinum. These deposition methods also make it possible to simplify the formation of the sub-layer 3 on the substrate 2 as well as better control of the chemical compositions of the sub-layer 3. They also make it possible to deposit a sub-layer 3 having mainly in volume a γ '-Νι ' 3ΑΙ phase, as well as possibly a γ-Ni phase, unlike known methods.

Several targets of different metallic materials can be used in parallel, simultaneously, during the deposition of an underlayer 3. This type of deposition can be carried out by co-evaporation or by co-sputtering: the speed, respectively of evaporation, or sputtering imposed on each target during the deposition of the sublayer 3 then determines the stoichiometry of said layer.
CLAIMS

1. Turbine part (1) comprising:

a substrate (2) in monocrystalline nickel-based superalloy, comprising rhenium, having a γ-γ 'Ni phase, and an average chromium mass fraction of less than 0.08;

a sublayer (3) of nickel-based metal superalloy covering the substrate (2);

characterized in that:

- The metallic superalloy sub-layer (3) comprises at least aluminum, nickel, chromium, silicon, hafnium and mainly has a γ'-ΝΪ3ΑΙ phase by volume.

2. Part according to claim 1, wherein the sublayer (3) also has a γ-Νι ' phase .

3. Part according to claim 1 or 2, wherein the average rhenium mass fraction of the substrate (2) is greater than 0.04.

4. Part according to one of claims 1 to 3, wherein the average mass fraction of platinum of the sublayer (3) is less than 0.05.

5. Part according to one of claims 1 to 4, wherein the average aluminum mass fraction of the underlayer (3) is between 0.06 and 0.25.

6. Part according to one of claims 1 to 5, wherein the average mass fraction of chromium of the sublayer (3) is between 0.07 and 0.20.

7. Part according to one of claims 1 to 6, wherein the average mass fraction of hafnium of the sublayer (3) is less than 5%.

8. Part according to one of claims 1 to 7, wherein the average silicon mass fraction of the sublayer (3) is less than 5%.

9. Part according to one of claims 1 to 8, wherein the underlayer

(3) further comprises at least one element selected from cobalt, molybdenum, tungsten, titanium, tantalum.

10. Part according to one of claims 1 to 9, comprising a protective layer (4) of aluminum oxide covering the sublayer (3).

1 1. Part according to claim 10, comprising a thermally insulating layer (9) of ceramic covering the protective layer

(4).

1 2. Part according to one of claims 1 to 11, wherein the thickness of the sublayer (3) is between 5 pm and 50 pm.

13. A method of manufacturing a turbine part (1) comprising a vacuum deposition step of an underlayer (3) of a nickel-based superalloy having predominantly by volume a phase γ′-Νί 3 ΑΙ, thus that possibly a γ-Νι ' phase , on a substrate (2) made of a nickel-based superalloy comprising rhenium and having a γ-γ' Ni phase.

14. The method of claim 13, wherein the deposition is carried out by a method chosen from physical vapor deposition, thermal spraying, Joule evaporation, pulsed laser ablation and cathodic sputtering.

15. The method of claim 13 or 14, wherein the sub-layer (3) is deposited by co-spraying and / or by co-evaporating targets of different metallic materials.