Turbine part made of superalloy comprising rhenium and/or ruthenium and associated manufacturing method

Abstract

The present invention concerns a turbine part comprising a substrate made of nickel-based monocrystalline superalloy, comprising chromium and at least one element chosen among rhenium and ruthenium, the substrate having a - phase, an average mass fraction of rhenium and of 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 a part of a surface of the substrate, characterised in that the sublayer has a - phase and an average atomic fraction of chromium greater than 5%, of aluminium between 10% and 20% and of platinum between 15% and 25%.

Claims

1. A process for manufacturing a turbine part comprising a single-crystal nickel-base superalloy substrate and a sublayer, the single-crystal nickel-base superalloy substrate having a - phase, comprising chromium with an average mass fraction of chromium less than or equal to 5%, and comprising at least one element selected from rhenium and ruthenium, with an average mass fraction of rhenium greater than or equal to 4% when there is no ruthenium, with an average mass fraction of ruthenium greater than or equal to 4% when there is no rhenium and with an average mass fraction of the total of rhenium and ruthenium greater than or equal to 4% when there is rhenium and ruthenium the sublayer covering at least part of a surface of the substrate, the sublayer having a - phase and an average atomic fraction: of chromium comprised between 5% and 10%, of aluminum comprised between 10% and 20%, of platinum comprised between 15% and 25%, wherein the process comprises at least the steps of: 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 comprised between 3% and 10%, heat treating the assembly formed by the substrate and the enrichment layer so that the enrichment layer diffuses at least partially into the substrate.

2. The process as claimed in claim 1, wherein, 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 comprised between 200 nm and 2 m and the platinum layer or layers having a total thickness comprised between 3 m and 10 m.

3. The process as claimed in claim 1, wherein, during step a) of depositing an enrichment layer, chromium and platinum are deposited simultaneously.

4. The process as claimed in claim 1, wherein the assembly formed by the substrate and the enrichment layer is heat treated at a temperature above 1000 C. for more than one hour.

5. The process as claimed in claim 1, wherein the deposition of the enrichment layer is carried out by a method selected from physical vapor deposition, thermal spraying, electron beam evaporation, pulsed laser ablation and cathode sputtering.

6. The process as claimed in claim 1, wherein the single-crystal nickel-base superalloy substrate has an average mass fraction of chromium less than or equal to 3%.

7. The process as claimed in claim 1, wherein the assembly formed by the substrate and the enrichment layer is heat treated at a temperature above 1000 C. for more than 2 hours.

Description

DESCRIPTION OF THE FIGURES

(1) Other features, aims and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and which should be read in conjunction with the appended drawings in which:

(2) FIG. 1, already commented on, schematically illustrates a cross-section of a turbine part in accordance with the state of the art, for example a turbine blade or a nozzle vane.

(3) FIG. 2 is a scanning electron microscopy photograph of the microstructure of a substrate and sublayer of the turbine part, before the part has been subjected to a series of thermal cycles.

(4) FIG. 3 is a scanning electron microscopy photograph of the microstructure of a substrate and a sublayer of the turbine part, after the part has been subjected to a series of thermal cycles.

(5) FIG. 4 schematically illustrates a process for manufacturing a part comprising a substrate and a sublayer, in accordance with an embodiment of the invention.

(6) FIG. 5 is a scanning electron microscopy photograph of a substrate and a sublayer of the part, before the part has been subjected to a series of thermal cycles.

(7) FIG. 6 is a scanning electron microscopy photograph of a substrate and a sublayer of the part, before the part has been subjected to a series of thermal cycles.

(8) Throughout the figures, similar elements bear the same reference marks.

DEFINITIONS

(9) The term superalloy refers to an alloy having, at high temperature and high pressure, very good resistance to oxidation, corrosion, creep and cyclic stresses (particularly mechanical or thermal stresses). Superalloys have a 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).

(10) A superalloy can have a two-phase microstructure comprising a first phase (called phase) forming a matrix, and a second phase (called phase) forming precipitates hardening in the matrix. The coexistence of these two phases is referred to as the - phase.

(11) The base of the superalloy refers to the main metal 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.

(12) Nickel-base superalloys have the advantage of providing a good compromise between oxidation resistance, high-temperature fracture resistance and weight, which justifies their use in the hottest parts of turbine engines.

(13) Nickel-base superalloys are made up of a phase (or matrix) of the -Ni face-centered cubic austenitic type, possibly containing additives in (Co, Cr, W, Mo)-substituted solid solution, and a phase (or precipitates) of the -Ni.sub.3X type, with X=Al, Ti or Ta. The phase has an ordered L12 structure, derived from the face-centered cubic structure, coherent with the matrix, i.e., having an atomic lattice very close thereto.

(14) Due to its ordered nature, the phase has the remarkable property of having a mechanical strength that increases with temperature up to about 800 C. The very strong coherence between the and phases gives a very high mechanical strength to nickel-base superalloys, which itself depends on the / ratio and the size of the hardening precipitates.

(15) A superalloy is, in all the embodiments of the invention, rich in rhenium and/or ruthenium, i.e., the average mass fraction of rhenium and ruthenium in the superalloy is greater than or equal to 4%, increasing the creep resistance of the superalloy parts as compared with superalloy parts without rhenium. A superalloy is also, in all the embodiments of the invention, low in chromium on average, i.e., the average mass fraction in the entire superalloy of chromium is less than 0.05, preferentially less than 0.03. Indeed, chromium depletion during rhenium and/or ruthenium enrichment of the superalloy allows a stable allotropic structure of the superalloy to be maintained, in particular a - phase.

(16) The term atomic fraction refers to the molar fraction, i.e., the ratio of the quantity of matter of an element or group of elements to the total quantity.

(17) 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

(18) FIG. 4 illustrates a process for manufacturing a part 1, comprising a substrate 2 and a sublayer 4. The substrate 2 used is of the type CMSX-4 plus (registered trademark) and has the chemical composition, in average atomic fraction, described in Table 1.

(19) TABLE-US-00001 TABLE 1 Cr Co Mo Ta W Cb Re Al Ti Hf Ni 3.5 10 0.6 8 6 0 4.8 5.7 0.85 0.1 Balance

(20) In a first step 401 of the process, an enrichment layer 11 is deposited on the substrate 2. The enrichment layer 11 has at least an average atomic fraction of platinum greater than 90% and an average atomic fraction of chromium comprised between 3% and 10%. The enrichment layer 11 comprises at least chromium and platinum, and preferentially chromium, platinum, hafnium and silicon. Preferentially, the enrichment layer 11 does not include nickel. The individual elements of the enrichment layer 11 may be alloyed.

(21) The different elements of the enrichment layer 11 may be deposited simultaneously. The enrichment layer 11 may also comprise several superimposed layers: each element may 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 or layers have a total thickness comprised between 200 nm and 2 m and the platinum layer or layers have a total thickness comprised between 3 m and 10 m. Thus, the quantity of metals diffused during the process in accordance with an embodiment of the invention is optimized.

(22) The deposition of the layer or layers forming the enrichment layer 11 can be carried out under vacuum, for example by a physical vapor deposition (PVD) process. Various PVD methods can be used to produce the enrichment layer 11, such as cathode sputtering, electron beam evaporation, laser ablation and electron-beam physical vapor deposition. The enrichment layer 11 may also be deposited by thermal spraying.

(23) In a second step 402 of the process, the assembly formed by the substrate 2 and the enrichment layer 11 is thermally treated so that the enrichment layer 11 diffuses at least partially into the substrate 2. Thus, a sublayer 4 is formed on the surface of the substrate 2. The heat treatment is preferentially carried out for more than one hour at a temperature comprised between 1000 C. and 1200 C., preferentially for more than two hours at a temperature comprised between 1000 C. and 1200 C., and even more preferentially substantially four hours at a temperature comprised between 1050 C. and 1150 C.

(24) In general, a sufficient quantity of platinum and chromium is deposited during step 401 so that, after heat treatment step 402, the average atomic fraction of platinum in the sublayer 4 is comprised between 15% and 25%, and so that the average atomic fraction of chromium in the sublayer 4 is greater than 5% and preferentially comprised between 5% and 20%. The quantity of platinum and chromium deposited in the enrichment layer 11 is therefore all the higher as the chromium and platinum atomic mole fraction of the substrate 2 is lower, which is typically the case for a substrate 2 enriched in rhenium and/or ruthenium.

(25) The thickness of the enrichment layer 11 is preferentially comprised between 100 nm and 20 m.

(26) FIG. 5 is a scanning electron microscopy photograph of the microstructure of a substrate 2 and a sublayer 4 of a part 1. The sublayer 4 is produced by the process shown in FIG. 4, in which an enrichment layer 11 comprising only chromium and platinum is deposited during step 401 of the process. The scale bar in FIG. 5 corresponds to a length equal to 20 m. The sublayer 4 has, in general, a - phase and an average atomic fraction of chromium greater than 5%, preferentially comprised between 5% and 20%, of aluminum comprised between 10% and 20%, of platinum comprised between 15% and 25%. In particular, the sublayer 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%.

(27) The sublayer 4 preferentially has exclusively a - phase. Indeed, the introduction of elements into the substrate 2 by the enrichment process described above make it possible not to cause a phase transition of the substrate 2, and thus to avoid mechanical stresses in the substrate 2 that could lead to the appearance of cracks 8. A substantially horizontal line divides the sublayer 4 into two superimposed parts: this line corresponds to the boundary between the substrate 2 and the enrichment layer 11, prior to the heat treatment step 402 during the manufacture of a part 1.

(28) The thickness of the sublayer 4 is typically comprised between 1 m and 100 m, and preferentially between 5 m and 50 m.

(29) In particular, the average atomic fraction of chromium in the sublayer 4 helps to promote the formation of -Al.sub.2O.sub.3 when the part is used in working conditions.

(30) With reference to FIG. 6, the sublayer 4 helps prevent cracking during extended heat treatment, representative of working conditions in a turbine. The scale bar corresponds to a length equal to 20 m. FIG. 6 is a scanning electron microscopy photograph of a part 1 comprising the substrate 2 and the sublayer 4, after the extended heat treatment. During the extended heat treatment, the part 1 is placed under air for 100 hours at 1050 C. and then for 10 hours at 1150 C. No cracks 8 are detectable in the substrate 2 after the extended heat treatment.