RU2773969C2 - Turbine part of alloy with max-phase content - Google Patents

Abstract

FIELD: powder metallurgy.

SUBSTANCE: invention relates to powder metallurgy, in particular to the manufacture of a turbine part. It can be used for the manufacture of a turbine blade or a nozzle blade. The turbine part contains polycrystal substrate containing grains of Ti₃AlC₂ phase, while the mass fraction of this phase in the alloy is more than 97%. An average length of grains is less than 50 mcm, an average ratio of a thickness to the length of grains is from 0.4 to 0.6, and an average volume of an elementary cell of Ti₃AlC₂ phase is less than 152.4

. To form the part, powders containing at least titanium, aluminum and carbon are mixed and homogenized, followed by reactive powder sintering. The resulting product of reactive sintering is brought to a powder-like state, and spark plasma sintering of powder is carried out.

EFFECT: obtaining a part with high mechanical strength and resistance to oxidation in the range of turbine operating temperatures.

15 cl, 9 dwg

Description

The field of technology to which the invention belongs

The invention relates to a turbine part, for example, a turbine blade or a nozzle blade used in the aircraft industry, in particular, to a turbine part containing a substrate, the material of which contains the MAX phase. The invention also relates to a method for manufacturing such a turbine part.

State of the art

In a turbojet engine, the combustion gases leaving the combustion chamber can reach high temperatures in excess of 1200°C, even 1600°C. Therefore, parts of a turbojet engine that come into contact with these combustion gases, such as turbine blades, must be able to maintain their mechanical properties at these high temperatures.

It is known that for this some parts of the turbojet engine are made of superalloy. Nickel based superalloys generally form a group of high tensile metal alloys capable of operating at temperatures relatively close to their melting point (typically a factor of 0.7 to 0.9 of their melting point).

However, these alloys are very dense and their mass limits the efficiency of turbines.

In this regard, the TiAl intermetallic alloy was used for the manufacture of turbine blades. This material is less dense than nickel-based superalloy and its mechanical properties allow TiAl parts to be installed in some areas of the turbine. Indeed parts of TiAl may have, for example, resistance to oxidation at temperatures up to about 750°C.

However, TiAl currently does not allow turbine parts to be made that have sufficient durability and oxidation resistance for temperatures above 800° C., in contrast to some nickel-based superalloys.

Therefore, materials containing the so-called MAX-phases were used in the production of turbine parts. Materials with MAX-phase are materials of the general formula M _n+1 AX _n , in which n is an integer from 1 to 3, M is a metal of the transition group (selected from Se, Ti, V, Cr, Zr, Nb, Mo, Hf , Ta), A is an element from the group A, i.e. selected from the group of Al, Si, P, Ga, Ge, As, Cd, In, Sn, Ti, Pb, and X is an element selected from carbon and nitrogen. The composition of the MAX-phase of the material provides the specific properties of the material in terms of oxidation, density and creep resistance, in particular in the operating temperature range of the turbine, for example, from 800 to 1200°C. In particular, it is known to use a material containing a Ti ₃ AlC ₂ phase in the manufacture of a turbine part. Indeed, aluminum in the Ti ₃ AlC ₂ phase makes it possible to form a protective layer of aluminum oxide, which protects the part from oxidation during turbine operation. The carbon in the Ti ₃ AlC ₂ phase gives the material optimum creep resistance in the turbine operating temperature range. Finally, titanium in the Ti ₃ AlC ₂ phase gives the material a low density compared to other materials containing the MAX phase.

FR 3 032 449 describes, for example, a material for use in the aeronautical field with increased mechanical density. This material contains the first MAX phase of the Ti ₃ AlC ₂ type and the second intermetallic phase of the TiAl ₃ type, while the volume fraction of the MAX phase is from 70 to 95%, the volume fraction of the intermetallic phase is from 5 to 30%.

However, the materials described in this source of information are subject to too much oxidation at 1100°C to be suitable for the manufacture of turbine parts in the aircraft industry.

Disclosure of the essence of the invention

One of the objectives of the invention is to provide a solution for manufacturing a turbine part from a MAX-phase material that is characterized by both specific increased mechanical strength and increased oxidation resistance in the operating temperature range of the turbine and is less dense compared to nickel-based superalloy materials.

This problem has been solved within the framework of the present invention in a turbine part containing a polycrystalline substrate, this substrate containing grains and at least one Ti ₃ AlC ₂ phase, while the mass fraction of this phase in the alloy exceeds 97%, and each grain has a length and width , wherein:

- the average grain length is less than 50 microns,

- the average ratio between the width and length of the grains is from 0.4 to 0.6 and

- the average unit cell volume of the Ti ₃ AlC ₂ phase is less than 152.4 Å ³ .

Since the average grain length is less than 50 μm, the average ratio between width and length is from 0.4 to 0.6, and the average unit cell volume of the Ti ₃ AlC ₂ phase is less than 152.4 Å ³ , the micromorphology of the Ti ₃ AlC ₂ phase provides increased resistance to oxidation in the operating temperature range of the turbine.

Preferably, the invention further comprises the following features, taken alone or in any of their technically possible combinations:

- the substrate contains titanium carbide, while its mass fraction in the substrate is less than 0.8%,

- the substrate contains aluminum oxide, while its mass fraction in the substrate is less than 3%,

- the substrate contains intermetallic compounds Ti _x Al _y , while their volume fraction in the substrate is less than 1%,

- the substrate contains phases containing iron and/or tungsten, while the sum of the average volume fraction of iron and tungsten in these phases is less than 2%,

- the relative density of the Ti ₃ AlC ₂ phase exceeds 96%.

Another object of the invention is a turbine blade containing the part described above.

Another object of the invention is a turbine nozzle apparatus containing the part described above.

Another subject of the invention is a turbine comprising a turbine blade and/or a turbine stator blade as described above.

Another object of the invention is a method for manufacturing a turbine part, wherein the part contains a polycrystalline substrate, this substrate contains grains and at least one Ti ₃ AlC ₂ phase, and the mass fraction of this phase in the alloy exceeds 97%, each grain has a length and width, average the grain length is less than 50 μm, the average ratio of width to length is from 0.4 to 0.6, the average unit cell volume of the Ti ₃ AlC ₂ phase is less than 152.4 Å ³ , according to the invention, the method contains a stage of spark plasma sintering.

Preferably, the invention further comprises the following features, taken alone or in any technically feasible combination:

- the temperature at the stage of spark plasma sintering is less than 1400°C,

- the pressure at the stage of spark plasma sintering is more than 60 MPa,

- at the stage of spark plasma sintering, heat treatment is applied at a maximum temperature with a maximum duration of less than ten minutes,

the spark plasma sintering step includes a cooling sub-step, wherein the cooling rate in this sub-step is less than 100° C. per minute,

- the method of manufacturing a part includes the following steps:

a) mixing and homogenizing powders containing at least titanium, aluminum and carbon,

b) reaction sintering of powders,

c) grinding the product of reaction sintering of powders to the state of a powder, while steps a) - c) are applied before the stage of spark plasma sintering of the grinding product.

Brief description of the drawings

Other features and advantages of the invention are set forth in the following description, which is purely illustrative and non-limiting and is explained by the attached figures, in which:

in fig. 1 schematically shows a detail of a turbine, such as a turbine blade or a nozzle blade, in cross section;

in fig. 2 shows a photograph of the microstructure of the substrate of a turbine part obtained by a scanning electron microscope;

in fig. 3 illustrates the weight gain of various substrates subjected to isothermal oxidation;

in fig. 4 shows the unit cell of the MAX-phase type 312;

in fig. 5 shows a diagram of the influence of the disproportion criterion of the volume of the unit cell and the relative density of the substrate on changes in the rate of steady-state creep of the substrate;

in fig. 6 shows a Larson-Miller creep plot plotted for different types of substrates;

in fig. 7 shows the change in the magnitude of the weight gain in several substrates characterized by different criteria for the disproportion of the volume of the unit cell during oxidation treatment;

in fig. 8 shows the influence of the mass fraction of titanium carbide on the weight gain of the substrates treated for oxidation;

in fig. 9 shows how the part is made.

Definition of concepts

The term "grain length L" refers to the maximum grain size along a straight line passing through the center of mass of a given grain.

The term "width 1 grain" refers to the minimum grain size along a straight line passing through the center of mass of this grain.

"Density" means the ratio of the mass of a given volume of substrate to the mass of the same volume of water at 4° C. and atmospheric pressure.

"Relative density" means the ratio of the density of a substrate to the calculated density of the same substrate.

"Larson-Miller parameter" means the parameter P given by formula (1):

where: T is the Kelvin temperature of the substrate, t _r is the time to failure of the substrate at a specific stress, and k is a constant.

"Stoichiometric compound" or "stoichiometric material" means a material of multiple elements, with the atomic fraction of each element expressed as an integer.

Implementation of the invention

According to FIG. 1 turbine part 1, such as blade 4, comprises a polycrystalline substrate 2. This substrate contains at least one Ti ₃ AlC ₂ phase. Shown in FIG. 1, the elements can be independently represented by elements of a turbine blade 4, a nozzle blade or any other element, part or detail of a turbine.

As shown in FIG. 2, the polycrystalline substrate 2 contains grains 3. The grains 3 of the substrate are characterized by certain morphological parameters. In the Ti ₃ AlC ₂ phase of the substrate 2, the length L of the grain 3 is on average less than 50 μm. In addition, the average grain shape factor is 3, i.e. the average ratio between the grain width 3 and its length l/L is 0.3 to 0.7, preferably 0.4 to 0.6, more preferably 0.45 to 0.55. Therefore, the microstructural parameters relating to the average grain length 3 and the average shape factor make it possible to increase the resistance of the substrate 2 to oxidation during turbine operation, as well as to increase the creep resistance. The really small grain size ensures an increase in the fraction of grain boundaries exposed to the surface of the substrate per unit area. However, the grain boundaries create a rapid and preferential diffusion of alloy elements, such as aluminum, which leads to the formation of an oxide layer. Aluminum can mostly diffuse, forming alumina on the surface. The alumina layer thus formed is very resistant and protective at high temperature, which makes it possible to limit or prevent the growth of the mass of the substrate 2.

The average grain shape factor in combination with the grain size also makes it possible to increase creep resistance while avoiding grain boundary slip. The scale bar at the bottom right of the photograph corresponds to a length of 10 µm.

In FIG. 3 shows the weight gain of various substrates after a treatment involving isothermal oxidation. Two types of substrates were oxidized: the first type of substrate 2 according to the invention, corresponding to the black stripes in FIG. 3, in which the average grain length 3 in the Ti ₃ AlC ₂ phase is essentially 10 µm, and a second type of substrate, not in accordance with the invention, in the form of gray stripes in FIG. 3 in which the average grain length in the Ti ₃ AlC ₂ phase is essentially 60 µm. Weight gain was measured after isothermal oxidation. Isothermal oxidation was carried out at different temperatures (800°C, 900°C and 1000°C) in air for 30 hours. For all oxidation temperatures, the substrates 2, in which the average grain length 3 was essentially 10 μm, were characterized by weight gain much smaller than the order of magnitude of the weight gain of the substrates, in which the average grain length is essentially 60 μm. Thus, the substrate 2, in which the average grain length 3 is less than 50 μm, has an improved resistance to oxidation.

In FIG. 4 shows the unit cell of the 312 type MAX phase. In general, the MAX phase (containing the elements M, A, and X) has a hexagonal structure. The hexagonal unit cell of the MAX-phase is formed by M ₆ X octahedrons, assembled layers, between which there are layers of A elements. The calculated volume of the Ti ₃ AlC ₂ unit cell is known and is equal to: V ₀ =153.45 Å ³ . According to an aspect of the invention, the average unit cell volume of the Ti ₃ AlC ₂ phase differs from the calculated volume. By means of the disproportion criterion of the unit cell volume, this parameter δ is determined using formula (2):

where: V _mes is equal to the average unit cell volume measured for the Ti ₃ AlC ₂ phase of substrate 2. This volume can be calculated after determining the unit cell parameters by the Rietveld structure establishment method based on X-Ray Diffraction (XRD), for example, when measuring in the angular range from 7 to 140°.The change in the parameter δ is caused in principle by possible contamination with chemical elements during the manufacture of the substrate 2. Also, this parameter δ may vary depending on the parameters of the manufacture of the substrate 2, such as pressure, temperature and /or the duration of processing of the substrate 2 in the manufacturing process.

In FIG. 5 shows a diagram of the influence of the parameter δ and the relative density of the substrate on changes in the steady-state creep rate of the substrate 2. The steady-state creep rate was measured on substrates 2 treated at a temperature of 900°C and subjected to a tensile stress of 140 MPa. The substrates 2 used in the cases shown in FIG. 5 measurements, had a volume fraction of the Ti ₃ AlC ₂ phase above 98% and had different relative densities. At a relative density of 97% (corresponding to the column in the center and the column on the right in Fig. 5), the steady-state creep rate is given for two parameters δ (δ=0.98% and δ=0.17%). The substrate was characterized by a steady creep rate higher at δ=0.17% and lower at δ=0.98%. In general, the substrate had a higher steady-state creep rate at δ<0.7%. Thus, the substrate 2 with δ>0.7% had a higher creep resistance. The δ parameter indicates the discrepancy between the actual or measured unit cell volume of the MAX phase and the calculated or reference unit cell volume. Consequently, in the case when δ increases, the volume of the unit cell of the MAX phase decreases, which indicates the convergence of different layers of elements A and octahedrons M ₆ X. The relationship between the parameter δ and the creep resistance of substrate 2 turned out to be unexpected. It may be possible to explain this phenomenon by slowing down the migration of dislocations, which then makes it possible to increase the creep resistance of this material. Preferably, the delta parameter is 0.7 to 2%, more preferably 0.92 to 1%. If we assume that V ₀ =153.45 Å ³ in the Ti ₃ AlC ₂ phase, then the average unit cell volume of the Ti ₃ AlC ₂ phase of substrate 2 will be less than 152.4 Å ³ . Preferably, the average unit cell volume of Ti ₃ AlC ₂ is from 150.38 to 152.37 Å ³ , more preferably from 151.91 to 152.03 Å ³ . The unit cell volume phenomena described above are preferably monitored when the average grain length is less than 50 µm and the average grain width to length ratio is between 0.4 and 0.6.

With δ equal to 0.98% (corresponding to the column on the left and the column in the center in Fig. 5), the steady-state creep rate is given for two different values of relative density ρ (ρ=92%, which corresponds to the left column, ρ=97% , which corresponds to the column in the center). Thus, the relative density ρ of the substrate 2, which is more than 96%, makes it possible to limit the creep rate with respect to the relative density of the substrate 2, which is less than 96%. Indeed, the volume of the material loaded during creep decreases with decreasing density. With an external stress, the forces - on the scale of the microstructure - increase when the relative density decreases. Therefore, the creep life is reduced when the density decreases.

In FIG. 6 shows creep in Larson-Miller imaging for different types of substrates. The specific voltage is shown as a function of the Larson-Miller parameter. Curve (a) corresponds to a known nickel-based polycrystalline superalloy substrate. Curve (b) corresponds to the substrate containing the phase Ti ₃ AlC ₂ , while the criterion δ disproportion of the unit cell is equal to 0.17%. Curve (c) corresponds to the substrate 2 according to the invention, which contains a Ti ₃ AlC ₂ phase and a unit cell disproportion criterion δ of 0.98%. Substrate feature 2 corresponding to curve (c) has a specific voltage similar to that of the nickel-based substrate; on the other hand, the feature of the substrate corresponding to curve (b) is characterized, at a given value of the Larson-Miller parameter, by a specific voltage of a lower magnitude than the specific voltage of the substrate 2 corresponding to curve (c). Thus, the substrate 2 used in part 1 has a higher mechanical strength than the known substrate with a Ti ₃ AlC ₂ phase.

In FIG. 7 shows the change in weight gain of several substrates with different δ parameters during oxidation treatment. Oxidation was caused by cyclic thermal treatment of each substrate at a temperature from 100 to 1000°C while holding at a temperature of 1000°C for 1 hour for 240 cycles. Weight gain per unit surface in an amount of 90 to 140 mg/cm ² was measured on substrates with a δ parameter of less than 0.7%. On the other hand, substrates 2 with a δ parameter greater than 0.7% were characterized by essentially zero weight gain. Thus, the substrates 2 according to the invention had an oxidation resistance under the temperature conditions of the turbine, which was higher than that of the known substrates.

In FIG. 8 shows the effect of the mass fraction of titanium carbide on the weight gain of the substrates after oxidation treatment. The substrates were oxidized under the control of hundreds of thermal cycles in air. Each cycle corresponded to the heat treatment of the substrate in the range from 100 to 1000°C with a temperature increase of 5°C/min. followed by treatment of the substrate at a temperature of 1000°C for 1 hour, followed by cooling from 1000 to 100°C.

Three substrates were subjected to heat treatment. Each of them had a different mass fraction of titanium carbide (TiC): 1.1% (shown in the column on the left in Fig. 8), 0.4% (shown in the column in the middle of Fig. 8) and 0% (shown in the column on the right in Fig. eight). Thus, the oxidation of the substrate can be significantly reduced by reducing the mass fraction of TiC in the substrate. Preferably, the substrate 2 according to an aspect of the invention contains a TiC mass fraction of less than 0.8% so as to reduce oxidation caused by the temperatures at which the turbine operates.

As shown in FIG. 9, the manufacturing method of part 1 may include the following steps.

In step 101 of the method for manufacturing the part 1, powders containing titanium, aluminum and carbon to be compacted are mixed. TiC _>0.95 , aluminum and titanium powders can, for example, be mixed in the following atomic fractional concentration: 1.9 at. %, 1.05 at. % and 1 at. %. For example, powders can be homogenized using a Turbula (registered trademark) type mixer or any type of 3D mixer. Preferably, the atomic fraction of the mixed aluminum powder is strictly greater than 1, and is preferably 1.03 to 1.08. In fact, the evaporation of aluminum during the subsequent reaction sintering reduces the atomic fraction of aluminum in the part obtained at the end of the process. Therefore, the atomic fraction of aluminum in the amount of from 1.03 to 1.08 in step 101 allows you to get a stoichiometric compound. Thus, according to an aspect of the invention, the substrate contains phases containing iron and/or tungsten, wherein the sum of the average volume fraction of iron and tungsten in said phases is less than 2%.

In method step 102, reaction sintering of the powders mixed in step 101 is carried out. Reaction sintering can take place in a protective atmosphere for 2 hours at 1450°C.

In process step 103, the articles obtained in step 102 are reduced to a powder state, for example by grinding.

In method step 104, spark plasma sintering (or SPS sintering: short for Spark Plasma Sintering) is applied. Spark plasma sintering is carried out, for example, at a temperature of 1360° C. for two minutes at 75 MPa while controlling cooling at a rate of -50° C. min ^-1 . The temperature in step 104 during SPS sintering is preferably less than 1400°C. Indeed, spark plasma sintering at temperatures below 1400°C avoids the decomposition of the Ti ₃ AlC ₂ phase. In addition, spark plasma sintering at a temperature below 1400°C can prevent the interaction and/or contamination of the product at step 103 with the material forming the mold of the spark plasma sintering device containing, for example, graphite. The pressure in the spark plasma sintering step is preferably more than 60 MPa. Indeed, such a pressure, exceeding the pressure during sintering by known methods, makes it possible to manufacture a part 1 with a relative density of the Ti ₃ AlC ₂ phase of more than 96%, in which the average grain length 3 is less than 50 μm, and the average grain width to length ratio is from 0, 4 to 0.6. Preferably, the spark plasma sintering step uses heat treatment at maximum temperature for less than ten minutes. Thus, excessive growth and deterioration of the properties of the grains 3 in the substrate 2 are avoided. Preferably, the target cooling rate in this sub-step is less than 100° C. min ^-1 . Thus, the accumulation of residual mechanical stresses in the substrate 2 during the cooling sub-stage is excluded. Residual stresses are a problem in the manufacture of parts, as they cause cracking of the material, for example, during the machining of the substrate. Thus, the risks of cracking during machining are reduced, as well as when using the method according to an aspect of the invention.

The manufacture of part 1 in the manner described above makes it possible to obtain the properties of a stoichiometric material for the substrate and to avoid or limit the inclusion of compounds that impair mechanical strength or material performance due to oxidation. Thus, according to an aspect of the invention, the mass fraction of alumina in the substrate is less than 3%. According to another aspect of the invention, the substrate contains intermetallic compounds Ti _x Al _y , while the volume fraction of these compounds is less than 1%.

Claims (22)

1. Detail (1) of the turbine containing a polycrystalline substrate (2), while the substrate (2) contains grains (3) of the Ti ₃ AlC ₂ phase, and the mass fraction of this phase in the alloy is more than 97%, while each grain (3 ) has a length and width, characterized in that: - the average grain length is less than 50 microns, - the average ratio of the width to the length of the grains is from 0.4 to 0.6 and - the average unit cell volume of the Ti ₃ AlC ₂ phase is less than 152.4

. 2. Part (1) of the turbine according to claim 1, in which the substrate (2) contains titanium carbide, while the mass fraction of titanium carbide in the substrate is less than 0.8%. 3. Part (1) of the turbine according to claim 1 or 2, in which the substrate contains aluminum oxide, while the mass fraction of aluminum oxide in the substrate is less than 3%. 4. Detail (1) of the turbine according to any one of paragraphs. 1-3, in which the substrate (2) contains one of the intermetallic compounds Ti _x Al _y , while the volume fraction of these Ti _x Al _y compounds in the substrate is less than 1%. 5. Detail (1) of the turbine according to any one of paragraphs. 1-4, in which the substrate contains phases containing iron and/or tungsten, while the sum of the average volume fraction of iron and tungsten in these phases is less than 2%. 6. Detail (1) of the turbine according to any one of paragraphs. 1-5, in which the relative density of the Ti ₃ AlC ₂ phase exceeds 96%. 7. Turbine blade, characterized in that it contains a part (1) according to any one of paragraphs. 1-6. 8. The nozzle apparatus of the turbine, characterized in that it contains a part (1) according to any one of paragraphs. 1-6. 9. A turbine, characterized in that it contains a working blade according to claim 7 and / or a turbine nozzle apparatus according to claim 8. 10. A method for manufacturing a part (1) of a turbine, wherein the part (1) contains a polycrystalline substrate (2), and the substrate (2) contains grains (3) of the Ti ₃ AlC ₂ phase, while the mass fraction of this phase in the alloy exceeds 97% , while each grain (3) has a length and width, while the average length of grains (3) is less than 50 μm, and the average ratio of width to length is from 0.4 to 0.6, and the average unit cell volume of the Ti ₃ phase AlC ₂ is less than 152.4

, which is characterized by the fact that it includes: a) a step of mixing and homogenizing powders containing at least titanium, aluminum and carbon, b) stage of reaction sintering of powders, c) the step of bringing into a powder state the product of reaction sintering obtained in step b), d) stage of spark plasma sintering of the powder obtained in stage c). 11. The method according to claim 10, wherein the temperature in the spark plasma sintering step is less than 1400°C. 12. The method according to claim 10 or 11, wherein the pressure in the spark plasma sintering step exceeds 60 MPa. 13. The method according to claim 11 or 12, wherein the spark plasma sintering step is heat treated at a maximum temperature for less than 10 minutes. 14. The method according to any one of paragraphs. 11-13, in which the spark plasma sintering step comprises a cooling sub-step in which the cooling rate is less than 100° C. per minute. 15. The method according to any one of paragraphs. 11-14, in which step c) powdering the reaction sinter product obtained in step b) is carried out by grinding.

Images