EP2163656B1 - High-temperature-resistant cobalt-base superalloy - Google Patents

Abstract

A cobalt-base super alloy contains (in wt.%) tungsten (25-28, preferably 26), aluminum (3-8, preferably 3.4), tantalum (0.5-6, preferably 5.1), molybdenum (0-3, preferably 2.8), carbon (0.01-0.2, preferably 0.2 or 0.02), hafnium (0.01-0.1, preferably 0.1 or 0.02), boron (0.001-0.05, preferably 0.05 or 0.002), silicon (0.01-0.1, preferably 0.1 or 0.01) and remainder of cobalt, and unavoidable impurities.

Description

Technical area

The invention relates to the field of materials technology. It relates to a cobalt-based superalloy with a y / y microstructure, which has very good mechanical properties and good oxidation resistance at high operating temperatures up to about 1000 ° C.

State of the art

Cobalt or nickel based superalloys are known in the art.

In particular, components of nickel-base superalloys, in which a γ / γ precipitation hardening mechanism is usually used to improve the high-temperature mechanical properties, have very good material strength at high temperatures, but also very good corrosion and oxidation resistance and creep properties. Due to these properties, when using such materials z. As in gas turbines, the inlet temperature of the gas turbine can be increased, whereby the efficiency of the gas turbine plant increases.

On the other hand, many cobalt base superalloys are solidified by carbide precipitates and / or solid solution hardening due to alloying of refractory elements, resulting in lower high temperature strength compared to the γ / γ nickel base superalloys. By secondary carbide precipitations in the temperature range of about 650 - 927 ° C, the ductility is also severely degraded. However, cobalt-base superalloys often have an advantage over nickel-base superalloys, improved hot corrosion resistance, and higher resistance to oxidation and wear.

For turbine applications, various cobalt-base casting alloys are commercially available, such as MAR-M302, MA-M509 and X-40, which have a comparatively high chromium content and, for. T. are alloyed with nickel. The nominal composition of these alloys is shown in Table 1. Table 1: Nominal composition of known commercial cobalt base superalloys Ni Cr Co W Ta Ti Mn Si C B Zr M303 - 21.5 58 10 9.0 - - - 0.85 0005 0.2 M509 10.0 23.5 55 7 3.5 0.2 - - 0.60 - 0.5 X-40 10.5 25.5 54 5.5 - - 0.75 0.75 12:50 - -

However, the mechanical properties, in particular the creep resistance, of these cobalt-base superalloys require improvement.

Recently, cobalt-based superalloys having a predominantly γ / γ 'microstructure have also become known which exhibit improved high-temperature strength compared with the abovementioned commercial cobalt-base superalloys.

• 27.6 Ni,
• 12.9 Ti,
• 8.7 Cr,
• 0.8 Mo,
• 2.6 Al,
• 0.2 W und
• 47.2 Co.

( D.H. Ping et al: Microstructural Evolution of a Newly Developed Strengthened Co-base Superalloy, Vacuum Nanoelectronics Conference, 2006 and the 50th International Field Emission Symposium., IVNC/IFES 2006, Technical Digest. 19th International Volume, Issue, July 2006, Pages 513-514 ).Such a known cobalt-base superalloy consists of (data in At.%):

• 27.6 Ni,
• 12.9 Ti,
• 8.7 Cr,
• 0.8 Mo,
• 2.6 Al,
• 0.2W and
• 47.2 Co.

( DH Ping et al: Microstructural Evolution of a Newly Developed Strengthened Co-base Superalloy, Vacuum Nanoelectronics Conference, 2006 and the 50th International Field Emission Symposium., IVNC / IFES 2006, Technical Digest. 19th International Volume, Issue, July 2006, Pages 513-514 ).

Also in this alloy are relatively high levels of chromium and nickel, and additionally contain titanium. The structure of this alloy consists mainly of the typical γ / γ structure with a hexagonal (Co, Ni) ₃ Ti compound with plate-like morphology, the latter having a negative influence on the high temperature properties and therefore the use of such alloys at temperatures below 800 ° C is limited.

In addition, Co-Al-W-based γ / γ superalloys have become known ( Akane Suzuki, Garret C. De Nolf, and Tresa M. Pollock: High Temperature Strength of Co-based γ / γ'-Superalloys, Mater. Res. Soc. Symp. Proc. Vol. 980, 2007, Materials Research Society ). The alloys studied there have 9 At. -% Al and 9-11 at. -% W, where optionally 2 At. -% Ta or 2 At. -% Re were added. From this document it can be seen that the addition of Ta to a ternary Co-Al-W alloy stabilizes the γ'-phase and there it is described that the ternary system (ie without Ta) produces approximately cubic γ'-precipitates with ca. 150 and 200 nm edge length, while in the alloy, the additionally 2 At. - contains% Ta, the structure has cube-shaped γ'-precipitates with approximately 400 nm edge length.

Presentation of the invention

The aim of the invention is to avoid the mentioned disadvantages of the prior art. The invention is based on the object to develop a cobalt-based superalloy, which has improved mechanical properties and good oxidation resistance especially at high temperatures up to about 1000 ° C. The alloy should also be advantageous for the production of monocrystalline components.

• 25-28 W,
• 3-8 Al,
• 0.5-6 Ta,
• 0-3 Mo,
• 0.01-0.2 C,
• 0.01-0.1 Hf,
• 0.001-0.05 B,
• 0.01-0.1 Si,

Rest Co und herstellungsbedingte VerunreinigungenAccording to the invention, this object is achieved in that the cobalt-base superalloy has the following chemical composition (in% by weight):

• 25-28 W,
• 3-8 Al,
• 0.5-6 Ta,
• 0-3 mo,
• 0.01-0.2 C,
• 0.01-0.1 Hf,
• 0.001-0.05 B,
• 0.01-0.1 Si,

Residual Co and manufacturing impurities

The alloy consists of a cubic face-centered γ-Co matrix phase and a high volume fraction of γ'-phase Co ₃ (Al, W), which is stabilized by Ta. The γ'-precipitates are very stable and lead to a material consolidation, which has a positive effect on the properties (creep properties, oxidation behavior), especially at high temperatures. This Co-superalloy has neither Cr nor Ni, but a relatively high proportion of W. This high proportion of tungsten (25-28% by weight) causes the γ'-phase to be further strengthened, thus improving the creep properties become. W adjusts the grid offset between the γ matrix and the γ 'phase, with a small lattice offset allowing the formation of a coherent microstructure.

Ta also acts as a precipitation hardener. 0.5 to 6% by weight of Ta, preferably 5.0-5.4% by weight of Ta, should be added. Ta increases the high-temperature strength. If more than 6% by weight of Ta is adjusted, the adverse oxidation resistance will be lowered.

The alloy contains 3-8% by weight of Al, preferably 3.1-3.4% by weight of Al. Thus, an Al ₂ O ₃ protective film is formed on the material surface, which enhances the high-temperature oxidation resistance.

B is an element which is available in small amounts from 0.001 to max. 0.05 wt.% Solidifies the grain boundaries of the cobalt base superalloy. Higher boron contents are critical, as they can lead to undesirable boron precipitations, which have an embrittling effect. In addition, B lowers the melting temperature of the Co alloy, so B contents above 0.05 wt% are not meaningful. The interaction of boron in the specified range with the other constituents, in particular with Ta, leads to good strength values.

Mo is a solid solution in the cobalt matrix. Mo affects the lattice offset between the γ-matrix and the γ'-phase and thus also the morphology of γ 'under creep stress.

C is in the specified range from 0.01 to max. 0.2% by weight is useful for carbide formation, which in turn increases the strength of the alloy. C also acts as a grain boundary consolidator. If more than 0.2% by weight of carbon is present, this disadvantageously leads to embrittlement.

Hf (in the specified range of 0.01-0.1% by weight) mainly solidifies the γ matrix and thus contributes to increasing the strength. In addition, Hf in combination with 0.01-0.1% by weight of Si has a favorable effect on the oxidation resistance. If the mentioned ranges are exceeded, this disadvantageously leads to embrittlement of the material.

If C, B, Hf and Si are at the lower limit of said ranges, it is advantageously possible to produce single crystal alloys, which leads to a further improvement in the properties of the Co alloys, in particular with regard to their use in gas turbines (high stress in Reference to temperature, oxidation, corrosion).

On the whole, the cobalt-base superalloy according to the invention, owing to its chemical composition (combination of the specified elements in the ranges indicated), has excellent properties at high temperatures up to about 1000 ° C., in particular good creep rupture strength, ie. H. good creep properties, and extremely high oxidation resistance.

Brief description of the drawings

Fig. 1

ein Gefügebild der erfindungsgemässen Legierung Co-1;

Fig. 2

die Abhängigkeit der Streckgrenze σ_0.2 der Legierung Co-1 und bekannter Vergleichslegierungen von der Temperatur im Bereich von Raumtemperatur bis ca. 1000 °C;

Fig. 3

die Abhängigkeit der Zugfestigkeit σ_UTS der Legierung Co-1 und bekannter Vergleichslegierungen von der Temperatur im Bereich von Raumtemperatur bis ca. 1000 °C;

Fig. 4

die Abhängigkeit der Bruchdehnung ε der Legierung Co-1 und bekannter Vergleichslegierungen von der Temperatur im Bereich von Raumtemperatur bis ca. 1000 °C und

Fig. 5

die Abhängigkeit der Spannung σ der erfindungsgemässen Legierungen Co-1, Co-4 und Co-5 und der bekannten Vergleichslegierung Mar-M509 vom Larson Miller Parameter.

In the drawings, embodiments of the invention are shown. Show it:

Fig. 1

a micrograph of the inventive alloy Co-1;

Fig. 2

the dependence of the yield strength σ _{0.2 of} the alloy Co-1 and known comparative alloys of the temperature in the range of room temperature to about 1000 ° C;

Fig. 3

the dependence of the tensile strength σ _{UTS of} the alloy Co-1 and known comparative alloys of the temperature in the range of room temperature to about 1000 ° C;

Fig. 4

the dependence of the elongation at break ε of the alloy Co-1 and known comparative alloys of the temperature in the range of room temperature to about 1000 ° C and

Fig. 5

the dependence of the stress σ of the inventive alloys Co-1, Co-4 and Co-5 and the known comparative alloy Mar-M509 Larson Miller parameters.

Ways to carry out the invention

The invention will be explained in more detail with reference to embodiments and the drawings.

There have been known the prior art commercial cobalt base superalloys Mar-M302, Mar-M509 and X-40 (composition see Table 1) and the Co-Al-W-Ta-γ / γ 'superalloy known in the literature with 9 at. -% Al, 10 At. -% W and 2 At. % Ta, balance Co, as well as the alloys according to the invention listed in Table 2 with regard to their mechanical properties at high temperatures.

The alloy constituents of the alloys Co-1 to Co-5 according to the invention are indicated in the table in% by weight: Table 2: Compositions of the investigated alloys according to the invention Co W al Ta C Hf Si B Not a word Co-1 rest 26 3.4 5.1 0.2 0.1 0.1 12:05 - Co-2 rest 27.25 8th 5.2 0.2 0.1 0.1 12:05 - Co-3 rest 26 3.4 0.5 0.2 0.1 12:05 12:05 2.8 Co-4 rest 25.5 3.1 5 0.2 0.1 12:05 12:05 - Co-5 rest 25.5 3.1 5.2 0.2 0.1 12:05 12:05 -

The comparison alloys Mar-M302, Mar-M509 and X-40 were investigated in the as cast state.

• Lösungsglühen bei 1200 °C/15 h unter Schutzgas/Luftabkühlung und
• Glühen bei1000 °C/72 h unter Schutzgas/Luftabkühlung (Ausscheidungsbehandlung).

The alloys according to the invention were subjected to the following heat treatment:

• Solution annealing at 1200 ° C / 15 h under inert gas / air cooling and
• Annealing at 1000 ° C / 72 h under inert gas / air cooling (precipitation treatment).

In Fig. 1 For the alloy Co-1 according to the invention, the microstructure obtained in this way is shown. The fine distribution of the precipitated γ'-phase in the γ-matrix can be seen very well. These γ'-precipitates are very similar to the γ'-phase, which is typical of nickel-base superalloys. It can be expected that the γ 'precipitates in this cobalt base superalloy are more stable than those in the nickel base superalloys. This is due to the presence of tungsten in the form of Co ₃ (Al, W), which has a low diffusion coefficient.

Fig. 2 shows the curve of the yield strength σ ₀₂ for the inventive alloy Co-1 as a function of the temperature in the range of room temperature to about 1000 ° C. The results for the in Table 1 The commercial comparative alloys listed and the known from the literature Co-Al-W-Ta alloy are also in Fig. 2 shown

The yield strength σ _{0.2 of} the alloy Co-1 is higher than the yield strength σ _{0.2 of} the three comparative commercial alloys over the entire temperature range studied, the difference being particularly pronounced at temperatures> 600 ° C. In the range of about 700-900 ° C, the yield strength of the cobalt-base superalloy Co-1 is about twice as large as the yield strength of the best known commercial alloy M302 investigated here. Although the known from the literature Co-Al-W-Ta alloy in the higher temperature range from about 650 ° C in relation to the yield strength σ _0.2 is superior to the commercial comparative alloys, so significantly improved values can be achieved with the present inventive alloy. This is mainly because, in addition to the already described advantages of the γ / γ'-structure of the cobalt base superalloys in the alloys of the invention by the additionally present elements C, B, Hf, Si and optionally Mo, additional solidification mechanisms (precipitation, grain boundaries -, solid solution hardening) come to fruition.

In Fig. 3 the dependence of the tensile strength σ _{US of} the alloy Co-1 and the known comparative alloys described in Table 1 from the temperature in the range from room temperature to about 1000 ° C is shown. In the temperature range from RT to about 600 C, the known superalloy M302 has the highest tensile strength values, from about 600 ° C, the inventive cobalt-based superalloy Co-1 is significantly better. At 900 ° C, the tensile strength of Co-1 is about twice as high as the tensile strength of M302 and even about 2.5 times as high as the tensile strength of M509 and X-40, respectively. This is due, on the one hand, to the finely divided γ'-phase, which solidifies the microstructure, and, on the other hand, to the additional solidification by the alloying elements C, B, Hf, Si. However, this is at the expense of breaking elongation, as from Fig. 4 can be seen.

In Fig. 4 is the dependence of the elongation at break ε of the alloy Co-1 and known comparative alloys of the temperature in the range of room temperature to about 1000 ° C shown. While at RT the elongation at break for alloy Co-1 is still above the values for the commercial alloys M509 and X-40, it is much lower at higher temperatures. The best elongation at break shows the alloy M302 in almost the entire temperature range investigated.

mit

• T: Temperatur in °K
• t: Zeit in Stunden.

Fig. 5 shows the dependence of the stress σ for the novel alloys Co-1, Co-4 and Co-5 and for the known comparative alloy Mar-M509 from the Larson-Miller parameter PLM, which describes the influence of aging time and temperature on the creep behavior. The Larson Miller parameter PLM is calculated as follows: $$\mathrm{PLM}=\mathrm{<figure-callout\; id="20"\; label="T"\; filenames="imgf0004.png"\; state="\{\{state\}\}">T</figure-callout>}\left(20+\mathrm{log\; t}\right){10}^{-3}$$

With

• T: temperature in ° K
• t: time in hours.

As outsourcing times were in Fig. 5 each used the break times. With a comparable Larson-Miller parameter, the alloys Co-1, Co-4 and Co-5 according to the invention endure consistently higher stresses than the comparative alloy, ie they have improved creep properties, due to the precipitation of the γ'-phase and the associated solidification and the additional above-mentioned solidification mechanisms is due.

From the cobalt-base superalloys according to the invention can advantageously be high-temperature components for gas turbines, such as blades, z. As guide vanes, or produce heat shields. Due to the good creep properties of the material, these components are particularly suitable for use at very high temperatures.

Of course, the invention is not limited to the embodiments described above. In particular, it is also advantageous to produce single-crystal components from the cobalt-base superalloys, specifically when the C and B contents (B and C are grain boundary strengtheners), but also the Hf and Si contents in comparison to those described above Examples are reduced and selected weight proportions, which are rather at the lower limit of the ranges given in claim 1 for these elements.

• 26 W, 3.4 Al, 5.1 Ta, 0.02 C, 0.02 Hf, 0.002 B, 0.01 Si, Rest Co und herstellungsbedingte Verunreinigungen.

This leads to a further improvement of the properties. An example of such a Co-based single crystal superalloy is an alloy having the following chemical composition (% by weight):

• 26 W, 3.4 Al, 5.1 Ta, 0.02 C, 0.02 Hf, 0.002 B, 0.01 Si, balance Co and manufacturing-related impurities.

• 0.01-0.03, vorzugsweise 0.02 C,
• 0.01-0.02, vorzugsweise 0.02 Hf,
• 0.001-0.003, vorzugsweise 0.002 B,
• 0.01-0.02, vorzugsweise 0.01 Si.

For single-crystal superalloys based on Co-W-Al-Ta, according to claim 1, the following ranges (indicating in% by weight) of the additional doping elements are advantageously to be selected:

• 0.01-0.03, preferably 0.02 C,
• 0.01-0.02, preferably 0.02 Hf,
• 0.001-0.003, preferably 0.002 B,
• 0.01-0.02, preferably 0.01 Si.

Claims (17)

1. Cobalt-base superalloy characterized by the following chemical composition (in % by weight): 25-28 W,

   3-8 Al,

   0.5-6 Ta,

   0-3 Mo,

   0.01-0.2 C,

   0.01-0.1 Hf,

   0.001-0.05 B,

   0.01-0.1 Si,

   remainder Co and unavoidable impurities.
2. Cobalt-base superalloy according to Claim 1, characterized by 25.5-27.25, preferably 25.5-26% by weight W.
3. Cobalt-base superalloy according to Claim 1, characterized by 3.1-3.4% by weight Al.
4. Cobalt-base superalloy according to Claim 1, characterized by 5-6, preferably 5.0-5.3% by weight Ta.
5. Cobalt-base superalloy according to Claim 1, characterized by 2.8% by weight Mo.
6. Cobalt-base superalloy according to Claim 1, characterized by 0.2% by weight C.
7. Cobalt-base superalloy according to Claim 1, characterized by 0.01-0.03, preferably 0.02% by weight C.
8. Cobalt-base superalloy according to Claim 1, characterized by 0.1% by weight Hf.
9. Cobalt-base superalloy according to Claim 1, characterized by 0.01-0.02, preferably 0.02% by weight Hf.
10. Cobalt-base superalloy according to Claim 1, characterized by 0.05% by weight B.
11. Cobalt-base superalloy according to Claim 1, characterized by 0.001-0.003, preferably 0.002% by weight B.
12. Cobalt-base superalloy according to Claim 1, characterized by 0.1% by weight Si.
13. Cobalt-base superalloy according to Claim 1, characterized by 0.05% by weight Si.
14. Cobalt-base superalloy according to Claim 1, characterized by 0.01-0.02, preferably 0.01% by weight Si.
15. Cobalt-base superalloy according to Claim 1, characterized by the following chemical composition (in % by weight):

    26 W,

    3.4 Al,

    5.1 Ta,

    0.2 C,

    0.1 Hf,

    0.05 B,

    0.1 Si,

    remainder Co and unavoidable impurities.
16. Cobalt-base superalloy in the form of a single-crystal alloy according to Claim 1, characterized by the following chemical composition (in % by weight):

    26 W,

    3.4 Al,

    5.1 Ta,

    0.02 C,

    0.02 Hf,

    0.002 B,

    0.01 Si,

    remainder Co and unavoidable impurities.
17. Use of the cobalt-base superalloy according to one of Claims 1-16 for producing a gas turbine component, preferably a blade or vane or a heat shield.

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