EP3175008B1 - Cobalt based alloy - Google Patents

Description

The invention relates to polycrystalline precipitation hardened and oxidation resistant γ / γ 'cobalt base superalloys for high temperature applications. The mechanical properties of the specified cobalt-based superalloys exceed those of conventional carbide-hardened cobalt alloys. Up to a temperature of 800 ° C similar and at temperatures above 800 ° C even higher hot strengths than the nickel-based γ / γ 'forging alloys are achieved. The creep strengths are also significantly higher. In comparison to γ / γ 'nickel-base superalloys, similar proportions of the γ' precipitation phase are achieved despite the low solvus temperature. Due to the large temperature range between the solidus and solvus temperatures, the precipitation-hardened γ / γ 'cobalt-base superalloys are particularly suitable as polycrystalline forging alloys.

Cobalt base and especially γ / γ 'nickel base superalloys are essential materials for a variety of components in jet engines of commercial aircraft or in stationary gas turbines for power conversion. Efforts to increase the efficiency of these turbines, reduce costs and reduce fossil fuel consumption can all be achieved through new materials that offer higher temperature resistance, longer service life and lower manufacturing and processing costs.

Conventional cobalt-base superalloys are used as high-temperature materials in aircraft engines and stationary gas turbines due to their high melting point, high wear resistance, good weldability and, in particular, their excellent hot gas corrosion and sulfidation resistance (see, for example, US Pat. Bürgel, Maier, Niendorf, Handbuch Hochtemperaturwerkstofftechnik, 4th revised edition 2011, Vieweg + Teubner Verlag, Springer Fachmedien Wiesbaden GmbH 2011 ).

However, because they are mixed crystal and carbide-cured, they are used only for light-weight and static components such as vanes because of their lower high-temperature strength than the precipitation-hardened γ / γ 'nickel-base superalloys. As materials for blades or turbine disks they are thus not used. With the discovery of the intermetallic γ'-phase Co ₃ (Al, W) with an L1 ₂ -crystal structure in the ternary Co-Al-W system in 2006, higher-strength, precipitation-hardened, biphasic γ / γ 'superalloys can now be produced on the basis of cobalt (γ: cubic face-centered cobalt mixed crystal) with the same microstructure as the γ / γ 'nickel-base superalloys used for decades are produced, as for example in Sato et al., Cobalt Base High-Temperature Alloys, Science 312 (2006) 90-91 is described. Compared to the polycrystalline γ / γ 'nickel-base superalloys, these have decisive advantages.

γ / γ 'Cobalt base superalloys generally have a very high solidus temperature in the temperature range of 1300 ° C to 1450 ° C in conjunction with a relatively low γ' solvus temperature in the temperature range of 900 ° C to 1150 ° C. Despite the relatively low γ 'solvus temperature, very high γ' volume fractions of more than 75% can be realized at temperatures up to 900 ° C (see, for example, US Pat. B. Bauer et al., Microstructure and creep strength of different γ / γ'-enhanced co-base superalloy variants, Scripta Materialia 63 (2010) 1197-1200 ). The ternary γ / γ 'cobalt-base superalloys Co-9Al-9W (in atomic%), for example, despite a γ' solvus temperature of about only 975 ° C has a relatively high γ 'excretion volume fraction of 58%. Owing to the large temperature range between solidus and γ 'solvus temperature (forging window), the comparatively low γ' solvus temperatures and the high γ 'volume fraction at application temperatures, the γ / γ' cobalt base superalloys are thus suitable in particular as forging alloys. Nickel base superalloys, in comparison, either have a low γ 'solvus temperature below 1100 ° C, combined with a low γ' volume fraction at use temperatures of up to 700 ° C (eg, Waspaloy: γ 'solvus temperature: 1038 ° C ( Semiatin et al., Deformation behavior of Waspaloy at hot-working temperatures, Scripta Materialia 50 (2004) 625-629 ); γ 'volume fraction at application temperature: 25% ( ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys, Ed. Davies et al, ASM International, Materials Park, OH 44073, USA ) or a high γ 'volume fraction at 700 ° C in conjunction with a significantly higher γ' solvus temperature (eg Udimet 720Li: γ 'solvus temperature: 1142 ° C; γ' volume fraction at application temperature: 45% ( Gu et al., Development of Ni-Co base alloys for high-temperature disk applications, Superalloys 2008, Ed. Roger C. Reed et al., The Minerals, Metals and Materials Society, Warrendale, PA, USA )). This has the consequence that the alloys are either malleable, but have a lower strength or that they still have a relatively high proportion of the precipitation phase at forging temperatures of 1000 ° C to 1150 ° C and thus difficult or no longer deformable are and can only be processed powder metallurgy. This significantly increases the costs.

Further, cobalt-based alloys are known to have higher hot-gas corrosion resistance than nickel-base alloys, since a liquid co-sulfur phase can occur only at 877 ° C, whereas a liquid Ni-S phase already occurs at 637 ° C ( please refer Bürgel, Maier, Niendorf, Handbook High Temperature Materials Technology, 4th Revised Edition 2011, Vieweg + Teubner Verlag, Springer Fachmedien Wiesbaden GmbH 2011 or ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys, Ed. Davies et al, ASM International, Materials Park, OH 44073, USA ). Increased hot gas corrosion resistance can thus lead to a lifetime extension. In addition, although pure cobalt shows less corrosion resistance than pure nickel in sulfuric acid, according to the literature in cobalt, only 10% by weight of chromium is necessary for passivation, whereas nickel requires 14% by weight of chromium (see, for example, US Pat ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys, Ed. Davies et al, ASM International, Materials Park, OH 44073, USA ).

In addition, it has been shown in recent years that with increased Co content in nickel-based forging alloys with γ / γ 'microstructure, the stacking fault energy decreases and thereby the twin density in the material increases, which leads to an additional hardening effect in the polycrystalline forging alloys and thus higher heat resistance can be achieved (see, eg Yuan et al., A new method for stronger turbine superalloys at service temperatures Scripta Mat. 66 (2012) 884-889 ). It is to be expected that cobalt-based forging alloys have an even higher twin density, so that this hardening effect can be further increased.

Despite increased research activities in the field of this new class of γ / γ 'cobalt base superalloys, only simple alloys with relatively few alloying elements and insufficient oxidation resistance have so far been developed and investigated. Cobalt base superalloys are known per se, for example Titus et al., "Creep and directional coarsening in single crystals of new γ-γ 'cobalt-base alloys", Scripta Mat. 66 (2012) 574-577 . US 2011/0268989 A1 . US 2010/0291406 A1 . EP 2 251 446 A1 . CA2 620 606 A1 . EP 1 925 683 A1 . US 2008/0185078 A1 . EP 2 163 656 A1 . US 2011/0062214 A1 . EP 2 298 486 A2 . EP 2 383 356 A1 . EP 2 045 345 A1 and EP 2 532 762 A1 ,

However, a good oxidation resistance in combination with good mechanical properties is essential to be able to use these new γ / γ 'cobalt base superalloys as a high-temperature material in the future.

The object of the invention is the development of polycrystalline, higher-strength, precipitation-hardened γ / γ 'cobalt-base superalloys, with very good oxidation properties, which can be processed by means of various forming processes, such as forging.

This object is achieved according to the invention by a cobalt-base superalloy comprising 32-45% by weight of Co, 28-40% by weight of Ni, 10-15% by weight of Cr, 2.5-5.5% by weight of Al, 6 , 5-16% by weight W, 0-9% by weight Ta, 0-8% by weight Ti, 0.1-1% by weight Si, 0-0.5% by weight B, 0-0.5 wt% C, 0-2 wt% Hf, 0-0.1 wt% Zr, 0-8 wt% Fe, 0-6 wt% Nb, 0 7 wt% Mo, 0-4 wt% Ge and a balance of unavoidable impurities.

In an advantageous variant, the cobalt-base superalloy comprises 32-45% by weight of Co, 28-40% by weight of Ni, 10-15% by weight of Cr, 2.5-5.5% by weight of Al, 6.5 -16 wt% W, 0-9 wt% Ta, 0-8 wt% Ti, 0.1-1 wt% Si, 0-0.5 wt% B, 0-0 , 5% by weight C, 0 to <2% by weight Hf, 0 to <0.1% by weight Zr, 0 to <8% by weight Fe, 0 to <6% by weight Nb, 0 to <7 wt .-% Mo, 0 to <4 wt .-% Ge and a balance of unavoidable impurities.
Advantageously, from the cobalt-base superalloy in another embodiment, 32-45 wt.% Co, 28-40 wt.% Ni, 10-15 wt.% Cr, 2.5-5.5 wt.% Al, 6 , 5-16 wt.% W, 0.2-9 wt.% Ta, 0.2-8 wt.% Ti, 0.1-1 wt.% Si, <0.5 wt. % B, <0.5 wt% C, 0-2 wt% Hf, 0-0.1 wt% Zr, 0-8 wt% Fe, 0-6 wt% Nb , 0-7 wt% Mo, 0-4 wt% Ge and a balance of unavoidable impurities.
In a further preferred embodiment, the cobalt base superalloy comprises 32-45 wt.% Co, 28-40 wt.% Ni, 10-15 wt.% Cr, 2.5-5.5 wt.% Al, 6, 5-16 wt% W, 0.2-9 wt% Ta, 0.2-8 wt% Ti, 0.1-1 wt% Si, <0.5 wt% B, <0.5 wt .-% C, 0 to <2 wt .-% Hf, 0 to <0.1 wt .-% Zr, 0 to <8 wt .-% Fe, 0 to <6 wt. -% Nb, 0 to <7 wt .-% Mo, 0 to <4 wt .-% Ge and a balance of unavoidable impurities.

Advantageously, the cobalt base superalloy comprising an aforesaid composition characterized by an intermetallic γ 'phase of the composition (Co, Ni) ₃ (Al, W, Ti, Ta), each clip containing at least one of the elements listed in parentheses , Advantageously, the intermetallic γ 'phase (precipitation phase) is contained with a volume fraction of more than 35%, preferably of more than 45%.
These oxidation resistant, precipitation hardened cobalt base superalloys, characterized by their composition, are composed of a variety of alloying elements. The reasons for the selected concentration ranges The alloying elements and their essential effects are described below:

Necessary alloying elements: Co (cobalt): 32-45% by weight

Co forms the cubic face-centered γ matrix phase as a basic element among other elements and is an important constituent of the hardening γ '- (Co, Ni) ₃ (Al, W, Ti, Ta) precipitation phase. Co also lowers the stacking fault energy.

Ni (nickel): 28-40% by weight

Ni in the specified range expands the γ / γ 'two-phase region to a sufficient extent, so that further alloying elements, in particular Cr, can be added to a sufficient extent. Cr contents from about 4% by weight destabilize the biphasic γ / γ 'microstructure in ternary Co-Al-W alloys, and further undesirable intermetallic phases are formed. Ni shifts the maximum possible concentration of Cr to higher concentrations. Furthermore, with Ni, the γ 'solvus temperature can be increased.

Cr (chromium): 10-15% by weight

In order to obtain a sufficient corrosion and oxidation resistance, the alloying element Cr should be added to the specified range. In addition, Cr acts as a mixed crystal hardener.

Al (aluminum): 2.5-5.5% by weight

Al forms the γ 'precipitation phase (Co, Ni) ₃ (Al, W, Ti, Ta), which contributes significantly to the increase in strength. Furthermore, Al increases the oxidation resistance. Higher levels of Al in the specified composition range can lead to the formation of additional intermetallic phases, such as CoAl, which can limit grain growth in forging alloys. As a result, smaller particle sizes and thus higher strengths can be achieved.

W (Tungsten): 6.5-16 wt%

W forms the γ 'precipitation phase Co ₃ (Al, W), which contributes decisively to the increase in strength and increases the creep resistance as a slowly diffusing element. Higher contents lead to too high a density and further undesirable intermetallic phases can form.

Si (silicon): 0.1-1% by weight

Si is a crucial element and significantly improves the oxidation resistance. However, excessive amounts of Si can lead to further undesirable intermetallic phases.

B (boron): <0.5% by weight

B acts as a grain boundary strengthening alloying element and improves the oxidation properties. Too high concentrations lead to too high a proportion of borides. Preferably, B is contained at more than 0.01% by weight.

C (carbon): <0.5% by weight

C acts as a grain boundary strengthening alloying element. In addition, C forms carbides. C is preferably contained with more than 0.01% by weight.

Required for high heat resistance: Ta (Tantalum): 0.2-9% by weight

Ta contributes to the formation of the γ 'precipitation phase, increases the γ' solvus temperature and the γ / γ 'lattice mismatch. Ta hardens the γ 'precipitation phase and leads to an increase in strength. In particular, when high toughness at 800 ° C is required, the two elements Ta and Ti are required.

Ti (titanium): 0.2-8 wt%

Ti contributes to the formation of the γ 'precipitation phase, increases the γ' solvus temperature and the γ / γ 'lattice mismatch. Ti hardens the γ 'precipitation phase and leads to an increase in strength. In particular, when high toughness at 800 ° C is required, the two elements Ta and Ti are required. Ti can largely replace W, thereby significantly reducing the density.

Optional alloy elements: Hf (hafnium): <2% by weight

Hf stabilizes the γ 'excretion phase. Preferably, Hf is contained at more than 0.2% by weight.

Zr (zirconium): <0.1% by weight

Zr serves to increase the grain boundary strength and to stabilize the γ 'precipitation phase. Preferably, Zr is included at more than 0.01% by weight.

Fe (iron): <8% by weight

Fe lowers the γ 'solvus temperature and can be used to adjust this especially for forging alloys. Fe is also a low cost element and can improve weldability. Too high concentrations destabilize the γ / γ 'microstructure. Preference is given to containing Fe more than 0.1% by weight.

Nb (niobium): <6% by weight

Nb contributes to the formation of the γ 'precipitation phase, leads to an increase in strength and increases the γ' solvus temperature. Higher concentrations within the given concentration range may lead to the formation of additional intermetallic phases which may limit grain growth in forging alloys. As a result, smaller particle sizes and thus higher strengths can be achieved. Preferably, Nb is contained at more than 0.1% by weight.

Mo (molybdenum): <7% by weight

Mo serves as a solid-solution-hardening element and can partially replace W, thereby decreasing the density. Higher concentrations lead to the formation of additional intermetallic phases, which can limit grain growth in forging alloys. As a result, smaller particle sizes and thus higher strengths can be achieved. Preferably, Mo is contained with more than 0.1 wt .-%.

Ge (germanium): <4% by weight

Ge forms the γ 'precipitation phase Co ₃ (Al, Ge, W), lowers the γ' solvus temperature and can be used to adjust this in particular for forging alloys. Ge preferably contains more than 0.1% by weight.

Fig.1

in einer Grafik den Zusammenhang zwischen dem Ausscheidungsanteil bei Anwendungstemperatur und der Solvustemperatur der γ'-Phase von γ/γ' Nickelbasissuperlegierungen im Vergleich zu Ausführungsbeispielen der Erfindung,

Fig. 2

die Mikrostruktur von beispielhaften Legierungen der Erfindung,

Fig. 3

eine EBSD-Messung zur Bestimmung der Korngröße und der Zwillingsdichte einer beispielhaften Legierung der Erfindung,

Fig. 4

in einer Grafik die Streckgrenze in Abhängigkeit der Temperatur für beispielhafte Legierungen der Erfindung,

Fig. 5

in einer Grafik die Kriechfestigkeit einer beispielhaften Legierung der Erfindung im Vergleich zu Nickelbasissuperlegierungen,

Fig. 6

Mikrostrukturbilder der ternären Legierung Co9Al9W im Vergleich einer beispielhaften Legierung der Erfindung,

Fig. 7

die Elementverteilung der Oxidschicht einer beispielhaften Legierung der Erfindung,

Fig. 8

REM-Aufnahmen einer beispielhaften Legierung der Erfindung,

Fig. 9

REM- und TEM-Aufnahmen einer beispielhaften Legierung der Erfindung und

Fig. 10

in einer Grafik die Streckgrenze in Abhängigkeit der Temperatur für eine weitere beispielhafte Legierung der Erfindung.

Embodiments of the invention are explained in more detail by a drawing and by the following information. Showing:

Fig.1

in a graph, the relationship between the precipitation rate at the application temperature and the solvus temperature of the γ'-phase of γ / γ 'nickel-base superalloys compared to embodiments of the invention,

Fig. 2

the microstructure of exemplary alloys of the invention,

Fig. 3

an EBSD measurement to determine the grain size and the twin density of an exemplary alloy of the invention,

Fig. 4

in a graph, the yield strength versus temperature for exemplary alloys of the invention,

Fig. 5

FIG. 4 is a graph of creep resistance of an exemplary alloy of the invention compared to nickel base superalloys; FIG.

Fig. 6

Microstructural images of the ternary alloy Co9Al9W compared to an exemplary alloy of the invention,

Fig. 7

the element distribution of the oxide layer of an exemplary alloy of the invention,

Fig. 8

SEM images of an exemplary alloy of the invention,

Fig. 9

REM and TEM images of an exemplary alloy of the invention and

Fig. 10

in a graph, the yield strength versus temperature for another exemplary alloy of the invention.

The compositions of some embodiments of the γ / γ 'cobalt base superalloys of the present invention, hereinafter referred to as CoWAlloy®, CoWAlloy1, and CoWAlloy2, as well as some reference alloys, are set forth in Table 1 below. Likewise, the properties of exemplary embodiments of the invention will be described in more detail below with reference to the figures and investigations. Table 1: Compositions of the γ / γ 'cobalt base superalloys described here CoWAlloy0, CoWAlloy1 and CoWAlloy2 as well as some polycrystalline, cobalt and nickel based reference alloys (% by weight). Co-based Co Ni al Cr W Ta Ti Hf Zr Si B C CoWAlloy 0 39.8 28.8 2.7 12.8 9.0 4.4 2.0 0.3 0.02 0.2 0,014 0.016 CoWAlloy 1 40.6 30.6 2.7 10.2 9.0 4.4 2.0 0.3 0.02 0.2 0,014 0.016 CoWAlloy 2 39.2 30.5 4.0 10.1 14.9 0.6 0.2 0.3 0.02 0.2 0,014 0.016 Co9Al9W Bal. - 3.6 - 24.6 - - - - - 0.06 - MarM 509 Bal. 10 - 24 7 3.5 0.2 - 0.5 - - 0.6 Ni-base Co Ni al Cr Not a word Ta Ti Hf Zr Si B C waspaloy 13.5 Bal. 1.3 19.5 4.3 - 3.0 - - - 0,006 0.08 Udimet 720Li 15.0 Bal. 2.5 16.0 3.0 - 5.0 - 0.05 - 0,018 0,025

Microstructure and Mechanical Properties:

The developed alloys described here have the distinct advantage compared to nickel-based forging alloys that despite the relatively low γ 'solvus temperatures of about 1050 ° C (CoWAlloy), 1070 ° C (CoWAlloy1) or 1030 ° C (CoWAlloy2) high Excretion volume ratios of more than 45% (CoWAlloy0) at 750 ° C can be achieved. Fig. 1 shows the relationship between the excretion fraction at the application temperature and the solvus temperature of the y'-phase of γ / γ 'nickel-base superalloys and the presently stated γ / γ' cobalt-base superalloy CoWAlloy0. Despite high precipitation volume fractions, the relatively low γ 'solvus temperatures facilitate easier forming at typical forging temperatures of 1000 ° C to 1150 ° C.

After hot rolling at a rolling temperature of 1100 ° C and a subsequent heat treatment of 1050 ° C / 4h + 900 ° C / 8h (CoWAlloy1) or 1000 ° C / 4h + 900 ° C / 4h + 750 ° C / 16h (CoWAlloy2) For example, average particle sizes of about 8 to 15 μm and a typical γ / γ 'microstructure can be set. This is immediately out Fig. 2 seen. Fig. 2 shows here in different resolution the microstructure of the γ / γ 'cobalt-base superalloys CoWAlloy1 a) and c) or CoWAlloy2 b) and d) in the heat-treated state.

Fig. 3 shows an electron back scattering diffraction (EBSD) measurement to determine the grain size and twin density of the CoWAlloy 2 γ / γ cobalt base superalloy described herein. The twin density of the alloy CoWAlloy2, which was determined by means of EBSD, is considerably higher at 55% compared to the nickel base superalloy Udimet 720Li with only 33%. This is due to the lower stacking fault energy of the cobalt base superalloys.

Fig. 4 shows the yield strength as a function of the temperature of the alloys CoWAlloy1 and CoWAlloy2 specified here in comparison with the nickel-based alloys Waspaloy and Udimet 720Li and with the cobalt alloy Mar-M509. The yield strengths determined by compression tests at room temperature with 1110 MPa (CoWAlloy1) and 995 MPa (CoWAlloy2) are in the range of the yield strengths of Waspaloy (1010 MPa) and Udimet (1155 MPa) and reach significantly higher values at 800 ° C (880 MPa (CoWAlloy1) compared to Waspaloy (680 MPa) and Udimet 720Li (about 800 MPa)).

Fig. 5 shows the creep strength of the γ / γ 'cobalt base superalloy CoWAlloy2 compared to the polycrystalline γ / γ' nickel base superalloys Waspaloy and Udimet 720LI at 700 ° C. Accordingly, the alloy CoWAlloy2 at 700 ° C also has a significantly higher creep resistance than the nickel-based alloys Waspaloy and Udimet 720Li.

Oxidation and corrosion behavior:

The oxidation behavior can be assessed on the basis of the oxide layer thicknesses formed at 900 ° C in 50 h. Fig. 6 shows for this purpose microstructural images of the oxide layers of the ternary alloy Co9Al9W (a) and the presently stated alloy CoWAlloy2 (b). The oxide layer thickness after annealing at 900 ° C for 50 h is at least 10 times smaller in the alloy CoWAlloy2 than in the ternary alloy Co9Al9W (see a with b). Compared to the ternary γ / γ 'cobalt base superalloy Co9Al9W ( Fig. 6a ), for example, the alloy CoWAlloy2 ( Fig. 6b ) (a much better oxidation resistance.

Fig. 7 shows the element distributions in the different oxide layers of the alloy CoWAlloy2 after annealing at 900 ° C for 50 h, determined by energy dispersive X-ray spectroscopy EDS in the scanning electron microscope SEM. The relatively good oxidation properties result from the protective oxide layers rich in Al, Si and Cr.

In particular, the cobalt-base superalloys of the present invention are characterized by being based on the element cobalt, hardened with the intermetallic γ 'phase (Co, Ni) ₃ (Al, W, Ti, Ta) to have better mechanical properties than conventional ones carbide-hardened cobalt-base superalloys have higher strengths than comparable polycrystalline γ / γ 'nickel-base superalloys at temperatures above 800 ° C, that they have higher creep strengths than comparable polycrystalline γ / γ' nickel-base superalloys at temperatures of 700 ° C, making them better Have oxidation properties as previous γ / γ 'Kobaltbasissuperlegierungen and / or that they have high γ' volume fractions at application temperatures of up to 850 ° C at comparatively low γ 'solvus temperatures and thus can be used as a forging alloy.

As a further embodiment of the invention, a γ / γ 'cobalt base superalloy is added with the addition of molybdenum (CoWAlloy3). The composition is shown again in Table 2 together with the other exemplary alloys CoWAlloy0, CoWAlloy1 and CoWAlloy2 described above. Compared to CoWAlloy2, the content of Mo is changed at the expense of Co. Mo, as already described, serves as a solid solution hardening element and can partially replace W, thereby reducing the density. In particular, Mo results in the formation of additional "grain boundary pinning" intermetallic phases which can limit grain growth in forging alloys. Table 2: Compositions of γ / γ 'cobalt base superalloy CoWAlloy3 together with CoWAlloy®, CoWAlloy® and CoWAlloy® (% by weight). Co-based Co Ni al Cr W Ta Ti Hf Zr Si B C Not a word CoWAlloy0 39.8 28.8 2.7 12.8 9.0 4.4 2.0 0.3 0.02 0.2 0,014 0.016 CoWAlloy1 40.6 30.6 2.7 10.2 9.0 4.4 2.0 0.3 0.02 0.2 0,014 0.016 CoWAlloy2 39.2 30.5 4.0 10.1 14.9 0.6 0.2 0.3 0.02 0.2 0,014 0.016 CoWAlloy3 37.9 30.3 4.0 10.1 14.9 0.6 0.2 0.3 0.02 0.2 0,014 0.016 1.55

Microstructure and properties:

As with the previously described CoWAlloy alloys 0, 1, 2, a relatively low solvus temperature of about 1050 ° C is expected for CoWAlloy3, at the same time relatively high solidus temperature, which is advantageous for the processing in particular by casting and forging, since these two temperatures span the window for processing and heat treatment. The alloy CoWAlloy3 was after a homogenization annealed at 1250 ° C for 3h at 1100 ° C for 1h and then hot rolled. The diameter was reduced in several passes from 40 mm to 15 mm. Subsequently, a recrystallization heat treatment was carried out to obtain a homogeneous, fine-grained texture. The simultaneous precipitation of the μ-phase allows a targeted variation of the grain size by a suitable choice of the heat treatment parameters.

Fig. 8 shows SEM micrographs of CoWAlloy3 after recrystallization for 4 h at (a) 1000 ° C and (b) 1100 ° C. The predominantly grain boundary white contrast phase is the W and Mo containing μ phase. It is clear that at a higher recrystallization temperature, the proportion of μ-phase decreases and at the same time the grain size increases significantly. The recrystallization at 1000 ° C leads to a μ-phase content of about 3.2% and a median grain size of about 5 microns. The same heat-treated CoWAlloy2 has a median of about 8 microns, which illustrates the grain boundary pinning effect of the μ-phase. Another two-stage heat treatment (900 ° C, 4 h + 750 ° C, 16 h) leads to the uniform excretion of γ 'phase in the co-mixed crystal. This shows Fig. 9 in which the γ / γ 'microstructure after a two-stage heat treatment (900 ° C, 4 h + 750 ° C, 16 h) is shown: (a) SEM with primary and secondary γ' fragments, (b) TEM Darkfield image with secondary and tertiary γ'-particles.

The γ 'particles are round, as in the comparative alloy CoWAlloy2, indicating a low lattice mismatch. The particle diameter is about 65 nm also in the range of the comparative alloy. A difference can be seen in the γ 'portion, which is about 37% lower than in CoWAlloy2. The reason for this can be assumed to be the formation of a μ phase Co ₇ (W, Mo) ₆ , which reduces the W content available in the co-mixed crystal to form γ '. However, this slightly lower phase content does not adversely affect the high temperature strength. Fig. 10 shows the yield stress above the temperature of the Mo-containing alloy CoWAlloy3 with grain boundary pinning μ-phase compared to CoWAlloy2.

Claims (4)

1. Cobolt-based superalloy consisting of 32-45% by weight of Co, 28-40% by weight of Ni, 10-15% by weight of Cr, 2.5-5.5% by weight of Al, 6.5-16% by weight of W, 0-9% by weight of Ta, 0-8% by weight of Ti, 0.1-1% by weight of Si, 0-0.5% by weight of B, 0-0.5% by weight of C, 0-2% by weight of Hf, 0-0.1% by weight of Zr, 0-8% by weight of Fe, 0-6% by weight of Nb, 0-7% by weight of Mo, 0-4% by weight of Ge and a balance of unavoidable impurities.
2. Cobolt-based superalloy according to Claim 1, comprising less than 2% by weight of Hf, less than 0.1% by weight of Zr, less than 8% by weight of Fe, less than <6% by weight of Nb, less than 7% by weight of Mo and less than 4% by weight of Ge.
3. Cobolt-based superalloy according to Claims 1 and 2, comprising at least 0.2% by weight of Ta, at least 0.2% by weight of Ti, less than 0.5% by weight of B and less than 0.5% by weight of C.
4. Cobolt-based superalloy, having a composition according to any of the preceding claims, characterized by an intermetallic γ' phase (Co,Ni)₃(Al, W, Ti, Ta).

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