COBALT-BASED SUPER ALLOY

Abstract

A cobalt-based superalloy includes the following: 32-45 wt.-% Co, 28-40 wt.-% Ni, 10-15 wt.-% Cr, 2.5-5.5 wt.-% 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 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 remainder of unavoidable impurities.

Description

The invention relates to polycrystalline, precipitation-hardened and oxidation-resistant γ/γ′ cobalt-based superalloys for high-temperature applications. The mechanical properties of the cobalt-based superalloys indicated are superior to those of conventional, carbide-hardened cobalt alloys. Up to a temperature of 800° C., similar hot strengths as those of nickel-based γ/γ′ forging alloys are achieved, and at temperatures above 800° C. even higher hot strengths than those of nickel-based γ/γ′ forging alloys are achieved. The creep strengths are likewise significantly higher. Compared to γ/γ′ nickel-based superalloys, similar proportions of the γ′ precipitate phase are achieved despite a lower solvus temperature. Owing to the large temperature range between solidus and solvus temperature, the precipitation-hardened γ/γ′ cobalt-based superalloys are particularly suitable as polycrystalline forging alloys.

Cobalt-based and in particular γ/γ′ nickel-based superalloys are essential materials for many components in jet engines of commercial aircraft or in stationary gas turbines for power conversion. New materials which have a greater heat resistance, longer life and lower production and processing costs make it possible to increase the efficiency of these turbines, lower the costs and reduce the consumption of fossil fuel.

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

However, since they are mixed crystal- and carbide-hardened, they are used only for components which are subject to relatively low stresses or are static, e.g. guide blades, because of their lower high-temperature strength compared to the precipitation-hardened γ/γ′ nickel-based superalloys. These are thus not used as materials for rotor blades or turbine disks. With the discovery of the intermetallic γ′ phase Co₃ (Al, W) having an L1₂ crystal structure in the ternary Co—Al—W system in 2006, it is now also possible to produce higher-strength, precipitation-hardened, two-phase γ/γ′ superalloys based on cobalt (γ: face-centered cubic cobalt mixed crystal) having the same microstructure as the γ/γ′ nickel-based superalloys which have been used for decades, as is described, for example, in Sato et al., Cobalt-Base High-Temperature Alloys, Science 312 (2006) 90-91. These have significant advantages compared to the polycrystalline γ/γ′ nickel-based superalloys.

γ/γ′ cobalt-based superalloys generally have a very high solidus temperature in the temperature range from 1300° C. to 1450° C. combined with a relatively low γ′ solvus temperature in the temperature range from 900° C. to 1150° C. Despite the relatively low γ′ solvus temperature, very high γ′ proportions by volume of over 75% can be realized at temperatures up to 900° C. (see, for example, Bauer et al., Microstructure and creep strength of different γ/γ′-strengthened Co-base superalloy variants, Scripta Materialia 63 (2010) 1197-1200). The ternary γ/γ′ cobalt-based superalloy Co-9Al-9W (reported in atom%) has, for example, a relatively high proportion by volume of γ′ precipitates of 58% despite a γ′ solvus temperature of only about 975° C. Owing to the large temperature range between solidus temperature and γ′ solvus temperature (forging window), the comparatively low γ′ solvus temperatures and the high γ′ proportion by volume at use temperatures, the γ/γ′ cobalt-based superalloys are thus particularly suitable as forging alloys. Nickel-based superalloys, on the other hand, have either a low γ′ solvus temperature below 1100° C., associated with a low γ′ proportion by volume at use temperatures of up to 700° C. (e.g. Waspaloy: γ′ solvus temperature: 1038° C. Semiatin et al., Deformation behavior of Waspaloy at hot-working temperatures, Scripta Materialia 50 (2004) 625-629); γ′ proportion by volume at use temperature: 25% (ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys, Ed. Davies et al, ASM International, Materials Park, Ohio 44073, USA)) or a high γ′ proportion by volume at 700° C. combined with a significantly higher γ′ solvus temperature (e.g. Udimet 720Li: γ′ solvus temperature: 1142° C.; γ′ proportion by volume at use 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 & Materials Society, Warrendale Pa., USA)). As a result, the alloys are either forgable but have a low strength or they still have a relatively high proportion of the precipitate phase at forging temperatures of from 1000° C. to 1150° C. and are thus deformable only with difficulty or not at all and can be processed only by a powder-metallurgical route. This significantly increases costs.

Furthermore, it is known that cobalt-based alloys can have a higher hot gas corrosion resistance than nickel-based alloys since a liquid Co-sulfur phase can appear only at 877° C., while a liquid Ni—S phase is formed at as low as 637° C. (see Bürgel, Maier, Niendorf, Handbuch Hochtemperaturwerkstofftechnik, 4^th 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, Ohio 44073, USA). An increased hot gas corrosion resistance can thus lead to a lengthening of the life. In addition, although pure cobalt displays a lower corrosion resistance than pure nickel in sulfuric acid, according to the literature only 10% by weight of chromium in cobalt is necessary for passivation, while in the case of nickel 14% by weight of chromium is required (see, for example, ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys, Ed. Davies et al, ASM International, Materials Park, Ohio 44073, USA).

In addition, it has been able to be shown over past years that the stacking fault energy decreases with increased Co content in nickel-based forging alloys having a γ/γ′ microstructure and the twin density in the material increases as a result, which leads to an additional hardening effect in the polycrystalline forging alloys and higher hot strengths can thus be achieved (see, for example, Yuan et al., A new method to strengthen turbine disc 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 increased further.

Despite increased research activity in the field of this new class of materials made up of the γ/γ′ cobalt-based superalloys, usually only simple alloys having relatively few alloy elements and an unsatisfactory oxidation resistance have been developed and examined to date (e.g. 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 2251446 A1, CA 2620606 A1, EP 1925683 A1, US 2008/0185078 A1, EP 2163656 A1, US 2011/0062214 A1, EP 2 298 486 A2).

A good oxidation resistance in combination with good mechanical properties is, however, essential for these new γ/γ′ cobalt-based superalloys to be able to be used in future as high-temperature material.

It is an object of the invention to develop polycrystalline, higher-strength, precipitation-hardened γ/γ′ cobalt-based superalloys which have very good oxidation properties and can be processed by means of various forming processes such as forging.

This object is achieved according to the invention by a cobalt-based 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 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.

In an advantageous variant, the cobalt-based 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% 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, from 0 to <2% by weight of Hf, from 0 to <0.1% by weight of Zr, from 0 to <8% by weight of Fe, from 0 to <6% by weight of Nb, from 0 to <7% by weight of Mo, from 0 to <4% by weight of Ge and a balance of unavoidable impurities.

In a further embodiment, the cobalt-based superalloy advantageously 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% by weight of W, 0.2-9% by weight of Ta, 0.2-8% by weight of Ti, 0.1-1% by weight of Si, <0.5% by weight of B, <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.

In a further preferred embodiment, the cobalt-based 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% by weight of W, 0.2-9% by weight of Ta, 0.2-8% by weight of Ti, 0.1-1% by weight of Si, <0.5% by weight of B, <0.5% by weight of C, from 0 to <2% by weight of Hf, from 0 to <0.1% by weight of Zr, from 0 to <8% by weight of Fe, from 0 to <6% by weight of Nb, from 0 to <7% by weight of Mo, from 0 to <4% by weight of Ge and a balance of unavoidable impurities.

The cobalt-based superalloy, which in particular comprises a composition as mentioned above, is advantageously characterized by an intermetallic γ′ phase having the composition (Co,Ni)₃(Al, W, Ti, Ta) where at least one of the elements shown in parentheses from each set of parentheses is present in each case. The intermetallic γ′ phase (precipitate phase) is advantageously present in a proportion by volume of more than 35%, preferably more than 45%.

These oxidation-resistant, precipitation-hardened cobalt-based superalloys characterized by their composition are made up of many alloy elements. The reasons for the chosen concentration ranges of the alloy elements and their significant modes of action are described below:

Necessary Alloy Elements:

Co (cobalt): 32-45% by weight

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

Ni (nickel): 28-40% by weight

Ni in the range indicated widens the γ/γ′ two-phase region to a sufficient extent, so that further alloy elements, in particular Cr, can be added in a sufficient amount. Proportions of Cr above about 4% by weight destabilize the two-phase 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, the γ′ solvus temperature can be increased by means of Ni.

Cr (chromium): 10-15% by weight

In order to attain a sufficient corrosion and oxidation resistance, the alloy element Cr should be added in the range indicated. In addition, Cr acts as mixed crystal hardener.

Al (aluminum): 2.5-5.5% by weight

Al forms the γ′ precipitate phase (Co,Ni)₃(Al, W, Ti, Ta), which contributes significantly to the increasing strength. Furthermore, Al increases the oxidation resistance. Relatively high proportions of Al in the indicated composition range can lead to formation of further intermetallic phases such as CoAl which can inhibit grain growth in forging alloys. This makes it possible to achieve smaller grain sizes and thus higher strengths.

W (tungsten): 6.5-16% by weight

W forms the γ′ precipitate phase Co₃(Al,W), which contributes significantly to the increasing strength and, as slowly diffusing element, increases the creep strength. Higher contents lead to an excessively high density and further undesirable intermetallic phases can be formed.

Si (silicon): 0.1-1% by weight

Si is a critical element and signicantly improves the oxidation resistance. However, excessively large amounts of Si can lead to further undesirable intermetallic phases.

B (boron): <0.5% by weight

B acts as grain boundary strengthening alloy element and improves the oxidation properties. Excessively high concentrations lead to an excessively high proportion of borides. B is preferably present in an amount of more than 0.01% by weight.

C (carbon): <0.5% by weight

C acts as grain boundary strengthening alloy element. In addition, C forms carbides. C is preferably present in an amount of more than 0.01% by weight.

Required for High Hot Strengths:

Ta (tantalum): 0.2-9% by weight

Ta contributes to the formation of the γ′ precipitate phase, increases the γ′ solvus temperature and the γ/γ′ lattice mismatching. Ta hardens the γ′ precipitate phase and leads to an increasing strength. The two elements Ta and Ti are necessary particularly when high hot strengths at 800° C. are required.

Ti (titanium): 0.2-8% by weight

Ti contributes to the formation of the γ′ precipitate phase, increases the γ′ solvus temperature and the γ/γ′ lattice mismatching. Ti hardens the γ′ precipitate phase and leads to an increasing strength. The two elements Ta and Ti are necessary particularly when high hot strengths at 800° C. are required. Ti can replace W to a substantial extent and in this way reduces the density significantly.

Optional Alloy Elements:

Hf (hafnium): <2% by weight

Hf stabilizes the γ′ precipitate phase. Hf is preferably present in an amount of more than 0.2% by weight.

Zr (zirconium): <0.1% by weight

Zr serves to increase the grain boundary strength and is able to stabilize the γ′ precipitate phase. Zr is preferably present in an amount of more than 0.01% by weight.

Fe (iron): <8% by weight

Fe lowers the γ′ solvus temperature and can be used for setting this, particularly in the case of forging alloys. Fe is also an inexpensive element and can improve the weldability. Excessively high concentrations destabilize the γ/γ′ microstructure. Fe is preferably present in an amount of more than 0.1% by weight.

Nb (niobium): <6% by weight

Nb contributes to the formation of the γ′ precipitate phase, leads to an increase in strength and increases the γ′ solvus temperature. Relatively high concentrations within the concentration range indicated can lead to formation of further intermetallic phases which can inhibit grain growth in forging alloys. Smaller grain sizes and thus higher strengths can be achieved as a result. Nb is preferably present in an amount of more than 0.1% by weight.

Mo (molybdenum): <7% by weight

Mo serves as mixed crystal hardening element and can partly replace W and thus decreases the density. Relatively high concentrations lead to formation of further intermetallic phases which can inhibit grain growth in forging alloys. Smaller grain sizes and thus higher strengths can be achieved as a result. Mo is preferably present in an amount of more than 0.1% by weight.

Ge (germanium): <4% by weight

Ge forms the γ′ precipitate phase Co₃(Al,Ge,W), lowers the γ′ solvus temperature and can be used for setting this, particularly in the case of forging alloys. Ge is preferably present in an amount of more than 0.1% by weight.

Working examples of the invention will be explained in more detail with the aid of a drawing and the following information. The drawing shows:

FIG. 1 a graph of the relationship between the proportion of precipitates at use temperature and the solvus temperature of the γ′ phase of γ/γ′ nickel-based superalloys compared to working examples of the invention,

FIG. 2 the microstructure of illustrative alloys of the invention,

FIG. 3 an EBSD measurement to determine the grain size and the twin density of an illustrative alloy of the invention,

FIG. 4 a graph of the yield point as a function of temperature for illustrative alloys of the invention,

FIG. 5 a graph of the creep strength of an illustrative alloy of the invention compared to nickel-based superalloys,

FIG. 6 images of the microstructure of the ternary alloy Co9Al9W compared to an illustrative alloy of the invention,

FIG. 7 the element distribution in the oxide layer of an illustrative alloy of the invention,

FIG. 8 scanning electron micrographs of an illustrative alloy of the invention,

FIG. 9 scanning electron micrographs and transmission electron micrographs of an illustrative alloy of the invention and

FIG. 10 a graph of the yield point as a function of temperature for a further illustrative alloy of the invention.

The compositions of some working examples of the γ/γ′ cobalt-based superalloys of the invention, hereinafter referred to as CoWAlloy0 and CoWAlloy1 and CoWAlloy2, and also some reference alloys are indicated in table 1 below. Likewise, the properties of working examples of the invention are described in more detail below with the aid of the figures and studies.

TABLE 1
Compositions of the γ/γ′ cobalt-based superalloys CoWAlloy0,
CoWAlloy1 and CoWAlloy2 described here and also some polycrystalline,
cobalt- and nickel-based reference alloys (figures in % by weight).
	Co	Ni	Al	Cr	W	Ta	Ti	Hf	Zr	Si	B	C
Co-based
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-based
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 alloys which have been developed and are described here have, compared to nickel-based forging alloys, the important advantage that high proportions by volume of precipitates of more than 45% (CoWAlloy0) can be achieved at 750° C. despite the relatively low γ′ solvus temperatures of about 1050° C. (CoWAlloy0), 1070° C. (CoWAlloy1) and 1030° C. (CoWAlloy2). To illustrate this, FIG. 1 shows the relationship between the proportion of precipitates at use temperature and the solvus temperature of the γ′ phase of γ/γ′ nickel-based superalloys and the γ/γ′ cobalt-based superalloy CoWAlloy0 reported here. Despite high proportions by volume of precipitates, easier deformation at typical forging temperatures of from 1000° C. to 1150° C. is made possible by the relatively low γ′ solvus temperatures.

After hot rolling at a rolled material temperature of 1100° C. and subsequent heat treatment at 1050° C./4 h+900° C./8 h (CoWAlloy1) or 1000° C./4 h+900° C./4 h+750° C./16 h (CoWAlloy2), average grain sizes of from about 8 to 15 μm and a typical γ/γ′ microstructure can be set. This can be seen directly from FIG. 2. Here, FIG. 2 shows, in different resolutions, the microstructure of the γ/γ′ cobalt-based superalloys CoWAlloy1 a) and c) and also CoWAlloy2 b) and d) in the heat-treated state.

FIG. 3 shows an EBSD (“electron backscattering diffraction”) measurement to determine the grain size and twin density of the γ/γ′ cobalt-based superalloy CoWAlloy2 described here. The twin density of the alloy CoWAlloy2 determined by means of EBSD is, at 55%, significantly higher than that of the nickel-based superalloy Udimet 720Li of only 33%. This is attributable to the lower stacking fault energy of the cobalt-based superalloys.

FIG. 4 shows the yield point as a function of temperature of the alloys CoWAlloy1 and CoWAlloy2 reported here compared to the nickel-based alloys Waspaloy and Udimet 720Li and to the cobalt alloy Mar-M509. The yield strengths determined by means of compressive tests are, at room temperature, 1110 MPa (CoWAlloy1) and 995 MPa (CoWAlloy2) and thus in the region of the yield strengths of Waspaloy (1010 MPa) and Udimet (1155 MPa), and at 800° C. sometimes achieve significantly higher values (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-based superalloy CoWAlloy2 compared to the polycrystalline γ/γ′ nickel-based superalloys Waspaloy and Udimet 720Li at 700° C. Accordingly, the alloy CoWAlloy2 also has, at 700° C., a significantly higher creep strength than the nickel-based alloys Waspaloy and Udimet 720Li.

Oxidation and Corrosion Behavior:

The oxidation behavior can be assessed by means of the oxide layer thicknesses formed at 900° C. in 50 hours. For this purpose, FIG. 6 shows images of the microstructure of the oxide layers on the ternary alloy Co9Al9W (a) and the abovementioned alloy CoWAlloy2 (b). The oxide layer thickness after heat treatment at 900° C. for 50 hours is a factor of at least 10 smaller in the case of the alloy CoWAlloy2 than for the ternary alloy Co9Al9W (cf. a with b). Compared to the ternary γ/γ′ cobalt-based superalloy Co9Al9W (FIG. 6a), the alloy CoWAlloy2 (FIG. 6b), for example, has a significantly better oxidation resistance.

FIG. 7 shows the element distributions in the various oxide layers of the alloy CoWAlloy2 after heat treatment at 900° C. for 50 hours, determined by energy-dispersive X-ray spectroscopy EDS in a scanning electron microscope SEM. The relatively good oxidation properties are due to the protective oxide layers rich in Al, Si and Cr.

The cobalt-based superalloys of the invention are characterized, in particular, in that they are based on the element cobalt, in that they are hardened by means of the intermetallic γ′ phase (Co,Ni)₃(Al, W, Ti, Ta), in that they have better mechanical properties than conventional, carbide-hardened cobalt-based superalloys, in that they have higher strengths than comparable, polycrystalline γ/γ′ nickel-based superalloys at temperatures above 800° C., in that they have higher creep strengths than comparable, polycrystalline γ/γ′ nickel-based superalloys at temperatures of 700° C., in that they have better oxidation properties than previous γ/γ′ cobalt-based superalloys and/or in that, at comparatively low γ′ solvus temperatures, they have high γ′ proportions by volume at use temperatures of up to 850° C. and thus can be used as forging alloy.

As further working example of the invention, a γ/γ′ cobalt-based superalloy having an addition of molybdenum (CoWAlloy3) will be reported. The composition is shown in table 2 again together with the further above-described illustrative alloys CoWAlloy0, CoWAlloy1 and CoWAlloy2. Compared to CoWAlloy2, the content of Mo has been changed at the expense of Co. Mo serves, as described above, as mixed crystal hardening element and can partly replace W, as a result of which the density is decreased. Mo leads, in particular, to formation of further “grain boundary pinning” intermetallic phases which can restrict grain growth in forging alloys.

TABLE 2
Compositions of the γ/γ′ cobalt-based superalloy CoWAlloy3 together
with CoWAlloy0, CoWAlloy1 and CoWAlloy2 (figures in % by weight)
Co-based	Co	Ni	Al	Cr	W	Ta	Ti	Hf	Zr	Si	B	C	Mo
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 in the case of the above-described CrWAlloy alloys 0, 1, 2, a relatively low solvus temperature of about 1050° C. combined with a relatively high solidus temperature, which is advantageous for processing, in particular by casting and forging, since these two temperatures span the window for processing and heat treatment, is expected for CoWAlloy3. The alloy CoWAlloy3 was subjected to an intermediate heat treatment at 1100° C. for one hour after a homogenizing heat treatment at 1250° C. for 3 hours and subsequently hot rolled. The diameter in a number of random samples was reduced here from 40 mm to 15 mm. A recrystallization heat treatment was subsequently carried out in order to obtain a homogeneous, fine-grain microstructure. The simultaneous precipitation of the μ phase allows targeted variation of the grain size by appropriate selection of the heat treatment parameters.

FIG. 8 shows scanning electron micrographs of the microstructure of CoWAlloy3 after recrystallization for 4 hours at (a) 1000° C. and (b) 1100° C. The phase having white contrast which is predominantly present at the grain boundaries is the W- and Mo-containing μ phase. It is clear that the proportion of p phase decreases significantly and at the same time the grain size increases significantly at higher recrystallization temperature. Recrystallization at 1000° C. leads to a proportion of μ phase of about 3.2% and a median grain size of about 5 μm. The CoWAlloy2 which has undergone the same heat treatment has a median of about 8 μm, which demonstrates the grain boundary pinning effect of the p phase. A further, two-stage heat treatment (900° C., 4 h+750° C., 16 h) leads to uniform precipitation of γ′ phase in the Co mixed crystal. This is shown by FIG. 9 in which the γ/γ′ microstructure after a two-stage heat treatment (900° C., 4 h+750° C., 16 h) is depicted: (a) scanning electron micrograph with primary and secondary γ′ particles, (b) dark-field transmission electron micrograph with secondary and tertiary γ′ particles.

The γ′ particles are, as in the case of the comparative alloy CoWAlloy2, round, which indicates low lattice mismatching. The particle diameter is about 65 nm and thus likewise in the range of the comparative alloy. One noticeable difference is the γ′ proportion, which is about 37% and thus lower than in the case of CoWAlloy2. The reason for this can be presumed to be the formation of a μ phase Co₇(W,Mo)₆, which reduces the W content available for γ′ formation in the Co mixed crystal. However, this somewhat lower phase content does not have an adverse effect on the high-temperature strength. To illustrate this, FIG. 10 shows the yield stress versus temperature of the Mo-containing alloy CoWAlloy3 with grain boundary pinning μ phase compared to CoWAlloy2.

Claims

1-5 (canceled)

6. A cobalt-based 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 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 remainder of unavoidable impurities.

7. The cobalt-based superalloy according to claim 6, which comprises:

from 0 to <2% by weight of Hf;

from 0 to <0.1% by weight of Zr;

from 0 to <8% by weight of Fe;

from 0 to <6% by weight of Nb;

from 0 to <7% by weight of Mo;

from 0 to <4% by weight of Ge.

8. The cobalt-based superalloy according to claim 6, comprising an intermetallic γ′ phase (Co,Ni)3(Al, W, Ti, Ta).

9. A cobalt-based 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 of W;

0.2-9% by weight of Ta;

0.2-8% by weight of Ti;

0.1-1% by weight of Si;

an amount of <0.5% by weight of B;

an amount of <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 remainder of unavoidable impurities.

10. The cobalt-based superalloy according to claim 9, comprising an intermetallic γ′ phase (Co,Ni)3(Al, W, Ti, Ta).

11. A cobalt-based 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 of W;

0.2-9% by weight of Ta;

0.2-8% by weight of Ti;

0.1-1% by weight of Si;

an amount of <0.5% by weight of B;

an amount <0.5% by weight of C;

from 0 to <2% by weight of Hf;

from 0 to <0.1% by weight of Zr;

from 0 to <8% by weight of Fe;

from 0 to <6% by weight of Nb; from 0 to <7% by weight of Mo;

from 0 to <4% by weight of Ge; and

a remainder of unavoidable impurities.

12. The cobalt-based superalloy according to claim 11, comprising an intermetallic γ′ phase (Co,Ni)3(Al, W, Ti, Ta).