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, 4th 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, 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, 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:
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.
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,
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
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,
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.
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.
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,
Number | Date | Country | Kind |
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10 2014 011 249.7 | Aug 2014 | DE | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2015/067697 | 7/31/2015 | WO | 00 |