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.

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).sub.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).sub.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).sub.3(Al, W, Ti, Ta).

Description

[0052] Working examples of the invention will be explained in more detail with the aid of a drawing and the following information. The drawing shows:

[0053] 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,

[0054] FIG. 2 the microstructure of illustrative alloys of the invention,

[0055] FIG. 3 an EBSD measurement to determine the grain size and the twin density of an illustrative alloy of the invention,

[0056] FIG. 4 a graph of the yield point as a function of temperature for illustrative alloys of the invention,

[0057] FIG. 5 a graph of the creep strength of an illustrative alloy of the invention compared to nickel-based superalloys,

[0058] FIG. 6 images of the microstructure of the ternary alloy Co9Al9W compared to an illustrative alloy of the invention,

[0059] FIG. 7 the element distribution in the oxide layer of an illustrative alloy of the invention,

[0060] FIG. 8 scanning electron micrographs of an illustrative alloy of the invention,

[0061] FIG. 9 scanning electron micrographs and transmission electron micrographs of an illustrative alloy of the invention and

[0062] FIG. 10 a graph of the yield point as a function of temperature for a further illustrative alloy of the invention.

[0063] 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-US-00001 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

[0064] Microstructure and Mechanical Properties:

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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)).

[0069] 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.

[0070] Oxidation and Corrosion Behavior:

[0071] 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.

[0072] 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.

[0073] 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).sub.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.

[0074] 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-US-00002 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

[0075] Microstructure and Properties:

[0076] 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.

[0077] 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.

[0078] 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 Co.sub.7(W,Mo).sub.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.