EP2229462A1

EP2229462A1 - Nickel base superalloy compositions, superalloy articles, and methods for stabilizing superalloy compositions

Info

Publication number: EP2229462A1
Application number: EP08867005A
Authority: EP
Inventors: Kevin Swayne O'hara; Laura Jill Carroll
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2007-12-26
Filing date: 2008-11-13
Publication date: 2010-09-22
Also published as: JP2011524943A; CN101910433B; JP5697454B2; CN101910433A; WO2009085420A1

Abstract

A stabilized superalloy composition comprises tungsten, molybdenum, and optionally rhenium, the superalloy composition being modified with a stabilizing amount of hafnium sufficient to decrease the formation of topologically close packed (TCP) phases in a superalloy microstructure at elevated temperatures with respect to a comparable unmodified superalloy composition. Articles exhibiting increased microstructure stability formed from hafnium-modified superalloy compositions may be utilized in gas turbine engines. Methods for stabilizing superalloy compositions at elevated temperatures include utilizing hafnium as a stabilizer to decrease the propensity to form TCP phases.

Description

NICKEL BASE SUPERALLOY COMPOSITIONS,

SUPERALLOY ARTICLES, AND METHODS FOR

STABILIZING SUPERALLOY COMPOSITIONS

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to nickel-base superalloy compositions, articles, and methods, and more particularly to such alloys for use as single crystal articles at elevated temperatures wherein the superalloy composition includes a stabilizing amount of hafnium.

[0002] A number of high temperature nickel base superalloys have been developed and reported for use in the form of single crystal articles at high temperature under severe load conditions. For example, such conditions exist in the turbine section of advanced gas turbine engines for aircraft use. Such single crystal articles are useful as airfoils in these turbine sections.

[0003] In general, recent advances in alloy strength for single crystal articles useful at such high temperatures and severe load conditions have been accomplished by the alloying with refractory elements for solid solution strengthening and increasing the volume fraction of the gamma prime phase. Unfortunately, both the presence of refractory elements, i.e., rhenium (Re), tungsten (W), tantalum (Ta), and molybdenum (Mo), and the increased volume fraction gamma prime render the alloy more susceptible to the precipitation of undesirable phases. Especially detrimental are phases known as the topologically close packed (TCP) phases, which form after exposure at temperatures above about 1800 ⁰F (982 ⁰C). TCP phases are brittle and their formation reduces solution strengthening potential of the alloy by removing solute elements from the desired alloy phase and concentrating them in the brittle phases so that intended strength and life goals are not met.

[0004] Thus, it is desirable to promote stability in high temperature superalloys that have a tendency to form brittle TCP phases. Also, because rhenium is costly and is in limited supply, it is desirable to provide stable high temperature superalloys having low or no rhenium content. This is particularly important because the other gamma- strengthening refractory elements, W and Mo, that are used to replace rhenium, more strongly promote the TCP instability. Additionally, they are not as potent solid solution strengtheners and thus require higher added amounts.

BRIEF DESCRIPTION OF THE INVENTION

[0005] The above-mentioned need or needs may be met by exemplary embodiments which provide a nickel-base superalloy having an improved combination of stress rupture life and microstructural stability with respect to the formation of TCP phases. The formation of undesirable TCP phases, beyond small nominal amounts, is affected by the composition and thermal history of the alloy, and once formed, invariably reduces the rupture life capability of the alloy.

[0006] It has been discovered that hafnium (Hf) acts as a stabilizer for nickel-base superalloys prone to formation of undesirable TCP phases. The hafnium- modified superalloys do not form the TCP phases to the extent that comparable unmodified nickel-base superalloys do under comparable conditions. Thus, the lowered propensity of TCP phase formation results in greater microstructure stability at high temperatures and increased alloying flexibility.

[0007] Embodiments disclosed herein include Hf-modified nickel-base superalloys for high temperature applications. Further embodiments disclosed herein include a single crystal article formed from a Hf-modified nickel-base superalloy having an improved microstructural stability at elevated temperatures. Further embodiments disclosed herein provide a method of improving the microstructural stability of alloys prone to form TCP phases.

[0008] In an exemplary embodiment, a stabilized superalloy composition comprises tungsten, molybdenum, and optionally rhenium. The superalloy composition is modified with a stabilizing amount of hafnium sufficient to decrease the formation of topologically close packed (TCP) phases in a superalloy microstructure at elevated temperatures with respect to a comparable unmodified superalloy composition. [0009] In an exemplary embodiment, there is provided a nickel base superalloy single crystal article exhibiting improved microstructural stability. The superalloy single crystal article is formed from a hafnium-modified superalloy composition including tungsten, molybdenum, and optionally rhenium, and a stabilizing amount of hafnium sufficient to decrease the formation of topologically close packed (TCP) phases in a superalloy microstructure at elevated temperatures with respect to a comparable unmodified superalloy composition.

[0010] In an exemplary embodiment, there is provided a method of improving the microstructural stability of a superalloy composition. The method includes evaluating a propensity of a superalloy composition to form topologically close packed (TCP) phases in a superalloy microstructure at elevated temperatures by determining an associated TCP number. The method further includes; if the TCP number exceeds a predetermined TCP number, increasing an amount of hafnium in the superalloy composition to an amount sufficient to provide a hafnium-modified superalloy composition, wherein the hafnium-modified superalloy composition exhibits improved microstructural stability at the elevated temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding part of the specification. The invention, however, may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:

[0012] FIG. 1 is a perspective view of a component article such as a gas turbine engine turbine blade.

[0013] FIG. 2 is a bar graph comparison of 2000 ⁰F/ 18 ksi stress rupture life of various alloys, normalized to a second-generation superalloy.

[0014] FIG. 3 is a bar graph comparison of 2100 ⁰F/ 10 ksi stress rupture life of various alloys, normalized to a second-generation superalloy. [0015] FIGS. 4-11 are a series of photomicrographs of the TCP phase in the dendrite primary core region after stress rupture testing at 2100 ⁰F/ 10 ksi for Alloys A, Al; B, Bl; C, Cl; and D, Dl, respectively.

[0016] FIG. 12 is a bar graph showing the relationship between the change in TCP number and increased rupture life.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Referring now to the drawings, FIG. 1 depicts a gas turbine blade 20. The gas turbine blade includes an airfoil 22 against which the flow of hot combustion gas impinges during service operation, a downwardly extending shaft 24, and an attachment in the form of a dovetail 26 which attaches the gas turbine blade 20 to a gas turbine disk (not shown) of the gas turbine engine. A platform 28 extends transversely outwardly at a location between the airfoil 22 and the shank 24 and dovetail 26. In an exemplary embodiment, gas turbine blade 20 comprises a single crystal nickel-base superalloy composition as disclosed herein.

[0018] It may be desirable to provide such nickel-base superalloy composition with reduced levels of rhenium (Re), although exemplary embodiments disclosed herein are not so limited. Exemplary embodiments may include about 1.5 wt % rhenium. Other exemplary embodiments may include up to about 6 wt % rhenium. Increased amounts of other strengthening alloying elements such as tungsten (W) and molybdenum (Mo) may be utilized to offset the lower levels of Re in advanced turbine engine blade alloys. However, in some cases, the increased amounts of refractory elements provide alloys with heightened propensity to form TCP phases. The presence of the TCP phases (needle-like phases) reduces creep life over the part life upon repeated exposure to high temperature environments. All percentages presented herein are percentages by weight, unless noted otherwise.

[0019] Initially, Hf at relatively low levels, 0.15 wt%, was added to single crystal superalloys to improve oxidation resistance. The addition of higher levels of hafnium (Hf) is known to improve the oxidation resistance of the coated superalloy and the adherence of thermal barrier coatings when utilized. As demonstrated by the embodiments disclosed herein, it has been discovered that the addition of Hf to nickel-base superalloys, in amounts greater than amounts typically used to strengthen grain boundaries, may be used to improve microstructure stability by reducing the formation of TCP phases. Embodiments disclosed herein provide superalloy compositions, modified by Hf additions, which promote TCP resistance. Increasing the TCP stability in an otherwise unstable alloy (e.g., TCP number greater than about 3), results in improvements in creep rupture strength.

EXAMPLE

TABLE 1

[0020] Four experimental pairs of comparable superalloy compositions were prepared and tested: A, Al; B, Bl; C, Cl; and D, Dl. See Table 1. One composition in each pair included 0.15 wt% Hf ("unmodified alloy"). The second composition in each pair included 0.60 wt% Hf ("Hf-modifϊed alloy"). For example, unmodified alloy A includes 0.15 wt% Hf; Hf-modified alloy Al includes 0.60 wt% Hf. The remaining alloying elements are nominally similar, with the balance being nickel and incidental impurities. Creep rupture data for the alloy pairs are shown in graphic form, normalized to a second-generation superalloy, in FIGS. 2 and 3. Photomicrographs of articles embodying each of the compositions taken after stress rupture testing at 2100 °F/10 ksi are shown in FIGS. 4-11.

[0021] Two of the unmodified alloys, A and C (Hf at 0.15 wt%), exhibited significant amounts of a deleterious TCP phase as shown in the photomicrographs. (See FIG. 4, FIG. 8, respectively). The formation of TCP phases is known to negatively affect the creep resistance of an alloy, with more remarkable degradation at greater volume fraction of TCP phases. TCP phases are refractory-rich needle — or dot-like phases (sigma, mu, or p) that deplete the superalloy matrix of refractory elements that are present to provide increased creep resistance.

[0022] As shown in FIGS. 5 and 9, respectively, the addition of an increased amount of Hf ( Hf content of about 0.60 wt%) in Hf-modified alloys Al and Cl inhibits the formation of the TCP phases during stressed exposures.

[0023] As shown in FIGS. 2 and 3, alloy pair B and Bl, have similar creep rupture lives, as do alloy pair D and Dl. The micrograph of alloy B (FIG. 6) shows that this unmodified alloy composition is not prone to formation of TCP phases. The increased level of Hf in Hf-modified alloy Bl does not significantly affect formation of TCP phases (FIG. 7). Similar results are obtained for the alloy pair D and Dl, as shown in FIGS. 10-11.

[0024] These results suggest that increasing the amount of Hf can stabilize those alloy compositions that have a propensity to be unstable due to TCP phase formation. In particular, compositions including relatively high levels of refractory elements or Cr, which can promote formation of TCP phases, may be stabilized by increased amounts of Hf.

[0025] FIG. 12 illustrates the affect of the increased Hf additions on the rupture life at 2000⁰F of alloy compositions that tend to form an appreciable amount of TCP e.g., TCP numbers greater than 3. The TCP number is an analytical value based on alloy composition utilized to predict TCP phase precipitation. High TCP numbers indicate a propensity to form TCP phases. TCP numbers of 4 or less are generally required for acceptable stress rupture life. As shown, an improvement in creep rupture life at high temperatures (>1800 ⁰F, 982 ⁰C) is observed when the alloy is modified by the addition of up to about 0.60 wt% Hf. As Figure 12 is based on a wide range of alloy compositions including alloy compositions with high refractory levels, beyond alloys A and C, this affect of higher Hf additions is possible for third- generation type alloys and beyond. For unmodified alloys having TCP numbers of 5 or greater (third-generation type alloys), the Hf modification has an even more profound effect. It has been shown that unstable alloys (those having a propensity to form TCP phases) may be stabilized with the addition of a sufficient amount of Hf. The embodiments disclosed herein in up to about 0.60 wt% Hf. It is believed that inclusion of Hf at lesser levels may be sufficient to provide the desired stabilizing effect.

[0026] An exemplary embodiment includes a method for increasing the micro structural stability of a nickel-base superalloy. In an exemplary method, an unmodified superalloy composition is evaluated for propensity to form TCP phases. If the unmodified superalloy composition exhibits a propensity to form TCP phases, for example a TCP number of greater than 3, the superalloy composition may be modified by the inclusion of a stabilizing amount of hafnium. In an exemplary embodiment, the stabilizing amount of hafnium may be up to about 0.60 wt%. In other exemplary embodiments, the stabilizing amount of hafnium may be less than 0.60 wt%. For some superalloy compositions, the stabilizing amount of Hf may be greater than 0.60 wt%. A "stabilizing amount" of hafnium may be considered to be an amount of hafnium able to provide a -modified superalloy composition with a lower propensity to form TCP phases as compared to a comparable unmodified superalloy composition. The propensity for an unmodified superalloy composition to form TCP phases may be evaluated by experimental or analytical methods. For example, an unmodified superalloy composition may tend to form TCP phases at a hafnium content of about 0.15%. If the hafnium content in a comparable modified superalloy composition is increased to about 0.60 %, the propensity to form TCP phases may be reduced. The increased hafnium content is referred to herein as a stabilizing amount of hafnium

[0027] The interaction of hafnium with the alloying elements to modify the refractory element phase partitioning provides unexpected and surprising results of improved properties of the Hf-modified nickel-base superalloy.

[0028] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

Claims

WHAT IS CLAIMED IS:

1. A stabilized superalloy composition comprising:

tungsten, molybdenum, and optionally rhenium, the superalloy composition being modified with a stabilizing amount of hafnium sufficient to decrease the formation of topologically close packed (TCP) phases in a superalloy micro structure at elevated temperatures with respect to a comparable unmodified superalloy composition.

2. The stabilized superalloy composition according to claim 1 including a hafnium content of up to about 0.6 wt %, wherein the unmodified superalloy composition includes about 0.15 wt% hafnium content.

3. The stabilized superalloy composition according to claim 1 wherein a TCP number associated with the stabilized superalloy composition is reduced by at least one as compared to the TCP number associated with the comparable unmodified superalloy composition.

4. The stabilized superalloy composition according to claim 1 wherein rhenium is present in amounts up to about 6 wt%.

5. The stabilized superalloy composition according to claim 1 wherein rhenium is present in amounts up to about 1.5 wt%.

6. The stabilized superalloy composition according to claim 1 comprising, in wt%:

about 6.2 Al, about 6.5 Ta, about 6.0 Cr, from about 6.0 to 6.5 W, from about 1.5 to 2.0 Mo, from about 0 to about 1.5 Re, about 7.5 Co, up to about 0.03 C, up to about 0.004 B, up to about 0.6 Hf, and a remainder including nickel and incidental impurities.

7. A nickel base superalloy single crystal article exhibiting improved micro structural stability, the superalloy single crystal article being formed from a hafnium-modified superalloy composition including tungsten, molybdenum, and optionally rhenium, and a stabilizing amount of hafnium sufficient to decrease the formation of topologically close packed (TCP) phases in a superalloy micro structure at elevated temperatures with respect to a comparable unmodified superalloy composition.

8. The nickel base superalloy single crystal article according to claim 7 comprising a component for a gas turbine engine.

9. The nickel base superalloy single crystal article according to claim 8 wherein the gas turbine engine component is a turbine blade or vane.

10. The nickel base superalloy single crystal article according to claim 7 wherein an associated TCP number is reduced by at least one as compared to the TCP number associated with the comparable unmodified superalloy composition.

11. A method of improving the microstructural stability of a superalloy composition, the method including:

evaluating a propensity of the superalloy composition to form topologically close packed (TCP) phases in a superalloy microstructure at elevated temperatures by determining an associated TCP number;

if the associated TCP number exceeds a predetermined value, increasing an amount of hafnium in the superalloy composition to an amount sufficient to provide a hafnium-modified superalloy composition, wherein the hafnium-modified superalloy composition exhibits improved microstructural stability at the elevated temperatures.

12. The method according to claim 11 wherein increasing an amount of hafnium in the superalloy composition includes providing at least a stabilizing amount of hafnium such that the TCP number associated with the hafnium-modified superalloy composition decreases by at least one.