| United States Patent | 3,887,363 |
| Smashey , et al. | June 3, 1975 |
Nickel-base superalloy cast article
Abstract
A nickel-base superalloy cast article is provided with improved stress rupture and creep properties as a result of providing its microstructure with aligned cellular dendrites in combination with the substantial absence of NiAl, carbon, carbides, and Ti. The superalloy consists, in atomic percent, essentially of 4-11 Cr, 5-16 Al, at least 0.5 Re, up to about 10 V, up to about 15 Co, up to about 5 Ta, up to about 5 W, up to about 1 Mo, up to about 2.5 Mn, up to about 2.5 Rh, with the balance nickel and incidental impurities.
| Inventors: | Smashey; Russell W. (Loveland, OH), Wukusick; Carl S. (Cincinnati, OH) |
| Assignee: |
General Electric Company
(Cincinnati,
OH)
|
| Appl. No.: | 05/426,092 |
| Filed: | December 18, 1973 |
| Current U.S. Class: | 148/404 ; 148/428; 420/445 |
| Current International Class: | C22C 19/05 (20060101); C22c 019/00 () |
| Field of Search: | 75/171,170 148/32,32.5 |
| 3526499 | September 1970 | Quigg et al. |
Attorney, Agent or Firm:
What is claimed is:
1. A cast Ni-base superalloy article the microstructure of which comprises aligned cellular dendrites and is characterized by the substantial absence of NiAl, carbon, carbides and Ti, the superalloy consisting, in atomic percent, essentially of 4-11 Cr, 5-16 Al, about 0.5-5 Re, up to about 10 V, up to about 15 Co, up to about 5 Ta, up to about 5 W, up to about 1 Mo, up to about 2.5 Mn, up to about 2.5 Rh, with the balance essentially nickel and incidental impurities.
2. The article of claim 1 in which the superalloy consists, in atomic percent, essentially of 3-8 Co, 4-9 Cr, 8-14 Al, 1-4 Ta, 1-7 V, 0.5-5 Re, up to about 2 W, up to about 1 Mo, up to about 1 Mn, up to about 1 Rh, with the balance essentially nickel and incidental impurities.
3. The article of claim 2 in which the V is 4-7 at. percent and the Re is 0.5-3 at. percent.
4. The article of claim 3 in which the superalloy consists, in atomic percent, essentially of 3-4 Co, 5-9 Cr, 11-13 Al, 1-3 Ta, 5-6 V, 0.5-2 Re, up to 1.5 W, up to 1 Mo, up to 0.5 Mn, with the balance nickel and incidental impurities.
5. A cast Ni-base superalloy characterized by the substantial absence of NiAl, carbon, carbides and Ti and consisting, in atomic percent, essentially of 4-11 Cr, 5-16 Al, about 0.5-5 Re, up to about 10 V, up to about 15 Co, up to about 5 Ta, up to about 5 W, up to about 1 Mo, up to about 2.5 Mn, up to about 2.5 Rh, with the balance essentially nickel and incidental impurities.
6. The alloy of claim 5 consisting, in atomic percent, essentially of 3-8 Co, 4-9 Cr, 8-14 Al, 1-4 Ta, 1-7 V, 0.5-5 Re, up to about 2 W, up to about 1 Mo, up to about 1 Mn, up to about 1 Rh, with the balance essentially nickel and incidental impurities.
7. The alloy of claim 6 in which the V is 4-7 at. percent and the Re is 0.5-3 at. percent.
8. The alloy of claim 7 consisting, in atomic percent, essentially of 3-4 Co, 5-9 Cr, 11-13 Al, 1-3 Ta, 5-6 V, 0.5-2 Re, up to 1.5 W, up to 1 Mo, up to 0.5 Mn, with the balance nickel and incidental impurities.
BACKGROUND OF THE INVENTION
This invention relates to nickel-base superalloys and to cast articles having an aligned cellular dendritic structure, for example as a result of unidirectional solidification.
More recent efforts in the development of nickel-base superalloys and their articles for use under strenuous operating conditions such as are found in gas turbine engines includes emphasis on composite eutectic alloys. Such alloys include reinforcing carbide members such as fibers which can be formed in situ during solidification of the alloy. One form of such solidification which has been used and has been widely reported is generally referred to as unidirectional solidification.
Creation of such carbide reinforced alloys obviously requires the addition of the element carbon. However, during the study of such alloys, it was recognzed that detrimental interaction can occur at the interface between the carbide and matrix. In addition, carbides can provide a source for crack initation.
It has been recognized that the gamma prime former Ti, when included in a nickel-base superalloy structure, depresses the alloy's incipient melting temperature and tends to promote the formation of a eutectic phase for example, the gamma-gamma prime eutectic. With Ti, the incipient melting temperature is about 2250.degree.F.
SUMMARY OF THE INVENTION
It is a principal object of the present invention to provide an improved cast nickel-base superalloy article, the microstructure of which is substantially free of carbon, carbides, titanium and phases which are detrimental to high temperature strength properties.
Another object is to provide a castable nickel-base superalloy substantially free of carbon and titanium and which is particulary useful in the casting of unidirectionally solidified articles.
These and other objects and advantages will be more clearly understood from the following detailed description, the drawings and the examples all of which are intended to be typical of rather than in any way limiting on the scope of the present invention.
Briefly, the present invention, in one form, provides a cast nickel-base superalloy article the microstructure of which comprises aligned cellular dendrites and is further characterized by the substantial absence of the detrimental NiAl phase, carbon, carbides and Ti. The superalloy associated with the present invention consists, in atomic percent, essentially of 4-11 Cr, 5-16 Al, at least 0.5 Re, up to about 10 V, up to about 15 Co, up to about 5 Ta, up to about 5 W, up to about 1 Mo, up to about 2.5 Mn, up to about 2.5 Rh, with the balance Ni and incidental impurities. Such incidental impurities may include Ti at less than 1 and C at less than 0.1 at. percent which, by weight, is less than about 0.017 percent. It is also preferred that the elements Zr and B be omitted from the composition, their presence being limited to those levels which result from normal pick-up of stray elements during melting and casting, for example up to about 0.03 percent Zr and 0.01 percent B by weight. For comparison purposes, the approximate percent by weight equivalent of this form of the invention consists essentially of 3.5-10 Cr; 2.2-7.2 Al; at least 1.5 Re; up to about 8.5 V; up to about 15 each of Co, Ta and W; up to about 1.5 Mo; up to about 2.5 Mn; up to about 4.5 Rh; with the balance Ni and incidental impurities.
In a preferred form of the present invention, the composition, and atomic percent, consist essentially of 3-8 Co, 4-9 Cr, 8-14 Al, 1-4 Ta, 1-7 V, 0.5-5 Re, up to about 2 W, up to about 1 each of Mo, Mn and Rh, with the balance essentially nickel and incidental impurities. For comparison purposes, the approximate percent by weight equivalent of this preferred form consists essentially of 3-8 Co; 3.5-8 Cr; 3.6-6.3 Al; 3-12 Ta; 0.8- 6 V; 1.5-15 Re; up to about 6 W; up to about 1.5 Mo; up to about 1 Mn; up to about 1.8 Rh; with the balance Ni and incidental impurities.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a photomicrographic view at 100 magnifications of the structure of example 105 within the scope of the present invention showing the aligned cellular dendrites in the transverse direction the absence of NiAl;
FIG. 2 is a photomicrographic view at 100 magnifications of the structure of example 105 within the scope of the present invention showing the aligned cellular dendrites in the longitudinal direction and the absence of NiAl;
FIG. 3 is a photomicrographic view at 100 magnifications in the transverse direction of the alloy of example 87 outside the scope of the present invention showing the presence of abundant gamma-gamma prime eutectic phase;
FIG. 4 is a photomicrographic view at 100 magnifications in the transverse direction of the alloy of example 145 outside the scope of the present invention showing the presence of NiAl phase; and
FIG. 5 is a graphical comparison of creep properties.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In order to provide an improved nickel-base superalloy article useful under such strenuous operating conditions as are found in the turbine section of a modern gas turbine engine, and to remove the carbide strengthening mechanism from the alloy of such article, it is necessary to design such alloy with significantly larger amounts of other strengthening elements. The principal strengthening mechanism remaining after carbide elimination is the gamma prime phase, which is predominantly Ni.sub.3 Al, in the gamma matrix, which is predominantly nickel. However, the gamma prime and the gamma phases can be strengthened with the addition of alloying elements. One of the problems which exists in the addition of significantly higher levels of alloying elements is that the detrimental NiAl phase can be forced to form, particularly at higher Al values. Further, the combination of elements can move the alloy into that portion of the alloy's phase diagram which causes the formation of gamma-gamma prime eutectic. The NiAl causes a signficant and dramatic reduction in high temperature properties and the gamma-gamma prime eutectic lowers the incipient melting point of the alloy.
The present invention defines a unique castable nickel-base superalloy which is not strengthened by any carbide mechanism and which includes a balance of alloying elements providing strength characteristics even greater than the carbide strengthened type superalloy structures. At the same time, its structure is extremely uniform, includes substantially no NiAl phase and avoids the gamma-gamma prime eutectic. As a result, the incipient melting temperature of the alloy associated with the present invention is at least about 100.degree.F higher than an ordinary superalloy's incipient melting temperature of about 2250.degree.F.In addition, F. In the inclusion of a balance of elements which strengthen the gamma prime precipitate phase, the gamma prime solution temperature is at least 100.degree. higher than that of the ordinary superalloy. Furthermore, the alloy is uniquely adapted for unidirectional solidification to provide the structure defined as an aligned cellular dendritic structure. Thus, an article having such a structure and made from the composition provided by the present invention has a higher temperature operating capability as well as overtemperature protection in that the chances of causing incipient melting of the article are reduced.
The present invention will be more fully understood from the following discussion of representative examples of alloy forms studied during the evaluation of the present invention. Such examples are grouped for ready comparison but are not intended to be limitations on the invention's scope.
The alloy associated with the present invention, in atomic percent, consists essentially of 4-11 Cr, 5-16 A1, at least 0.5 Re, up to about 10 V, up to about 15 Co, up to about 5 each of Ta and W, up to about 1 Mo, up to about 2.5 each of Mn and Rh, with the balance essentially nickel and incidental impurties. However, the preferred form of such alloy, in atomic percent, consists essentially of 3-8 Co, 4-9 Cr, 8-14 Al, 1-4 Ta, 1-7 V, 0.5-5 and more preferably 0.5-3 Re, up to about 2 W, up to about 1 each of Mo, Mn and Rh, with the balance essentially Ni and incidental impurities. The following Table I lists the compositions of selected forms of such alloy within the preferred range of the present invention and Table II lists some of each form's mechanical property data. None of the elements C, Ti, B or Zr, usually found in nickel-base superalloys, were added and are to be specifically avoided, except in impurity amounts, according to the present invention. Unless otherwise specified throughout this specification, all compositions are in atomic percent.
TABLE I __________________________________________________________________________ Preferred form of Invention Composition (Atomic %) Balance Ni __________________________________________________________________________ Example Co Cr Al Ta V Re W Mo Mn Rh __________________________________________________________________________ 105 3.5 5.4 12.7 2.2 5.5 2.0 106 3.5 5.4 12.8 1.4 5.5 2.0 0.9 118 3.5 5.4 12.7 2.2 5.5 1.5 0.5 122 3.5 5.4 12.7 2.2 5.5 1.0 1.0 123 3.5 5.4 12.7 2.2 5.5 0.5 1.5 124 3.5 5.4 12.7 2.7 5.5 1.5 125 3.5 5.4 12.7 3.2 5.5 1.0 127 3.5 5.4 12.7 2.2 5.5 1.5 0.5 128 3.5 5.4 12.7 2.2 5.5 1.0 1.0 133 3.5 5.4 12.7 2.2 5.5 1.5 0.5 134 3.5 5.4 12.7 2.2 5.5 1.0 1.0 136 3.5 5.9 12.7 2.2 5.5 1.5 146 3.5 7.0 12.2 2.1 5.3 2.0 147 3.5 8.5 11.7 2.1 5.0 2.0 __________________________________________________________________________
TABLE II __________________________________________________________________________ Mechanical Properties of Preferred Form Stress Rupture Tensile (1200.degree.F) 1650.degree.F/60 ksi 1800.degree.F/35 ksi Ultimate Yield R.A. Example Life(hrs) R.A. Life(hrs) R.A. (ksi) (ksi) (%) __________________________________________________________________________ 105 246 21 281 39 171 138 4 106 373 24 172 179 139 12 118 163 19 147 25 122 204 32 184 42 157 136 14 123 98 12 175 40 142 125 8 124 301 24 270 49 139 131 12 125 189 24 130 24 127 253 30 195 39 128 203 37 148 49 133 189 24 159 128 18 134 121 16 162 50 136 247 6 173 26 146 174 3 202 26 147 140 20 247 34 __________________________________________________________________________
As used in tables herein, the terms "ksi" means "thousands of pounds per square inch" and "RA" means "Reduction in Area". All of the data were obtained by testing in air under the conditions identified.
In order to provide cast stress rupture, tensile and creep specimens for each of the alloys evaluated in connection with the present invention, each alloy form was cast and unidirectionally solidified at the rate of about 20 inches per hour to create the aligned cellular dendritic structure which characterizes the article of the present invention. Such structure is aligned predominantly in the <001> direction, which is equivalent to the <100> and <010> direction. Photomicrographic studies of each of the alloys in Table I showed no NiAl phase present. Referring to the drawings, FIGS. 1 and 2 are photomicrographs at 100 magnifications of example 105, typical of the microstructure of the present invention. They show the aligned cellular dendritic structure which resulted from unidirectional solidification, FIG. 1 being in the transverse direction and FIG. 2 being in the longitudinal direction. The elongated dendrites are more clearly shown in FIG. 2. The absence of the dark NiAl phase, shown in FIG. 4, to be discussed later, is particularly evident in FIGS. 1 and 2.
As was mentioned before, NiAl phase is dramatically detrimental to stress rupture properties and hence one of the important characteristics of the present invention is that no NiAl is present in the alloy's microstructure. The data of Table II clearly shows the significantly improved stress rupture properties of the present invention at no sacrifice of tensile properties even though no carbide strengthening is present and the gamma prime strengthener Ti has not been included as an alloying addition.
the present invention specifically excludes the alloying addition of the elements C, Ti, B and Zr. As has been discussed, the element C, although it plays a significant part in ordinary nickel-base superalloys in the carbide strengthening mechanism, can provide a source for crack initiation. Its elimination, except perhaps as an impurity in very small amounts, defines the alloy associated with the present invention as a different kind than the more classical types of nickel-base superalloys.
The elements Zr and B can function in nickel-base superalloys as grain boundary modifiers but have a tendency to lower melting temperature. Therefore, Zr and B are not included as alloying additions in the present invention and are present only as residual elements which can be picked up during normal melting practices. For example, up to about 0.03 percent Zr and up to about 0.01 percent B, by weight, can be tolerated by the present invention without seriously affecting its characteristics.
Only a trace or very small amounts of Ti, for example up to about 1 atomic percent, can be tolerated by the present invention because of the tendency of Ti to form the gamma-gamma prime eutectic phase and to lower the melting temperature. During the evaluation of the present invention, a variety of Ni-base superalloys including varying amounts of Ti were made and tested. A typical one which forms the gamma-gamma prime eutectic in abundance is example 87, the composition of which is, in atomic percent, 6.7 Co; 9 Cr; 1 Mo; 2.4 W; 9.3 Al; 5.8 Ti; 1.6 Ta; 0.3 Zr; 0.17 B; 0.25 C with the balance essentially Ni and incidental impurities. Particularly because of the presence of Ti and C, the alloy composition is outside of the scope of the present invention. FIG. 3 of the drawings is a photomicrograph at 100 magnifications in the transverse direction of the example 87 after unidirectional solidification. FIG. 3 shows the presence of large amounts of the gamma-gamma prime eutectic which is the lighter constituent in the photomicrograph. The incipient melting temperature of example 87 is about 2250.degree.F or at least about 100.degree.F lower than that of the present invention.
Although Ti generally is an essential element in other nickel-base superalloys as a strong gamma prime former, it has been eliminated from the present invention except in trace or residual amounts less than 1 at. percent. Accordingly, a significant feature of the present invention is the substantial elimination of the elements C, Zr, B and Ti normally found in ordinary nickel-base superalloys.
Because of the virtual elimination of the strong gamma prime former Ti, a relatively large amount of Al, which in itself is a strong gamma prime former, is included in the alloy composition associated with the present invention. In this type of alloy, less than 5 at. percent Al does not form sufficient gamma prime and therefore leads to a weak structure. Greater than about 16 at. percent Al, even with a careful balance of other elements, tends to drop out NiAl or excess eutectic and in some alloys tends to reduce incipient melting temperature. In addition to its being a strong gamma prime former, Al also improves oxidation resistance. Its preferred range is 8-14 at. percent.
Substituting for the eliminated Ti is the element V, a gamma prime former without titanium's tendency toward the formation of the gamma-gamma prime eutectic phase which can lower melting temperature. V also provides some solid solution strengthening. In atomic percent, V is included in the range of up to 10 percent although 1-7 percent is preferred. Greater than about 10 percent will have a tendency toward the rejection of NiAl and thus dramatically reduce stress rupture properties. When higher strength is desired, it is specifically preferred that V be included in the range of about 4-7 at. percent.
An important element which is required to be included in the present invention is Re for solid solution strengthening and precipitation hardening. It affects both the gamma prime precipitate as well as the gamma matrix. At least 0.5 at. percent Re, equivalent to at least about 1.5 percent Re by weight, is required for its significant effect in strengthening the matrix, particularly to increase high temperature stress rupture life. In addition, it also affects the gamma prime in that it has a tendency to force hardeners such as Ta and V into the gamma prime. In addition to this function, Re can substitute in amounts up to about 2.5 at. percent for such elements as W, Mn, Ta, Mo, and Cr, all of which tend to partition between the gamma prime precipitate and the gamma matrix. Thus, Re is included in the present invention within the range of 0.5-5 at. percent and preferably in the range of 0.5-3 percent. As shown by the examples of the following Table III, Re in the specific range of 0.5-2 at. percent is particularly desirable for increasing high temperature stress rupture properties and while considering alloy cost. Comparison of example 123 with example 110 shows that the absence of Re is not compensated for by an increase in W to maintain the 1800.degree.F stress rupture properties of example 123.
TABLE III __________________________________________________________________________ Effect of Re on Properties Composition (Atomic %) Stress Rupture Life (hrs) Base: 3.5 Co, 12.7 Al, Balance Ni Example Cr Ta V Re W 1650.degree.F/60 ksi 1800.degree.F/35 ksi __________________________________________________________________________ 105 5.4 2.2 5.5 2.0 246 281 124 5.4 2.7 5.5 1.5 301 270 136 5.9 2.2 5.5 1.5 247 173 125 5.4 3.2 5.5 1.0 189 130 131 5.4 2.2 6.5 1.0 150 134 126 5.4 3.7 5.5 0.5 111 96 132 5.4 2.2 7.0 0.5 101 36 123 5.4 2.2 5.5 0.5 1.5 98 175 110 5.7 2.2 5.5 -- 2.1 110 96 __________________________________________________________________________
Ta can be included in the present invention up to 5 at. percent and is preferably included in the range of 1-4 at. percent. Ta in the type of alloy to which the present invention relates partitions between the gamma prime precipitate and the gamma matrix. Thus, it is both a gamma prime former as well as a solid solution strengthener. Also, it has a tendency to increase incipient melting temperature.
Two elements which act similarly to Ta are W and Mo. Although W can be included up to about 5 at. percent, it is preferred that such element be maintained in the range of up to about 2 percent for improved properties. Mo, which can be included up to about 1 at. percent, in the absence of Ti will partition to gamma prime. However, it has a tendency to impair corrosion and oxidation resistance. Therefore, it is included only up to about 1 at. percent.
Required primarily for improvement in oxidation resistance is Cr which can be included in the range of about 4-11 at. percent and preferably in the range of about 4-9 at. percent. Less than 4 percent insufficient for oxidation resistance; greater than 11 percent tends to introduce alloy instability. At such higher levels, the alloy is either too weak or is unstable. Therefore, it is preferred that Cr be included in the range of 4-9 at. percent with higher amounts being tolerable provided other elements, within the range of the present invention, are balanced to avoid forcing the formation of NiAl or other undesirable phases such as sigma, eta and mu. The effect of such unbalance is shown in the following Table IV.
TABLE IV __________________________________________________________________________ Effect of Re and Cr on Properties Composition (Atomic %) Stress Rupture Life (hrs) Base: 3.5 Co, Balance Ni Example Cr Al Ta V Re 1650.degree.F/60 ksi 1800.degree.F/35 ksi __________________________________________________________________________ 136 5.9 12.7 2.2 5.5 1.5 247 173 146 7.0 12.2 2.1 5.3 2.0 174 202 142 8.0 12.7 2.2 5.5 1.0 121 95 147 8.5 11.7 2.1 5.0 2.0 140 247 143 9.0 12.7 2.2 5.5 1.0 101 102 144 10.0 12.7 2.2 5.5 1.0 2 3 148 10.0 11.2 2.0 4.7 2.0 33 48 145 11.0 12.7 2.2 5.5 1.0 1 3 __________________________________________________________________________
Photomicrographic studies of the examples of Table IV showed that only examples 144, 145 and 148 exhibited the undesirable NiAl structure. The dramatic difference in properties can be seen from the stress rupture data presented in Table IV. Referring to the drawings, FIG. 4 is a photomicrograph at 100 magnifications in the transverse direction of the structure of example 145 showing a large amount of the dark NiAl detrimental phase which produced the dramatic reduction in stress rupture properties in examples 144, 145 and 148 even though example 148 included 2 at. percent Re. For this reason, the present invention is characterized by the absence of NiAl in its microstructure which also has the aligned cellular dentrites.
The element Co can be included in the present invention as a substitute for nickel in an amount up to about 15 at. percent. It has a slight tendency toward the increase of melting temperature and lowers the stacking fault energy. Preferably, Co is included in the range of about 3-8 at. percent.
Mn and Rh can be included as partial substitutes for Re in the present invention. However, they are not as effective as is Re. Each of Mn and Rh can be included within the present invention in amounts up to 2.5 at. percent but preferably are included in amounts up to 1 at. percent each. The effect of additions of Mn, Rh and Mo at various levels of Re is shown in Table V.
TABLE V __________________________________________________________________________ Effect of Mo, Mn and Rh on Properties Composition (Atomic %) Stress Rupture Life (hrs) Base: 3.5 Co;5.4 Cr;12.7 Al;2.2 Ta;5.5 V;Bal Ni Example Re Mn Mo Rh 1650.degree.F/60 ksi 1800.degree.F/35 ksi __________________________________________________________________________ 118 1.5 0.5 163 146 119 1.0 1.0 131 111 120 0.5 1.5 73 42 127 1.5 0.5 253 195 128 1.0 1.0 203 148 129 0.5 1.5 129 79 133 1.5 0.5 189 134 1.0 1.0 121 162 135 0.5 1.5 117 84 __________________________________________________________________________
In addition to a remarkable improvement in stress rupture properties, the present invention provides significant improvement in creep properties. This is shown in the data on which FIG. 5 is based, comparing example 106 with a known cast nickel-base superalloy now in production use in gas turbine engines and included within the scope of U.S. Pat. No. 3,615,376 - Ross, issued Oct. 26, 1971.
From all of these data, it can be seen that the present invention provides a different kind of alloy which is particularly useful in the formation of articles having improved high temperature properties as a result of the combination of the balance of elements and the processing to provide aligned cellular dendrites in the article's microstructure. Ordinary nickel-base superalloys include carbon which is then available for the formation of various types of carbides. The strength mechanism and microstructure of such ordinary alloy heavily involves carbide formation and accumulation at various points in the microstucture. The literature in respect to nickel-base superalloys includes very complete discussions of this type of microstructure and its problems and benefits based on carbides. Without carbon, there is defined a completely different kind of alloy, the properties of which depend on the gamma prime, gamma, eutectic and other phases, some of which can be detrimental or undesirable. For example, NiAl, which is sometimes called beta phase, is dramatically destructive toward stress rupture properties; the gamma-gamma prime eutectic tends to lower incipient melting temperature and hence it is to be maintained at as low a level as is practical. To obtain high temperature strength which otherwise has been provided by the absent carbides, the type of alloy involved with the present invention must include significantly larger or different alloying additions to strengthen both the gamma prime intermetallic precipitate as well as the gamma matrix while removing the tendency toward NiAl formation and reducing the gamma-gamma prime eutectic formation. Thus, the present invention adds as much Al and Cr as possbile while maintaining such stability and balancing the alloy's stability with other alloying additions to avoid the formation of NiAl.
The cast article of the present invention is characterized not only by its aligned cellular dendritic structure and the absence of carbides and NiAl, but also by the fact that it does not include alloying additions of Ti, Zr and B normally added to nickel-base superalloys. In addition, it includes ralatively large amounts of Re which has been found to provide improved strength both for the gamma matrix as well as for the gamma prime precipitate. Because the alloy has a narrower solidus-liquidus range, it is easier to process by unidirectional solidification and therefore such processing can be conducted at higher rates. Its improved stress rupture properties are attained without a sacrifice of tensile properties which are as good or better than ordinary superalloy tensile strength and ductilities.
Although the present invention has been described in connection with specific examples and embodiments, it will be understood by those skilled in the art the variations and modifications of which the invention is capable within its broad scope.
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