Title:
NICKEL-BASE SUPERALLOY RESISTANT TO OXIDATION-EROSION
United States Patent 3754902
Abstract:
Nickel-base superalloys are described which oxidize selectively to form a single protective oxide Al2 O3 which is particularly adherent to the alloy surface. Alloys of the nickel-chromium-aluminum type are disclosed in a basic chemistry corresponding to, by weight, 4-7 percent aluminum, 12-21 percent chromium, 19-25 percent aluminum plus chromium, 0.01-0.5 percent yttrium or similar material, balance nickel with the usual alloying ingredients associated with the superalloys.
US Patent References:
Nickel chromium alloys having high creep strength at high temperatures
Gresham et al. - July 1955 - 2712498
Nickel-base alloys
Thielemann - January 1965 - 3164465
Nickel base alloy
Wlodek - February 1967 - 3304176
Sulfidation and oxidation resistant cobalt-base alloy
Roush - August 1968 - 3399058
Nickel chromium alloys having high creep strength at high temperatures
Gresham et al. - July 1955 - 2712498
Nickel-base alloys
Thielemann - January 1965 - 3164465
Nickel base alloy
Wlodek - February 1967 - 3304176
Sulfidation and oxidation resistant cobalt-base alloy
Roush - August 1968 - 3399058
Inventors:
Boone, Donald H. (North Haven, CT)
Duhl, David N. (Newington, CT)
Goward, George W. (North Haven, CT)
Duhl, David N. (Newington, CT)
Goward, George W. (North Haven, CT)
Application Number:
04/734706
Publication Date:
08/28/1973
Filing Date:
06/05/1968
View Patent Images:
Export Citation:
Assignee:
United Aircraft Corporation (East Hartford, CT)
Primary Class:
Other Classes:
420/446
International Classes:
C22C19/00; C22C32/00; F01D25/00; C22B61/00
Field of Search:
75/122.5,171
Primary Examiner:
Quarforth, Carl D.
Assistant Examiner:
Tate R. L.
Claims:
We claim
1. An oxidation resistant alloy which consists essentially of, by weight:
2. An alloy according to claim 1 also including, by weight:
1. An oxidation resistant alloy which consists essentially of, by weight:
2. An alloy according to claim 1 also including, by weight:
Description:
BACKGROUND OF THE INVENTION
The present invention relates to the nickel-base super-alloys particularly those adapted to use in high temperature oxidizing environments including gas turbine engines. As utilized in the fabrication of blades, vanes and other gas turbine engine components, the chemistry of the typical superalloy represents, as it must, a compromise between the optimum mechanical properties of the material and its optimum oxidation-erosion resistance. In the scale of such things, the typical nickel-base superalloys, such as B-1900, are reasonably resistant to oxidation-erosion in the temperature range up to about 1,600°-1,800°F. Above these temperatures, however, the oxidation rates in most applications are unacceptably rapid and protective coatings are necessarily utilized to prolong the useful life of the alloy.
The typical coating provides the superalloy with a surface layer characterized by increased oxidation resistance. In the gas turbine engine industry this protective layer is often formed by reacting aluminum with the surface of the alloy to form an aluminide zone which upon oxidation at high temperature in turn reacts to form an oxide layer through which the transport rates of the reacting species are low. In this regard see the U.S. Pat. to Joseph, No. 3,102,044. Although increased lifetimes can be attained by this approach, oxidation is still the usual form of failure which limits component lifetime in most applications.
Coatings are prone to failure by a variety of mechanisms and, upon such failure, the substrate alloy is again subject to an unacceptable rapid oxidation-erosion. Aluminide coatings are frequently subject to failure because of compositional changes resulting from the loss of the oxide forming component due to spalling of the oxide. Because of the complex nature of the alloys such as B-1900, the protective oxides are in fact mixtures of oxides (e.g. Cr 2 O 3 , Al 2 O 3 , NiO, spinels, etc.) rather than a single oxide. And it has been discovered that such mixtures are less efficient barriers than the single pure oxide and more subject to spalling under the thermal cycling conditions encountered in jet engines. THe cyclic process of spallation and reformation renders such alloys even less oxidation resistant than they would be under purely isothermal conditions.
Furthermore, aluminide coatings are a source of fracture initiation in fatigue and for a specific design either decrease the volume of the load carrying material or increase the weight of the particular component. Thus, the use of aluminide coatings to extend the life and operating temperature of the nickel-base superalloys is usually not without some trade-off of mechanical properties coupled, of course, with an added economic burden.
In view of the foregoing the desirability and utility of a superalloy having mechanical properties at least equivalent to the conventional alloys together with significantly-improved oxidation-erosion resistance is immediately evident.
SUMMARY OF THE INVENTION
The present invention may be broadly described as a nickel-base alloy of the gamma-gamma prime type or, more particularly, a superalloy of the nickel-chromium aluminum type.
It is the principal object of the present invention to provide a nickel-base superalloy with greatly improved oxidation-erosion resistance and yet with adequate mechanical properties rendering it suitable for use in gas turbine engine hardware and other rigorous environments. Accordingly, a relatively specific alloy composition is described wherein the chromium and aluminum contents are adjusted so as to form a protective layer of a single oxide of alumina upon oxidation and which includes a reactive element such as yttrium for increased adherence of the single protective oxide. And the chemistry is so selectred that the alloy is retained within the two-phase gamma-gamma prime phase field.
Broadly stated in terms of the alloy composition, this invention encompasses the alloys containing, by weight, 4-7 percent aluminum, 12-21 percent chromium, 19-25 percent chromium plus aluminum, 0.01-0.5 percent yttrium and/or similar materials, balance nickel together with one or more of the optional alloying ingredients conventionally associated with the nickel-base superalloys.
A particularly preferred composition is that hereinafter identified as Alloy 350. This alloy has particular utility in the formation of single crystal gas turbine engine components and has a chemistry as follows:
Ingredient Percent by Weight Chromium 15.5 - 17 Aluminum 5.3 - 6 Titanium 1.7 - 2.2 Tungsten 7.5 - 8.5 Yttrium 0.01 - 0.3 Nickel Balance
For the more conventional casting techniques, Alloy 350P is preferred at the following composition:
Ingredient Percent by Weight Chromium 15.5 - 17 Aluminum 5.3 - 6 Titanium 1.7 - 2.2 Tungsten 7.5 - 8.5 Yttrium 0.01 - 0.3 Carbon 0.03 - 0.08 Boron 0.01 - 0.02 Zirconium 0.08 - 0.12 Nickel Balance
It will be noted that Alloy 350 and Alloy 350P differ only in the inclusion in Alloy 350P of carbon, boron and zirconium to promote ductility and fabricability in the polycrystalline components.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph illustrating the oxidation-erosion of Alloy 350 as compared to the conventional nickel-base superalloys in both the coated and uncoated condition. In this test comparative specimens were exposed to a high velocity propane flame at 2,100°F.
FIG. 2 is a graph comparing the stress-rupture characteristics of Alloy 350 at various temperatures as compared to cast Udimet 700.
FIG. 3 is a graph which dramatically demonstrates the oxidation-erosion behavior of various nickel-chromium-aluminum alloys with and without a reactive metal addition as compared to representative contemporary materials. Tests were conducted in a JP5-R flame at 2,100°F at a gas velocity of about Mach 0.3.
FIG. 4 is a plot of weight change/time comparing a subject alloy with Hastelloy X at 2,100°F in dynamic oxidation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the description which follows reference will be made to various of the conventional or contemporary nickel-base alloys. Representative alloys of this general type are those identified in the industry as follows, percent by weight:
Alloy Nominal Composition Hastelloy X 22%Cr, 1.5%Co, 9%Mo, .1%C, 18.5%Fe, .6%W, balance Ni. Udimet 700 15%Cr, 18.5%Co, 3.5%Ti, 4.25%Al, 5.2%Mo, .08%C, .03%B, balance Ni. B-1900 8%Cr, 10%Co, 1%Ti, 6%Al, 6%Mo, .1%C, 4.3%Ta, .015%B, .07%Zr, balance Ni.
The typical nickel-base superalloy is essentially a nickel-chromium solid solution (gamma phase) hardened by the addition of aluminum and, most frequently, titanium to precipitate an intermetallic compound (gamma prime phase), typically Ni 3 (Al,Ti). In the preferred embodiment the gamma prime phase Ni 3 (Al,Ti) is ordered face-centered-cubic with aluminum and titanium atoms at the corners of the unit cell and nickel at the face centers.
Current alloys also normally contain cobalt to raise the solvus temperature of the gamma prime phase, refractory metal additions for solution strengthening, and carbon, boron and zirconium to promote ductility and fabricability.
The present alloys are of the nickel-chromium-aluminum type or, in a generic sense, of the nickel-chromium-aluminum-yttrium type. They have the unique characteristic of being able to form the single oxide, Al 2 O 3 , upon oxidation as a result of the careful control of the chromium and aluminum levels in the alloys coupled with the ability to tenaciously retain this oxide by virtue of the adherence effects promoted by the addition of a reactive element such as yttrium, scandium, thorium, lanthanum and the other rare earth elements. The excellent oxidation resistance so imparted suggests the possible use of these alloys in the uncoated condition, thereby eliminating many of the problems associated with the use of an aluminide coating. This is clearly demonstrated by the oxidation-erosion tests of FIG. 1. It will be observed that there is a significant improvement in the oxidation-erosion resistance of Alloy 350 and the associated compositions when compared with the contemporary nickel-base superalloys. Even more signficant is the comparable level of oxidation resistance obtained with the uncoated alloys of the present invention when compared to the aluminide coated conventional alloys. In the case of the Alloy 350 type alloys, the alloy continues to oxidize at a low and predictable rate for an as-yet indeterminate period of time. As shown in the data, this is not true in the case of the conventional coated alloy which upon failure of the coating reverts to the more rapid rate of the uncoated alloy.
In terms of its mechanical properties Alloy 350 exhibits tensile, creep and fatigue properties equivalent to cast Udimet 700 as indicated in FIG. 2 and the following Table.
Table I
Alloy 350
Temp. .2% Yield Ultimate %Elonga- (°F) Strength (psi) Strength (psi) tion R.A. R.T. 136,000 164,000 8.4 8.5 1400 141,500 157,500 7.5 14.4 1800 41,500 60,000 17.1 29.7 Room Temperature (R.T.) Elastic Modulus Alloy 350 17 × 10 6 psi Cast Udimet 700 32 × 10 6 psi Low Cycle Fatigue Life at 1400°F E = 1.6% Alloy 350 469 cycles Cast Udimet 700 167 cycles
The excellent thermal fatigue resistance can be attributed to the unusually low elastic modulus of the material in columnar-grained or single crystal form.
The basic precept involved in the present invention involves the maintenance of a critical relationship between the chromium and aluminum contents of the alloy. Specifically, the chromium and aluminum contents are adjusted to form only the single oxide of aluminum upon oxidation while maintaining the overall alloy for structural purposes within the two-phase gamma-gamma prime phase field. Coupled with the above phenomenon is the inclusion of a reactive element such as yttrium to promote adherence of the single oxide so formed. A variety of tests within the basic composition were performed to establish the criticality of the compositional limits established, as selectively summarized in FIGS. 2-4.
As previously set forth, the basic composition involved here is as follows, by weight:
Nickel base -- 4-7 percent aluminum -- 12-21 percent chromium -- 19-25 percent chromium plus aluminum -- 0.01-0.5 percent reactive metal, including yttrium and scandium.
Various of the basic alloy compositions tested include the following:
Ni -- 16%Cr -- 5%Al -- 0.5%Y
Ni -- 20%Cr -- 5% Al -- 0.5%Y
Ni -- 25%Cr -- 6%Al -- 1%Y
Ni -- 25%Cr -- 6%Al -- 0.85%Zr
Ni -- 16%Cr -- 5%Al -- 0.3%Sc
Ni -- 18%Cr -- 6%Al -- 10%W -- 0.5%Y
Ni -- 16%Cr -- 6%Al -- 10%W -- 0.25%Sc
Ni -- 16%Cr -- 8%Al -- 4%Ta -- 0.5%Y
Ni -- 16%Cr -- 6%Al -- 4%Ta -- 4%W -- 2%Ti -- 0.1%Sc
Ni -- 16%Cr -- 10%Co -- 5%W -- 4%Ta -- 1.5%Mo -- 5%Al -- 2%Ti -- 0.1%Sc
All of the above alloys possess superior oxidation-erosion resistance when compared to the contemporary alloys. The latter five alloys contain tungsten, tantalum and titanium, elements usually added to the nickel-base superalloys for strengthening purposes.
Alloys of the composition Ni--16%Cr -- 5%Al -- 0.1%Y and Ni -- 16%Cr -- 5%Al -- 0.5%Y have been fabricated to 40 mil sheet by forging and rolling. This sheet has been tested in stress rupture to compare its properties with Hastelloy X, a widely used sheet alloy in the gas turbine engine industry. The results indicate performance generally equivalent to or better than Hastelloy X. And two points should be emphasized in connection with these particular tests: (a) these alloys were tested in the as-rolled condition without application of any optimizing heat treatment; and (b) these alloys were, in fact, among the weakest in the series under consideration, lacking the usual strengtheners, such as tungsten, tantalum and titanium. Even in the basic form, however, these alloys exhibited a strength comparable to a widely used sheet alloy but with significantly superior oxidation resistance.
The addition of these elements commonly used to strengthen the nickel-base alloys does not appear to degrade the oxidation-erosion resistance observed for the simpler alloys. The preferred composition of Alloy 350 is: Ni -- 15.5-17%Cr -- 5.3-6%Al -- 1.7-2.2%Ti -- 7.5-8.5%W -- 0.01-0.3%Y. It will be noted that the elements carbon, boron and zirconium are not required in Alloy 350 for single crystal components formed by directional solidification techniques. The techniques and advantages of the directional solidification processes are discussed in detail in the U.S. Pat. to VerSnyder No. 3,260.505 and in the copending application of Piearcey, Ser. No. 540,114, now U.S. Pat. No. 3,494,709. This patent and this application share a common assignee with the present invention.
Carbon, boron and zirconium are normally included in the alloys cast into polycrystalline form, however, to promote ductility and fabricability. Carbon is believed to achieve this end by precipitating at grain boundaries as a modified Cr 23 C 6 . The Cr 23 C 6 particles decrease the tendency of the alloy to form intergranular cracks by suppressing grain boundary sliding. But the carbon also reacts to form other carbide phases (MC, M 6 C and M 7 C 3 ) depending upon composition and heat treatment.
Many of the nickel-base superalloys even in an uncoated condition exhibit poor resistance to thermally-induced, low-cycle fatigue which in many cases can be traced to the presence of large MC carbides. Because of the slow solidification rates and shallow solidification front thermal gradients in the directional solidification processes, the growth of the detrimental MC-type carbides is fostered. However, since the single crystal articles formed by such processes are characterized by an absence of grain boundaries the carbon may advantageously be eliminated. The precracking phenomena associated with the MC-type carbides produced in the directional solidification process is described in detail in the copending application to Gell et al., Ser. No. 725,889 filed May 1, 1968, which shares a common assignee with the present invention. As reported therein, the carbon content of the nickel-base alloys used to produce single crystal engine components will advantageously be held below about 100 parts per million at which level the MC-type carbide formation is prohibited. The elimination of carbon in these components greatly enhances the fatigue life thereof by removing a prime site for fatigue crack initiation. In the polycrystalline alloy modification, however, carbon, boron and zirconium will advantageously be included.
In the preceding description a number of examples and preferred embodiments are discussed. Those versed in the art will recognize that a number of variations in alloy composition are possible, including the following:
1. Substitution of other refractory metals such as molybdenum, tantalum, columbium, rhenium or hafnium for all or part of the tungsten content of the alloy.
2. Substitution of columbium for all or part of the titanium present in the alloy.
3. Substitution of other reactive metals such as scandium, thorium, lanthanum or other rare earths, or mixtures such as misch metal, for the yttrium content in whole or in part.
4. The partial substitution of cobalt for nickel in the alloy.
5. The addition of an inert oxide dispersant such as thoria for strengthening.
As previously indicated, the optional additions to the alloys of the present invention, including those variations just described, do not interfere with the basic objects of this invention. This will remain true as long as the addition does not alter the basic structure of the alloy from a two-phase gamma-gamma prime phase field; and as long as the addition does not interfere with the oxidation behavior of the alloy insofar as it forms a single adherent surface oxide upon exposure to a high temperature oxidizing environment.
In terms of promoting oxide adherence to the substrate alloy any metal having an affinity for oxygen approximating or exceeding that of aluminum is workable. However, as used herein, the term "reactive metal" has reference to the elements yttrium, scandium, thorium, lanthanum and the rare earths from lanthanum (atomic number 57) to lutetium (atomic number 71). Recent experiments have indicated that it may be advantageous in some instances in terms of mechanical properties to change the reactive metal from yttrium with its low solubility to an element such as scandium of higher solubility. This would where desired eliminate the presence of a reactive metal rich phase such as Ni 5 Y such as has been found in some alloys.
It will be recognized, therefore, that while the present invention has been described in detail in connection with certain examples and particular preferred embodiments, numerous variations with respect thereto, some of which have been mentioned in the preceding discussion, will be evident to those skilled in the art. Furthermore, while in terms of utility, the various alloys have been described in connection with gas turbine engine components, the usefulness of these alloys is not so confined. These alloys provide exceptional oxidation-erosion resistance in conjunction with good mechanical properties such as creep and fatigue resistance, and good tensile strength and ductility. As a monocrystal or directionally solidified component with the [100] orientation parallel to the stress axis the low modulus of this material will provide excellent thermal fatigue resistance. The alloys are, accordingly, suitable for any part required to operate at elevated temperatures in an oxidizing atmosphere.
The present invention relates to the nickel-base super-alloys particularly those adapted to use in high temperature oxidizing environments including gas turbine engines. As utilized in the fabrication of blades, vanes and other gas turbine engine components, the chemistry of the typical superalloy represents, as it must, a compromise between the optimum mechanical properties of the material and its optimum oxidation-erosion resistance. In the scale of such things, the typical nickel-base superalloys, such as B-1900, are reasonably resistant to oxidation-erosion in the temperature range up to about 1,600°-1,800°F. Above these temperatures, however, the oxidation rates in most applications are unacceptably rapid and protective coatings are necessarily utilized to prolong the useful life of the alloy.
The typical coating provides the superalloy with a surface layer characterized by increased oxidation resistance. In the gas turbine engine industry this protective layer is often formed by reacting aluminum with the surface of the alloy to form an aluminide zone which upon oxidation at high temperature in turn reacts to form an oxide layer through which the transport rates of the reacting species are low. In this regard see the U.S. Pat. to Joseph, No. 3,102,044. Although increased lifetimes can be attained by this approach, oxidation is still the usual form of failure which limits component lifetime in most applications.
Coatings are prone to failure by a variety of mechanisms and, upon such failure, the substrate alloy is again subject to an unacceptable rapid oxidation-erosion. Aluminide coatings are frequently subject to failure because of compositional changes resulting from the loss of the oxide forming component due to spalling of the oxide. Because of the complex nature of the alloys such as B-1900, the protective oxides are in fact mixtures of oxides (e.g. Cr 2 O 3 , Al 2 O 3 , NiO, spinels, etc.) rather than a single oxide. And it has been discovered that such mixtures are less efficient barriers than the single pure oxide and more subject to spalling under the thermal cycling conditions encountered in jet engines. THe cyclic process of spallation and reformation renders such alloys even less oxidation resistant than they would be under purely isothermal conditions.
Furthermore, aluminide coatings are a source of fracture initiation in fatigue and for a specific design either decrease the volume of the load carrying material or increase the weight of the particular component. Thus, the use of aluminide coatings to extend the life and operating temperature of the nickel-base superalloys is usually not without some trade-off of mechanical properties coupled, of course, with an added economic burden.
In view of the foregoing the desirability and utility of a superalloy having mechanical properties at least equivalent to the conventional alloys together with significantly-improved oxidation-erosion resistance is immediately evident.
SUMMARY OF THE INVENTION
The present invention may be broadly described as a nickel-base alloy of the gamma-gamma prime type or, more particularly, a superalloy of the nickel-chromium aluminum type.
It is the principal object of the present invention to provide a nickel-base superalloy with greatly improved oxidation-erosion resistance and yet with adequate mechanical properties rendering it suitable for use in gas turbine engine hardware and other rigorous environments. Accordingly, a relatively specific alloy composition is described wherein the chromium and aluminum contents are adjusted so as to form a protective layer of a single oxide of alumina upon oxidation and which includes a reactive element such as yttrium for increased adherence of the single protective oxide. And the chemistry is so selectred that the alloy is retained within the two-phase gamma-gamma prime phase field.
Broadly stated in terms of the alloy composition, this invention encompasses the alloys containing, by weight, 4-7 percent aluminum, 12-21 percent chromium, 19-25 percent chromium plus aluminum, 0.01-0.5 percent yttrium and/or similar materials, balance nickel together with one or more of the optional alloying ingredients conventionally associated with the nickel-base superalloys.
A particularly preferred composition is that hereinafter identified as Alloy 350. This alloy has particular utility in the formation of single crystal gas turbine engine components and has a chemistry as follows:
Ingredient Percent by Weight Chromium 15.5 - 17 Aluminum 5.3 - 6 Titanium 1.7 - 2.2 Tungsten 7.5 - 8.5 Yttrium 0.01 - 0.3 Nickel Balance
For the more conventional casting techniques, Alloy 350P is preferred at the following composition:
Ingredient Percent by Weight Chromium 15.5 - 17 Aluminum 5.3 - 6 Titanium 1.7 - 2.2 Tungsten 7.5 - 8.5 Yttrium 0.01 - 0.3 Carbon 0.03 - 0.08 Boron 0.01 - 0.02 Zirconium 0.08 - 0.12 Nickel Balance
It will be noted that Alloy 350 and Alloy 350P differ only in the inclusion in Alloy 350P of carbon, boron and zirconium to promote ductility and fabricability in the polycrystalline components.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph illustrating the oxidation-erosion of Alloy 350 as compared to the conventional nickel-base superalloys in both the coated and uncoated condition. In this test comparative specimens were exposed to a high velocity propane flame at 2,100°F.
FIG. 2 is a graph comparing the stress-rupture characteristics of Alloy 350 at various temperatures as compared to cast Udimet 700.
FIG. 3 is a graph which dramatically demonstrates the oxidation-erosion behavior of various nickel-chromium-aluminum alloys with and without a reactive metal addition as compared to representative contemporary materials. Tests were conducted in a JP5-R flame at 2,100°F at a gas velocity of about Mach 0.3.
FIG. 4 is a plot of weight change/time comparing a subject alloy with Hastelloy X at 2,100°F in dynamic oxidation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the description which follows reference will be made to various of the conventional or contemporary nickel-base alloys. Representative alloys of this general type are those identified in the industry as follows, percent by weight:
Alloy Nominal Composition Hastelloy X 22%Cr, 1.5%Co, 9%Mo, .1%C, 18.5%Fe, .6%W, balance Ni. Udimet 700 15%Cr, 18.5%Co, 3.5%Ti, 4.25%Al, 5.2%Mo, .08%C, .03%B, balance Ni. B-1900 8%Cr, 10%Co, 1%Ti, 6%Al, 6%Mo, .1%C, 4.3%Ta, .015%B, .07%Zr, balance Ni.
The typical nickel-base superalloy is essentially a nickel-chromium solid solution (gamma phase) hardened by the addition of aluminum and, most frequently, titanium to precipitate an intermetallic compound (gamma prime phase), typically Ni 3 (Al,Ti). In the preferred embodiment the gamma prime phase Ni 3 (Al,Ti) is ordered face-centered-cubic with aluminum and titanium atoms at the corners of the unit cell and nickel at the face centers.
Current alloys also normally contain cobalt to raise the solvus temperature of the gamma prime phase, refractory metal additions for solution strengthening, and carbon, boron and zirconium to promote ductility and fabricability.
The present alloys are of the nickel-chromium-aluminum type or, in a generic sense, of the nickel-chromium-aluminum-yttrium type. They have the unique characteristic of being able to form the single oxide, Al 2 O 3 , upon oxidation as a result of the careful control of the chromium and aluminum levels in the alloys coupled with the ability to tenaciously retain this oxide by virtue of the adherence effects promoted by the addition of a reactive element such as yttrium, scandium, thorium, lanthanum and the other rare earth elements. The excellent oxidation resistance so imparted suggests the possible use of these alloys in the uncoated condition, thereby eliminating many of the problems associated with the use of an aluminide coating. This is clearly demonstrated by the oxidation-erosion tests of FIG. 1. It will be observed that there is a significant improvement in the oxidation-erosion resistance of Alloy 350 and the associated compositions when compared with the contemporary nickel-base superalloys. Even more signficant is the comparable level of oxidation resistance obtained with the uncoated alloys of the present invention when compared to the aluminide coated conventional alloys. In the case of the Alloy 350 type alloys, the alloy continues to oxidize at a low and predictable rate for an as-yet indeterminate period of time. As shown in the data, this is not true in the case of the conventional coated alloy which upon failure of the coating reverts to the more rapid rate of the uncoated alloy.
In terms of its mechanical properties Alloy 350 exhibits tensile, creep and fatigue properties equivalent to cast Udimet 700 as indicated in FIG. 2 and the following Table.
Table I
Alloy 350
Temp. .2% Yield Ultimate %Elonga- (°F) Strength (psi) Strength (psi) tion R.A. R.T. 136,000 164,000 8.4 8.5 1400 141,500 157,500 7.5 14.4 1800 41,500 60,000 17.1 29.7 Room Temperature (R.T.) Elastic Modulus Alloy 350 17 × 10 6 psi Cast Udimet 700 32 × 10 6 psi Low Cycle Fatigue Life at 1400°F E = 1.6% Alloy 350 469 cycles Cast Udimet 700 167 cycles
The excellent thermal fatigue resistance can be attributed to the unusually low elastic modulus of the material in columnar-grained or single crystal form.
The basic precept involved in the present invention involves the maintenance of a critical relationship between the chromium and aluminum contents of the alloy. Specifically, the chromium and aluminum contents are adjusted to form only the single oxide of aluminum upon oxidation while maintaining the overall alloy for structural purposes within the two-phase gamma-gamma prime phase field. Coupled with the above phenomenon is the inclusion of a reactive element such as yttrium to promote adherence of the single oxide so formed. A variety of tests within the basic composition were performed to establish the criticality of the compositional limits established, as selectively summarized in FIGS. 2-4.
As previously set forth, the basic composition involved here is as follows, by weight:
Nickel base -- 4-7 percent aluminum -- 12-21 percent chromium -- 19-25 percent chromium plus aluminum -- 0.01-0.5 percent reactive metal, including yttrium and scandium.
Various of the basic alloy compositions tested include the following:
Ni -- 16%Cr -- 5%Al -- 0.5%Y
Ni -- 20%Cr -- 5% Al -- 0.5%Y
Ni -- 25%Cr -- 6%Al -- 1%Y
Ni -- 25%Cr -- 6%Al -- 0.85%Zr
Ni -- 16%Cr -- 5%Al -- 0.3%Sc
Ni -- 18%Cr -- 6%Al -- 10%W -- 0.5%Y
Ni -- 16%Cr -- 6%Al -- 10%W -- 0.25%Sc
Ni -- 16%Cr -- 8%Al -- 4%Ta -- 0.5%Y
Ni -- 16%Cr -- 6%Al -- 4%Ta -- 4%W -- 2%Ti -- 0.1%Sc
Ni -- 16%Cr -- 10%Co -- 5%W -- 4%Ta -- 1.5%Mo -- 5%Al -- 2%Ti -- 0.1%Sc
All of the above alloys possess superior oxidation-erosion resistance when compared to the contemporary alloys. The latter five alloys contain tungsten, tantalum and titanium, elements usually added to the nickel-base superalloys for strengthening purposes.
Alloys of the composition Ni--16%Cr -- 5%Al -- 0.1%Y and Ni -- 16%Cr -- 5%Al -- 0.5%Y have been fabricated to 40 mil sheet by forging and rolling. This sheet has been tested in stress rupture to compare its properties with Hastelloy X, a widely used sheet alloy in the gas turbine engine industry. The results indicate performance generally equivalent to or better than Hastelloy X. And two points should be emphasized in connection with these particular tests: (a) these alloys were tested in the as-rolled condition without application of any optimizing heat treatment; and (b) these alloys were, in fact, among the weakest in the series under consideration, lacking the usual strengtheners, such as tungsten, tantalum and titanium. Even in the basic form, however, these alloys exhibited a strength comparable to a widely used sheet alloy but with significantly superior oxidation resistance.
The addition of these elements commonly used to strengthen the nickel-base alloys does not appear to degrade the oxidation-erosion resistance observed for the simpler alloys. The preferred composition of Alloy 350 is: Ni -- 15.5-17%Cr -- 5.3-6%Al -- 1.7-2.2%Ti -- 7.5-8.5%W -- 0.01-0.3%Y. It will be noted that the elements carbon, boron and zirconium are not required in Alloy 350 for single crystal components formed by directional solidification techniques. The techniques and advantages of the directional solidification processes are discussed in detail in the U.S. Pat. to VerSnyder No. 3,260.505 and in the copending application of Piearcey, Ser. No. 540,114, now U.S. Pat. No. 3,494,709. This patent and this application share a common assignee with the present invention.
Carbon, boron and zirconium are normally included in the alloys cast into polycrystalline form, however, to promote ductility and fabricability. Carbon is believed to achieve this end by precipitating at grain boundaries as a modified Cr 23 C 6 . The Cr 23 C 6 particles decrease the tendency of the alloy to form intergranular cracks by suppressing grain boundary sliding. But the carbon also reacts to form other carbide phases (MC, M 6 C and M 7 C 3 ) depending upon composition and heat treatment.
Many of the nickel-base superalloys even in an uncoated condition exhibit poor resistance to thermally-induced, low-cycle fatigue which in many cases can be traced to the presence of large MC carbides. Because of the slow solidification rates and shallow solidification front thermal gradients in the directional solidification processes, the growth of the detrimental MC-type carbides is fostered. However, since the single crystal articles formed by such processes are characterized by an absence of grain boundaries the carbon may advantageously be eliminated. The precracking phenomena associated with the MC-type carbides produced in the directional solidification process is described in detail in the copending application to Gell et al., Ser. No. 725,889 filed May 1, 1968, which shares a common assignee with the present invention. As reported therein, the carbon content of the nickel-base alloys used to produce single crystal engine components will advantageously be held below about 100 parts per million at which level the MC-type carbide formation is prohibited. The elimination of carbon in these components greatly enhances the fatigue life thereof by removing a prime site for fatigue crack initiation. In the polycrystalline alloy modification, however, carbon, boron and zirconium will advantageously be included.
In the preceding description a number of examples and preferred embodiments are discussed. Those versed in the art will recognize that a number of variations in alloy composition are possible, including the following:
1. Substitution of other refractory metals such as molybdenum, tantalum, columbium, rhenium or hafnium for all or part of the tungsten content of the alloy.
2. Substitution of columbium for all or part of the titanium present in the alloy.
3. Substitution of other reactive metals such as scandium, thorium, lanthanum or other rare earths, or mixtures such as misch metal, for the yttrium content in whole or in part.
4. The partial substitution of cobalt for nickel in the alloy.
5. The addition of an inert oxide dispersant such as thoria for strengthening.
As previously indicated, the optional additions to the alloys of the present invention, including those variations just described, do not interfere with the basic objects of this invention. This will remain true as long as the addition does not alter the basic structure of the alloy from a two-phase gamma-gamma prime phase field; and as long as the addition does not interfere with the oxidation behavior of the alloy insofar as it forms a single adherent surface oxide upon exposure to a high temperature oxidizing environment.
In terms of promoting oxide adherence to the substrate alloy any metal having an affinity for oxygen approximating or exceeding that of aluminum is workable. However, as used herein, the term "reactive metal" has reference to the elements yttrium, scandium, thorium, lanthanum and the rare earths from lanthanum (atomic number 57) to lutetium (atomic number 71). Recent experiments have indicated that it may be advantageous in some instances in terms of mechanical properties to change the reactive metal from yttrium with its low solubility to an element such as scandium of higher solubility. This would where desired eliminate the presence of a reactive metal rich phase such as Ni 5 Y such as has been found in some alloys.
It will be recognized, therefore, that while the present invention has been described in detail in connection with certain examples and particular preferred embodiments, numerous variations with respect thereto, some of which have been mentioned in the preceding discussion, will be evident to those skilled in the art. Furthermore, while in terms of utility, the various alloys have been described in connection with gas turbine engine components, the usefulness of these alloys is not so confined. These alloys provide exceptional oxidation-erosion resistance in conjunction with good mechanical properties such as creep and fatigue resistance, and good tensile strength and ductility. As a monocrystal or directionally solidified component with the [100] orientation parallel to the stress axis the low modulus of this material will provide excellent thermal fatigue resistance. The alloys are, accordingly, suitable for any part required to operate at elevated temperatures in an oxidizing atmosphere.