Abstract:
A nickel-base alloy that exhibits a desirable balance of mechanical properties, environmental properties, and microstructural stability suitable for gas turbine engine applications. The nickel-base alloy is in the form of a single-crystal casting consisting of, by weight, 5.75% to 6.5% aluminum, 4% to 5% tantalum, 2% to 6% chromium, 5.5% to 7% tungsten, 1.5% to 3% molybdenum, 4% to 5% rhenium, up to 1.0% niobium, 10% to 16% cobalt, up to 1% titanium, 0.01% to 0.05% carbon, up to 0.005% boron, up to 0.01% yttrium, 0.5% to 1.0% hafnium, the balance nickel and incidental impurities. The alloy has a density of not more than 0.320 lbs/in 3  (about 8.87 g/cm 3 ), and contains a combined amount of aluminum, tungsten, molybdenum, niobium, titanium, and hafnium specified relative to the combined amount of tantalum and rhenium.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention generally relates to nickel-base alloys. More particularly, this invention relates to a nickel-base superalloy capable of being cast as a single-crystal article and exhibiting desirable properties for use in gas turbine engine applications.  
         [0002]     The superalloy commercially known as René N6, disclosed in commonly-assigned U.S. Pat. No. 5,455,120, has a nominal composition of, by weight, about 12.5% Co, 4.2% Cr, 7.2% Ta, 5.75% Al, 5.8% W, 5.4% Re, 1.4% Mo, 0.2% Hf, 0.05% C, 0.004% B, 0.01% Y, the balance nickel and incidental impurities. N6 is well known to have a number of very desirable properties for gas turbine engine applications, such as the high pressure turbine blades and vanes of aircraft gas turbine engines.  
         [0003]     As with the formulation of other superalloys, the composition of N6 is characterized by controlled concentrations of certain critical alloying elements to achieve a desired mix of properties. For use in gas turbine engine applications, such properties include high temperature creep strength, oxidation and corrosion resistance, resistance to low and high cycle fatigue (LCF and HCF), and single-crystal castability. While N6 performs well in applications within gas turbine engines, improvements would be desirable. Often of interest is the desire to reduce weight and cost, the latter of which is due in part to alloying constituents such as tantalum and rhenium. However, obtaining such reductions is difficult in view of the desire to maintain or often improve other properties of the alloy, including oxidation resistance and microstructural stability. With regard to the latter issue, microstructural instability is known to occur in various high strength superalloys when protected with coatings containing relatively high levels of aluminum, such as a diffusion aluminide environmental coating or bond coat. In particular, aluminum migration out of such coatings and into the underlying superalloy substrate can result in the formation of topologically close-packed (TCP) phases that, if present at sufficiently high levels, reduce the load-carrying capability of the alloy. The incidence of TCP phases has been associated with superalloys that contain significant amounts of refractory elements such as rhenium, tungsten, tantalum, hafnium, molybdenum, niobium, and zirconium.  
       BRIEF SUMMARY OF THE INVENTION  
       [0004]     The present invention provides a nickel-base alloy that exhibits a desirable balance of high-temperature strength (including creep resistance), oxidation and corrosion resistance, resistance to low and high cycle fatigue, castability, and microstructural stability so as to be suitable for components of gas turbine engines, such as the high pressure turbine blades and vanes of gas turbine engines. These properties are achieved with an alloy that is lower in density than the superalloy known as René N6, and in which relatively higher levels of aluminum, tungsten, molybdenum, niobium (columbium), titanium, and hafnium are present and relatively lower levels of tantalum and rhenium are present as compared to N6.  
         [0005]     According to the invention, the nickel-base alloy is in the form of a single-crystal casting consisting of, by weight, 5.75% to 6.5% aluminum, 4% to 5% tantalum, 2% to 6% chromium, 5.5% to 7% tungsten, 1.5% to 3% molybdenum, 4% to 5% rhenium, up to 1.0% niobium, 10% to 16% cobalt, up to 1% titanium, 0.01% to 0.05% carbon, up to 0.005% boron, up to 0.01% yttrium, 0.5% to 1.0% hafnium, the balance nickel and incidental impurities. Importantly, the density of the alloy is not more than and preferably less than 0.320 lbs/in 3  (about 8.87 g/cm 3 ). In preferred embodiments, molybdenum is present in an amount greater than 1.5 weight percent, molybdenum is present in an amount greater than 1.5 weight percent, rhenium and tantalum are each present in an amount less than 5 weight percent, and hafnium is present in an amount greater than 0.5 weight percent.  
         [0006]     As noted above, the nickel-base alloy of the present invention nominally contains more aluminum, tungsten, molybdenum, niobium, titanium, and hafnium and less tantalum and rhenium than N6. For the purpose of characterizing the alloy, these groups of alloys are designated as “delta addition” and “delta reduction” elements, respectively. According to the present invention, the ratio of delta addition to delta reduction elements (hereinafter, delta ratio) is less than 1.0. The limitation that the value of the delta ratio is less than unity reflects the determination of this invention that, contrary to conventional wisdom, the combination of (Al+Mo+W+Ti+Hf+Nb) is, by weight, a more potent strengthener than (Re+Ta).  
         [0007]     The alloy of this invention is believed to have properties comparable to, and in some instances better than, those of the N6 alloy, particularly with the above-noted restriction on the delta ratio, which allows for lower tantalum and rhenium contents. Consequently, the alloy of this invention provides an excellent lower density (weight) and potentially lower-cost alternative to N6.  
         [0008]     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  depicts a high pressure turbine blade that can be formed from the nickel-base superalloy of the present invention.  
         [0010]      FIGS. 2 through 7  are graphs plotting yield strength, tensile strength, 1800° F. rupture, 2000° F. rupture, 2100F rupture, and 1600° F. high cycle fatigue life of alloys prepared during investigations leading to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]     The present invention was the result of an effort to develop a nickel-base alloy having properties comparable to the nickel-base alloy commercially known as René N6, but with a chemistry that reduces the density and cost of the alloy while maintaining or improving high temperature strength (including creep resistance), oxidation resistance, fatigue resistance, castability, and microstructural stability (resistance to TCP formation) for use in such applications as the hot gas flow path of gas turbine engines. As an example,  FIG. 1  depicts a high pressure turbine (HPT) blade  10  having an airfoil  12 , a dovetail  14  by which the blade  10  is anchored to a turbine disk (not shown), and a platform  16  therebetween. While the advantages of this invention will be described with reference to components of a gas turbine, such as the high pressure turbine blade  10  shown in  FIG. 1 , the teachings of this invention are generally applicable to other components that require high temperature capabilities.  
         [0012]     In a first round of investigations, alloys having the approximate chemistries set forth in Table I below were formulated. Specimens of various sizes were machined from single crystal slab castings that had been solution heat treated at about 2370° F. (about 1300° C.) for about six hours and then aged at about 1975° F. (about 1080° C.) for about four hours. The densities of the alloys were in the range of about 0.315 to 0.319 lbs/in 3  (about 8.73 to about 8.84 g/cm 3 ), which is significantly less than the density of N6 (0.323 lbs/in 3 ; about 8.95 g/cm 3 ). For reference, Table I also includes the nominal composition for N6.  
                                                                                                                                         TABLE I                                   Al   Ta   Cr   W   Mo   Re   Nb   Co   Ti   C   B   Y   Hf   Ni                                    N6   5.75   7.2   4.2   5.8   1.4   5.4   —   12.5   —   0.05   0.004   0.01   0.2   bal.       1   6.25   4.0   4.2   6.0   1.5   4.0   1.50   10.0   0.3   0.03   0.004   0.004   0.6   61.6       2   6.25   5.0   4.2   7.0   1.5   4.0   0.75   10.0   0.3   0.03   0.004   0.004   0.6   60.4       3   6.00   5.0   4.2   6.0   1.5   5.0   1.50   10.0   0.3   0.03   0.004   0.004   0.6   59.9       4   6.25   4.0   4.2   7.0   1.5   4.0   1.50   10.0   0.3   0.03   0.004   0.004   0.6   60.6       5   6.25   5.0   4.2   7.0   1.5   5.0   0.75   10.0   0.3   0.03   0.004   0.004   0.6   59.4       6   6.25   5.0   4.2   6.0   1.5   5.0   1.50   10.0   0.3   0.03   0.004   0.004   0.6   59.6       7   6.00   5.0   4.2   7.0   1.5   4.0   1.50   10.0   0.3   0.03   0.004   0.004   0.6   59.9       8   6.00   4.0   4.2   7.0   1.5   5.0   1.50   10.0   0.3   0.03   0.004   0.004   0.6   59.9       9   6.00   4.0   4.2   7.0   1.5   5.0   0.75   10.0   0.3   0.03   0.004   0.004   0.6   60.6       10    6.25   4.0   4.2   6.0   1.5   5.0   0.75   10.0   0.3   0.03   0.004   0.004   0.6   61.4       11    6.00   5.0   4.2   6.0   1.5   4.0   0.75   10.0   0.3   0.03   0.004   0.004   0.6   61.6       12    6.00   4.0   4.2   6.0   1.5   4.0   0.75   10.0   0.3   0.03   0.004   0.004   0.6   62.6       13    6.125   4.5   4.2   6.5   1.5   4.5   1.13   10.0   0.3   0.03   0.004   0.004   0.6   60.6       14    6.125   5.5   4.2   6.5   1.5   4.5   1.13   10.0   0.3   0.03   0.004   0.004   0.6   59.6                  
 
         [0013]     The above alloying levels were selected to evaluate the affects of adding the relatively lighter elements titanium and niobium, increasing the levels of tungsten, hafnium, and relatively lighter elements such as aluminum and molybdenum, and reducing the levels of heavier elements such as tantalum and rhenium in alloys based on N6. The approach of the investigation was also to maintain the total gamma-prime precipitation hardening phase while evaluating the affects of altering the amounts of rhenium, molybdenum, and tungsten that go into the gamma phase. As known in the art, the high-temperature strength of a nickel-base superalloy is directly related to the volume fraction of the gamma-prime phase, which in turn is directly related to the total amount of the gamma prime-forming elements (aluminum, titanium, tantalum, niobium, and hafnium) present. Based on these relationships, the composition and volume fraction of the gamma-prime phase and the amounts of the gamma prime-forming elements required to maintain a given strength level can be approximately estimated based on the starting chemistry of the alloy and some basic assumptions about the phases that form. It was initially viewed that an alloy having the desired level of creep strength for a HPT blade should contain at least as much gamma prime-forming elements (about 15.8 atomic percent of aluminum, tantalum, niobium, titanium, and hafnium combined) and as much gamma-forming elements (about 4.7 atomic percent of rhenium, molybdenum, and tungsten combined) as nominally contained in N6. However, other properties important to HPT blades and other hot gas flow path components, such as fatigue life, castability, metallurgical stability, and oxidation resistance, cannot be predicted from amounts of these and other elements.  
         [0014]     Tensile and yield strengths of the alloys are summarized in  FIGS. 2 and 3 , in which “N6 avg” identifies historical averages for N6. The data indicate that yield and tensile strengths of the specimens were similar to and generally higher than, respectively, N6.  
         [0015]      FIGS. 4-6  are graphs plotting time to stress rupture for Alloys 1-14 in comparison to historical averages for N6 (“N6 avg”). Samples from each alloy were machined to form conventional creep test specimens and stress rupture tested in accordance with ASTM E139 at stress and temperature combinations of about 40 ksi (about 276 MPa) and about 1800° F. (about 980° C.), about 20 ksi (about 138 MPa) and about 2000° F. (about 1090° C.), and about 13 ksi (about 90 MPa) and about 2100° F. (about 1150° C.). Specimens from all but one alloy exhibited stress rupture strength approaching N6 at 1800° F., and Alloys 9, 10, and 11 exhibited the best overall stress rupture performance at the three test temperatures.  
         [0016]      FIG. 7  is a graph plotting axial-axial high cycle fatigue (HCF) life at about 1600° F. (about 870° C.) for Alloys 9, 10, and 11 in comparison to N6 baseline data. The HCF tests were conducted under the stress-controlled condition and about 60 Hz cyclic loading. The data indicate that the HCF lives of Alloys 9, 10, and 11 were equal or better than the N6 baseline at the temperature tested.  
         [0017]     On the basis of Alloys 9, 10, 11, an alloy having the approximate broad and nominal compositions (by weight) summarized in Table II is believed to have properties similar to N6 and therefore suitable for use as an alloy for hot gas path components of gas turbine engines, as well as other applications in which similar properties are required. The densities of Alloys 9, 10, and 11 were about 0.319 lbs/in 3  (about 8.82 g/cm 3 ), about 0.316 lbs/in 3  (about 8.74 g/cm 3 ), and about 0.317 lbs/in 3  (about 8.77 g/cm 3 ), respectively. It is believed that an alloy within the ranges set forth in Table II can be satisfactorily heat treated using the treatment described above.  
                                                                                   TABLE II                                   BROAD   PREFERRED   NOMINAL                                        Al   5.75 to   6.5   6.00 to   6.5   6.125           Ta   4.0 to   5.0   4.0 to   5.0   4.5           Cr   2.0 to   6.0   2.0 to   6.0   4.2           W   5.5 to   7.0   5.5 to   7.0   6.5           Mo   1.5 to   3.0   1.5 to   3.0   1.5           Re   4.0 to   5.0   4.0 to   5.0   4.5           Nb   0 to   1.0   0.5 to   1.0   0.75           Co   10.0 to   16.0   10.0 to   12.0   10.0           Ti   0 to   1.0   0.25 to   1.0   0.3           C   0.01 to   0.05   0.01 to   0.05   0.03           B   0 to   0.005   0.001 to   0.005   0.004           Y   0 to   0.01   0 to   0.01   0.004           Hf   0.5 to   1.0   0.5 to   1.0   0.60                Ni   balance   balance   balance                      
 
         [0018]     It should be noted that the relative amounts of niobium, tantalum, tungsten, and rhenium appeared to be particularly important for both strength and microstructural stability (resistance to TCP formation). Specifically, data obtained with the alloys of Table I coated with diffusion aluminide coatings showed that certain alloys of Table I appeared to exhibit lower incidence of TCP phases if they contained, by weight, 0.75% (e.g., less than 1%) niobium, 6% (e.g., less than 7%) tungsten, and 4% (e.g., less than 5%) tantalum. On this basis, it was theorized that strength and microstructural stability could be further promoted by adjusting the compositions set forth in Table 11 to contain, by weight, about 4.0% tantalum, about 6.0% tungsten, and about 5.0% rhenium.  
         [0019]     Because of their excellent mechanical properties and low densities, Alloys 9, 10, and 11 were further characterized on the basis of, in comparison to the nominal composition of N6 (Table 1), their nominal decreases in the levels of the relatively heavy elements tantalum and rhenium and their nominal increases in the levels of aluminum, tungsten, molybdenum, niobium, titanium, and hafnium levels (of which all but tungsten and hafnium are significantly less dense than tantalum and rhenium). The sums of these increased and decreased levels, designated herein as delta additions and delta reductions, respectively, are summarized for Alloys 9, 10, and 11 in Table III, as is the ratio of delta additions to delta reductions for each alloy (delta ratio).  
                                                         TABLE III                                   ALLOY 9   ALLOY 10   ALLOY 11   ALLOY 14                                    ΔAl   0.25   0.50   0.25   0.375       ΔW   1.2   0.2   0.2   0.7       ΔMo   0.1   0.1   0.1   0.1       ΔNb   0.75   0.75   0.75   1.13       ΔTi   0.3   0.3   0.3   0.3       ΔHf   0.4   0.4   0.4   0.4       Delta Addition   3.0   2.25   2.0   3.005       ΔTa   3.2   3.2   2.2   1.7       ΔRe   0.4   0.4   1.4   0.9       Delta Reduction   3.6   3.6   3.6   2.6       Delta Ratio   0.83   0.62   0.55   1.15                  
 
         [0020]     From the above, it can be seen that the delta ratios of Alloys 9, 10, and 11 were less than one. For comparison, the delta ratio of Alloy 14 is also calculated in Table Ill. In view of these results, it was concluded that the delta ratios of the alloys could be used as indicators of their abilities to exhibit desirable mechanical properties relative to their densities, with lower delta ratios evidencing lower densities as compared to N6. Based on the nominal composition of N6 as set forth in Table I, the calculation of the delta ratio for alloys within the scope of this invention can be made using the following formulas: 
 
Delta Ratio=(Al-5.75+W-5.8+Mo-1.4+Nb+Ti+Hf-0.2)/(7.2-Ta+5.4-Re) 
 
or (Al+W+Mo+Nb+Ti+Hf−13.15)/(12.6−Ta—Re) 
 
         [0021]     A second round of alloys was then identified for further testing based on the knowledge gained from the first round. The alloying levels for the second round of alloys identified as Alloys 15-25 in Table IV below were selected to evaluate the affects of limiting tantalum and niobium levels to the minimum present in Alloys 9, 10, and 11, reducing the tungsten content, and increasing the molybdenum content. In part, the approach taken with Alloys 15-25 is to approximately maintain levels of gamma prime-forming elements similar to that nominally contained in N6 (about 15.8 atomic percent of aluminum, tantalum, niobium, titanium, and hafnium combined) without increasing density. Alloys 15-25 also reflect the intent to maintain a delta ratio of less than one.  
                                                                                                                                         TABLE IV                                   Al   Ta   Cr   W   Mo   Re   Nb   Co   Ti   Ru   C   B   Y   Hf                                    N6   5.7   7.2   4.2   5.5   1.4   5.4   —   12.5   —   —   0.0   0.004   0.01   0.2       15   6.1   4.0   4.2   5.5   2.25   4.5   0.7   10.0   0.3   0.0   0.0   0.004   0.004   0.6       16   6.1   4.0   4.2   6.0   3.00   4.5   0.7   10.0   0.3   0.0   0.0   0.004   0.004   0.6       17   6.1   4.0   4.2   6.0   2.25   4.5   0.7   10.0   0.3   0.0   0.0   0.004   0.004   0.6       18   6.1   4.0   4.2   5.5   3.00   5.0   0.7   10.0   0.3   0.0   0.0   0.004   0.004   0.6       19   6.1   4.0   4.2   5.5   2.25   5.0   0.7   10.0   0.3   0.0   0.0   0.004   0.004   0.6       20   6.1   4.0   4.2   6.0   3.00   5.0   0.7   10.0   0.3   0.0   0.0   0.004   0.004   0.6       21   6.1   4.0   4.2   6.0   2.25   5.0   0.7   10.0   0.3   0.0   0.0   0.004   0.004   0.6       22   6.1   4.0   4.2   6.0   3.00   5.0   0.7   10.0   0.3   0.0   0.0   0.004   0.004   0.6       23   6.1   4.0   4.2   6.0   1.50   5.0   0.4   10.0   0.5   0.0   0.0   0.004   0.004   0.6       24   6.1   4.0   4.2   6.0   1.50   5.0   0.7   10.0   0.5   0.0   0.0   0.004   0.004   0.6       25   6.1   4.0   4.0   6.5   2.25   5.0   0.7   10.0   0.3   0.0   0.0   0.004   0.004   0.6                  
 
         [0022]     Alloys 15-25 can generally be summarized as covering the ranges and nominal compositions (by weight) summarized in Table V below.  
                                             TABLE V                                   RANGE   NOMINAL                                        Al   6.00 to 6.25   6.1           Ta   4.0 to 5.0   4.0           Cr   2.0 to 6.0   4.2           W   6.0 to 7.0   6.0           Mo   1.5 to 3.0   2.25           Re   4.0 to 5.0   4.75           Nb   0.5 to 1.0   0.75           Co   10.0 to 12.0   10.0           Ti   0.25 to 1.0    0.3           C   0.01 to 0.05   0.03           B   0.001 to 0.005   0.004           Y     0 to 0.01   0.004           Hf   0.5 to 1.0   0.60           Ni   balance   balance                      
 
         [0023]     Preparation of Alloys 15-25 for testing is currently underway.  
         [0024]     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.