Abstract:
A nickel-based superalloy containing 12.0 to 16.0% by weight of Cr, 4.0 to 9.0% by weight of Co, 3.4 to 4.6% by weight of Al, 0.5 to 1.6% by weight of Nb, 0.05 to 0.16% by weight of C, 0.005 to 0.025% by weight of B, and at least one of Ti, Ta and Mo. Amounts of Ti, Ta and Mo are ones calculated by the equations (1) and (2), wherein TiEq is 4.0 to 6.0 and MoEq is 5.0 to 8.0.  
     Ti Eq =Ti% by weight+ 0.5153 ×Nb% by weight+ 0.2647 ×Ta% by weight  ( 1 )  
     Mo Eq =Mo% by weight+ 0.5217   ×x W% by weight+ 0.5303 ×Ta% by weight+ 1.0326 ×Nb% by weight.  ( 2 )

Description:
DETAILED DESCRIPTION OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a Ni-base superalloy and a gas turbine blade made of cast Ni-base superalloy.  
           [0003]    2. Description of Prior Art  
           [0004]    In power engines such as jet engines, land-based gas turbines, etc., turbine inlet temperatures are being elevated more and more so as to increase efficiency of the turbines. Therefore, it is one of the most important objects to develop turbine blades material that withstands high temperatures.  
           [0005]    The main properties required for turbine blades are high creep rupture strength, high ductility, superior resistance to oxidation in high temperature combustion gas atmosphere and high corrosion resistance. In order to satisfy these properties, nickel base superalloys are used as turbine blade materials at present.  
           [0006]    There are conventional cast alloys, unidirectional solidification alloys of columnar grains and single crystal nickel base alloys as nickel base superalloys. Among these, conventional cast alloys have the highest casting yield of the blades. Thus, the technique is appropriate for manufacturing land-based gas turbine blades. See Japanese Patent Laid-open Hei 6 (1994)- 57359 . However, the normal cast steel is still insufficient in its high temperature creep rupture strength. Thus, there have not been proposed alloys that have high temperature creep rupture strength, corrosion resistance and oxidation resistance.  
           [0007]    There are single crystal alloys or unidirectional solidification alloys that have superior creep rupture strength, but these alloys contain a smaller chromium content and contain larger amounts of tungsten and tantalum which have high solid solution strengthening so as to improve creep rupture strength. Therefore, these alloys are insufficient in corrosion resistance at high temperatures. From the viewpoint of corrosion resistance, these alloys that contain relatively large amount of impurities are not suitable for land based gas turbines.  
           [0008]    An object of the present invention is to provide a nickel base superalloy for normal casting or unidirectional casting, which has improved high temperature creep rupture strength, oxidation resistance and corrosion resistance, and also provide a gas turbine blade made of the alloy. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0009]    [0009]FIG. 1 shows relationship between MoEq and TiEq values.  
         [0010]    [0010]FIG. 2 is a bar graph showing creep rupture time in creep rupture tests.  
         [0011]    [0011]FIG. 3 is a bar graph showing creep rupture time in creep rupture tests.  
         [0012]    [0012]FIG. 4 is a bar graph showing oxidation loss in high temperature oxidation tests.  
         [0013]    [0013]FIG. 5 is a bar graph showing corrosion loss in high temperature corrosion tests.  
         [0014]    [0014]FIG. 6 is a perspective view of a gas turbine.  
         [0015]    [0015]FIG. 7 is a perspective view of a gas turbine blade. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0016]    The nickel base superalloy of the present invention contains, 12.0 to 16.0% by weight of Cr, 4.0 to 9.0% by weight of Co, 3.4 to 4.6% by weight of Al, 0.5 to 1.6% by weight of Nb, 0.05 to 0.16% by weight of C, 0.005 to 0.025% by weight of B, and Ti, Ta, Mo and W.  
         [0017]    In addition to the above ingredients, there are contained, 0 to 2.0% by weight of Hf, 0 to 0.5% by weight of Re, 0 to 0.05% by weight of Zr, 0 to 0.005% by weight of O, 0 to 0.005% by weight of N, 0 to 0.01% by weight of Si, 0 to 0.2% by weight of Mn, 0 to 0.01% by weight of P, and 0 to 0.01% by weight of S.  
         [0018]    The remaining is substantially nickel and unavoidable impurities that may be introduced at the time of making the alloy.  
         [0019]    The nickel base alloy of the present invention has a composition calculated by the following equations.  
         TiEq=Ti% by weight+0.5153×Nb% by weight+0.2647×Ta% by weight  
         MoEq=Mo% by weight+0.5217×W% by weight+0.5303×Ta% by weight+1.0326×Nb% by weight  
         [0020]    The nickel base alloy of the present invention has a structure wherein γ′ phase precipitates in austenite matrix. The γ′ phase is an intermetallic compound, which may be Ni3(Al,Ti), Ni3(Al,Nb), Ni3(Al,Ta,Ti), etc, based on alloy compositions.  
         [0021]    TiEq that relates to stability of matrix and creep rupture strength is a sum of Ti numbers that are calculated by summing [Ti] % by weight, Ti equivalent of [Nb] % by weight and Ti equivalent of [Ta] % by weight. In order to precipitate γ′ phase in γ phase matrix, in other words, in order to prevent precipitation of brittle phases such as TCP phase, aphase or η phase, TiEq value should be 6.0 or less. The smaller the TiEq, the better the stability of matrix becomes. But, if TiEq is too small, the creep rupture strength will be lower. Thus, TiEq should be 4.0 or more. More preferably, TiEq should be within a range of from 4.0 to 5.0 so that particularly high creep rupture strength is expected.  
         [0022]    MoEq that also relates to stability of matrix and creep rupture strength is a sum of Mo numbers that are calculated by summing [Mo] % by weight, Mo equivalent of [W] % by weight, Mo equivalent of [Ta] % by weight, and Mo equivalent of [Nb] % by weight. In order to stabilize matrix, MoEq should be 8.0 or less. The smaller the MoEq, the better the stability of matrix becomes. But, if MoEq is too small, creep rupture strength will be lower. Thus, MoEq should be 5.0 or more. More preferably, 5.5 to 7.5 of MoEq should be selected.  
         [0023]    In the nickel base alloy of the invention, a preferable range of W is 3.5 to 4.5% by weight, Mo is 1.5 to 2.5% by weight, Ta is 2.0 to 3.4% by weight and Ti is 3.0 to 4.0% by weight. Accordingly, the present invention provides nickel base heat resisting alloys that contain the above elements in the specified ranges.  
         [0024]    In the following, functions and reasons of contents will be explained.  
         [0025]    Cr; 12.0 to 16.0% by weight: Cr is effective to improve corrosion resistance at high temperatures, and is truly effective at an amount of 12.0% by weight or more. Since the alloy of the invention contains Co, Mo, W, Ta, etc, an excess amount of Cr may precipitate brittle TCP phase to lower high temperature strength. Thus, the maximum amount of Cr is 16.0% by weight to take balance between the properties and ingredients. In this composition, superior high temperature strength and corrosion resistance are attained.  
         [0026]    Co; 4.0 to 9.0% by Weight  
         [0027]    Co makes easy solid solution treatment by lowering precipitation temperature of γ′ phase, and strengthen γ′ phase by solid solution and improve high temperature corrosion resistance. These improvements are found when the amount of cobalt is 4.0% by weight or more. If Co exceeds 9.0% by weight, the alloy of the invention loses balance between the ingredients and properties because W, Mo Co, Ta, etc are added, thereby to suppress the precipitation of γ′ phase to lower high temperature strength. Therefore, the upper limit of Co should be 9.0% by weight. In considering balance between easiness of solid solution heat treatment and strength, a preferable range is within 6.0 to 8.0% by weight.  
         [0028]    W; 3.5 to 4.5% by Weight  
         [0029]    W dissolves in γ phase and precipitated γ′ phase as solid solution to increase creep rupture strength by solid solution strengthening. In order to attain these advantages, W is necessary to be 3.5% by weight or more. Since W has large density, it increases specific gravity (density) of alloy and decreases corrosion at high temperatures. When W amount exceeds 4.5% by weight, needle-like W precipitates to lower creep rupture strength, corrosion at high temperatures and toughness. In considering the balance between high temperature strength, corrosion resistance and stability of structure matrix at high temperatures, a preferable range of W is 3.8 to 4.4% by weight.  
         [0030]    Mo; 1.5 to 2.5% by Weight  
         [0031]    Mo has the similar function to that of W, which elevates solid solubility temperature of γ′ phase to improve creep rupture strength. In order to attain the function, at least 1.5% by weight of Mo is necessary. Since Mo has smaller density than W, it is possible to lessen specific gravity (density) of alloy. On the other hand, Mo lowers oxidation resistance and corrosion resistance, the upper limit of Mo is 2.5% by weight. In considering balance between strength, corrosion resistance and oxidation resistance at high temperatures, a preferable range of Mo is 1.6 to 2.3% by weight.  
         [0032]    Ta; 2.0 to 3.4% by Weight  
         [0033]    Ta dissolves in γ′ phase in the form of Ni3(Al,Ta) to solid-strengthen the alloy, thereby increasing creep rupture strength. In order to attain this effect, at least 2.0% by weight of Ta is preferable. On the other hand, if Ta exceeds 3.4% by weight, it becomes supersaturated thereby to precipitate [Ni, Ta] or needle like σ phase. As a result, the alloy has lowered creep rupture strength. Therefore, the upper limit of Ta is 3.4% by weight. In considering balance between high temperature strength and stability of structure matrix, a preferable range is 2.5 to 3.2% by weight.  
         [0034]    Ti; 3.0 to 4.0% by Weight  
         [0035]    Ti dissolves in γ′ phase as Ni(Al,Ti) solid to strengthen the matrix, but it does not have good effect as Ta does. Ti has a remarkable effect to improve cession resistance at high temperatures. In order to attain high temperature corrosion resistance, at least 3% by weight is necessary. However, if Ti exceeds 4.0% by weight, oxidation resistance of alloy decreases drastically. Thus, the upper limit of Ti is 4.0% by weight. In considering balance between high temperature strength and oxidation resistance, a preferable range is 3.2 to 3.6% by weight.  
         [0036]    Nb; 0.5 to 1.6% by Weight  
         [0037]    Nb is an element that solid-dissolves in γ′ phase in the form of Ni3(Al,Nb) to strengthen the matrix, but it does not have an effect as Ta does. On the contrary, it remarkably improves corrosion resistance at high temperatures. In order to attain corrosion resistance, at least 0.5% by weight of Nb is necessary. However, if the amount exceeds 1.6% by weight, strength will decrease and oxidation resistance will be lowered. Thus, the upper limit is 1.6% by weight. In considering balance between high temperature strength, oxidation resistance and corrosion resistance, a preferable amount will be from 1.0 to 1.5% by weight.  
         [0038]    Al; 3.4 to 4.6% by Weight  
         [0039]    Al is an element for constituting the γ′ reinforcing phase, i.e. Ni3Al that improves creep rupture strength. The element also remarkably improves oxidation resistance. In order to attain the properties, at least 3.4% by weight of Al is necessary. If the amount of Al exceeds 4.6% by weight, excessive γ′ phase precipitates to lower strength and degrades corrosion resistance because it forms composite oxides with Cr. Accordingly, a preferable amount of Al is 3.4 to 4.6% by weight. In considering balance between high temperature strength and oxidation resistance, a more preferable range is 3.6 to 4.4% by weight.  
         [0040]    C; 0.05 to 0.16% by Weight  
         [0041]    C may segregate at the grain boundaries to strengthen the grain boundaries, and at the same time a part of it forms TiC, TaC, etc. that precipitate as blocks. In order to effect segregation at grain boundaries to strengthen grain boundaries, at least 0.05% by weight of C is necessary. If an amount of C exceeds 0.16% by weight, excessive amount of carbides are formed to lower creep rupture strength and ductility at high temperatures, and corrosion resistance as well. In considering balance between strength, ductility and corrosion resistance, a more preferable range is 0.1 to 0.16% by weight.  
         [0042]    B; 0.005 to 0.025% by Weight  
         [0043]    B segregates at grain boundaries to strengthen grain boundaries, and a part of it forms borides such as (Cr,Ni,Ti,Mo)3B2, etc. that precipitate at grain boundaries. In order to effect segregation at grain boundaries, at least 0.005% by weight is necessary. However, since the borides have remarkably low melting points that lowers a melting point of the alloy and narrower the solid-solution heat treatment temperature range, an amount of B should be no more than 0.025% by weight. In considering balance between strength and solid-solution treatment, a more preferable range of B is 0.01 to 0.02% by weight.  
         [0044]    Hf; 0 to 2.0% by Weight  
         [0045]    This element does not serve for enhancing strength of the alloy, but it has a function to improve corrosion resistance and oxidation resistance at high temperatures. That is, it improves bonding of a protective oxide layer of Cr2O3, Al2O3, etc. by partitioning between the oxide layer and the surface of the alloy. Therefore, if corrosion resistance and oxidation resistance is desired, addition of Hf is recommended. If an amount of Hf is too large, a melting point of alloy will lower and the range of solid-solution treatment will be narrowed. The upper limit should be 2.0% by weight. In case of normal casting alloys, effect of Hf is not found in the least. Therefore, addition of Hf is not recommended. Thus, the upper limit of Hf should be 0.1% by weight. On the other hand, in unidirectional solidification casting, remarkable effect of Hf is found, and hence at least 0.7% by weight of Hf is desired.  
         [0046]    Re; 0 to 0.5% by Weight  
         [0047]    Almost all of Re dissolves in γ phase matrix and improves creep rupture strength and corrosion resistance. However, since Re is expensive and has a large density to increase specific gravity (density) of alloy, Re is added if necessary. In the alloy of the present invention that contains a large amount of Cr, needle like α-W or α-Re precipitates when an amount of Re exceeds 0.5% by weight, to thereby lower creep rupture strength and ductility. Thus, the upper limit should be 0.5% by weight.  
         [0048]    Zr; 0 to 0.05% by Weight  
         [0049]    Zr segregates at the grain boundaries to improve strength at the boundaries more or less. Most of Zr forms intermetallic compound with Ni to form Ni3Zr at grain boundaries. The intermetallic compound lowers ductility of the alloy and it has a low melting point to thereby lower melting point of the alloy that leads to a narrow solid-solution treatment range. Zr has no useful effect, and the upper limit is 0.05% by weight.  
         [0050]    O; 0 to 0.005% by Weight  
         [0051]    N; 0 to 0.005% by Weight  
         [0052]    O and N are elements mainly introduced into the alloy from raw materials in general. O may be carried in alloys in a crucible. O or N introduced into alloys are present in the crucible in the form of oxides such as Al 2 O 3  or nitrides such as TiN or AlN. If these compounds are present in castings, they become starting points of cracks, thereby to lower creep rupture strength or to be a cause of stress-strain cracks. Particularly, O appears in the surface of castings that are surface defects to lower a yield of castings. Accordingly, O and N should be as little as possible. O and N should not exceed 0.005% by weight.  
         [0053]    Si; 0 to 0.01% by Weight  
         [0054]    Si is introduced into casting by raw materials. In the present invention, since Si is not effective element, it should be as little as possible. Even if it is contained, the upper limit is 0.01% by weight.  
         [0055]    Mn; 0 to 0.2% by Weight  
         [0056]    Mn is introduced into castings by raw materials, too. As same as Si, Mn is not effective in the alloys of the present invention. Therefore, it should be as a little as possible. The upper limit is 0.2% by weight.  
         [0057]    P; 0 to 0.01% by Weight  
         [0058]    P is an impurity that should be as little as possible. The upper limit is 0.01% by weight.  
         [0059]    S; 0 to 0.01% by Weight  
         [0060]    S is an impurity that should be as little as possible. The upper limit is 0.01% by weight.  
         [0061]    According to the present invention, there is provided a nickel-based superalloy comprising Cr, Co, W, Mo, Ta, Ti, Al, Nb, C and B in ranges of optimum amounts. Concretely, the nickel-based supperalloy comprises 13.0 to 15.0% by weight of Cr, 6.0 to 8.0% by weight of Co, 3.8 to 4.4% by weight of W, 1.6 to 2.3% by weight of Mo, 2.3 to 3.2% by weight of Ta, 3.2 to 3.6% by weight of Ti, 3.6 to 4.4% by weight of Al, 1.0 to 1.5% by weight of Nb, 0.10 to 0.16% by weight of C and 0.01 to 0.02% by weight of B.  
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0062]    [0062]FIG. 6 shows a perspective view of a land-based gas turbine. In FIG. 6, numeral  1  denotes first stage blade, numeral  2  second stage blade and numeral  3  third stage blade. Among the blades, the first stage blade is subjected to highest temperature and the second stage blade second highest temperature. FIG. 7 shows a perspective view of a blade of a land-based gas turbine. In a normal gas turbine, the height of the blade is about ten and several centimeters. In the present invention, the turbine blade is made of a normal casting material of the nickel-based superalloy. If necessary, the blade is made by unidirectional casting alloy.  
         [0063]    In the following, test pieces were prepared by machining out them from conventional casting.  
         [0064]    In table 1, there are shown chemical compositions of the alloys of the present invention (A1 to A28). In table 2, there are shown chemical compositions of comparative alloys (B1 to B28) and conventional alloys (C1 to C3).  
         [0065]    Each alloy was prepared by melting and casting using a vacuum induction furnace with a refractory crucible having a volume of 15 kg. Each ingot had a diameter of 80 mm and a length of 300 mm. Then, the ingot was vacuum melted in an alumina crucible and cast in a ceramic mold heated at 1000° C. to make a casting of a diameter of 20 mm and a length of 150 mm. After casting, solid-solution heat treatment and aging heat treatment at conditions shown in Table 3 were carried out.  
         [0066]    Test pieces for creep rupture test each of which has a diameter of 6.0 mm in 30 mm of a gauge length, test pieces for high temperature oxidation test each having a length of 25 mm, a width of 10 mm, and a thickness of 1.5 mm, and test pieces for high temperature corrosion test each having a diameter of 8.0 mm and a length of 40.0 mm. Micro structure of each test piece was examined with a scanning type electron microscope to evaluate stability of the matrix structure.  
         [0067]    In Table 4 there are shown test conditions done on each test piece for evaluation of properties.  
         [0068]    Creep rupture test was conducted under the conditions of 1123K-314 MPa and 1255K-138 MPa. High temperature oxidation test was conducted under the condition of 1373K, which was repeated 12 times after holding test pieces for 20 hours. High temperature corrosion test was conducted under the condition where the test piece was exposed to combustion gas containing 80 ppm of NaCl and the corrosion test under the condition 1173K was repeated 10 times in 7 hours to measure weight change.  
         [0069]    In Table 5, there are shown TiEq and MoEq values and stability of structure matrix of alloys of the present invention. FIG. 1 shows relationship between TiEq values and MoEq values with respect to alloys (A1 to A28) of the present invention.  
         [0070]    In Table 5 and FIG. 1,  represents alloys whose abnormal structure matrix was observed and ◯ represents alloys whose abnormality was not observed. The abnormal structure matrix is that TCP phase or nphase when structure observation was made after heat treatment. As is apparent from FIG. 1, when TiEq and MoEq values are chosen to be in the ranges of the present invention, alloys with superior in structure matrix are obtained.  
         [0071]    Table 6 and FIGS.  2  to  5  show test results of evaluation of properties of the alloys used in the experiments. Creep rupture test was conducted by measuring rupture time. Since there are relationship between creep rupture time and rupture strength, alloys having longer rupture time can be considered as alloys having higher rupture strength. FIG. 2 shows creep rupture time under the condition of 1123K-314 MPa, FIG. 3 creep rupture time under 1255K-138 MPa, FIG. 4 oxidation loss under high temperature oxidation and FIG. 5 corrosion loss under high temperature corrosion test, FIGS.  2  to  5  being all bar graphs.  
                                                   TABLE 1-1                       Item   Alloy No.   Cr   Co   Ti   Al   Mo   W   Ta   Nb                   Invention Alloys   A1   13.42   6.59   3.06   3.60   1.52   4.02   2.50   1.00           A2   14.07   7.99   3.09   4.22   1.98   4.23   2.99   1.47           A3   13.65   4.56   3.59   3.57   1.51   4.26   2.96   0.51           A4   14.23   7.10   3.44   4.21   2.03   3.77   2.83   1.21           A5   14.30   8.37   3.47   3.41   1.55   3.69   2.97   1.01           A6   13.66   4.44   3.38   3.54   1.98   3.97   3.20   1.50           A7   14.02   4.55   3.01   3.42   2.22   4.27   2.52   0.97           A8   14.17   8.45   3.03   3.94   1.54   3.98   3.21   0.53           A9   13.56   5.27   3.54   3.41   2.40   4.34   2.02   1.48           A10   13.96   8.04   3.56   3.60   2.20   3.72   2.47   0.52           A11   13.57   7.01   3.43   4.40   2.01   3.69   2.57   1.47           A12   14.50   6.37   3.09   4.42   1.79   4.10   2.70   1.23           A13   14.30   7.59   3.12   3.90   2.41   4.24   2.50   1.02           A14   13.76   7.95   3.49   3.86   1.99   4.28   3.11   1.18           A15   13.22   5.99   3.59   3.53   1.50   4.27   2.97   0.67           A16   14.29   6.55   3.63   3.81   2.22   4.10   2.69   1.01           A17   13.81   7.21   3.09   3.91   1.96   4.28   3.10   1.22           A18   13.43   6.01   3.27   3.58   1.53   3.97   2.53   1.02           A19   14.00   7.02   3.35   4.21   1.80   4.15   2.75   1.20           A20   14.00   7.97   3.35   3.96   2.00   4.30   2.97   1.03           A21   14.50   6.71   3.27   3.80   1.80   4.11   2.69   1.23           A22   13.67   7.94   3.45   4.40   1.97   4.26   3.12   1.20           A23   13.40   6.02   3.25   3.65   2.41   3.96   2.58   1.03           A24   14.06   7.00   3.36   4.22   1.82   4.16   2.77   1.18           A25   14.08   7.89   3.31   3.99   2.22   4.29   2.99   1.01           A26   14.49   6.74   3.25   4.41   1.83   4.12   2.71   1.22           A27   14.31   7.62   3.11   3.89   2.05   4.23   2.55   1.20           A28   14.03   7.95   3.36   3.97   2.00   4.29   2.96   1.02                  
 
         [0072]    [0072]                                                       TABLE 1-2                           Alloy                                           Item   No.   Hf   Re   P   S   C   B   O   N   Ni                   Invention alloys   A1   0.01   0.008   0.003   0.005   0.14   0.011   0.00   0.004   64.11           A2   0.04   0.007   0.003   0.004   0.12   0.017   0.002   0.002   60.00           A3   0.02   0.006   0.003   0.003   0.12   0.016   0.003   0.003   65.22           A4   0.09   0.008   0.004   0.005   0.13   0.019   0.002   0.004   63.74           A5   0.06   0.005   0.003   0.004   0.13   0.011   0.001   0.004   60.98           A6   0.01   0.005   0.003   0.003   0.13   0.013   0.001   0.003   64.76           A7   0.09   0.006   0.003   0.005   0.11   0.011   0.001   0.002   65.39           A8   0.01   0.007   0.003   0.003   0.08   0.016   0.003   0.004   62.22           A9   0.05   0.006   0.003   0.003   0.11   0.011   0.002   0.003   65.02           A10   0.00   0.007   0.003   0.003   0.08   0.017   0.002   0.004   61.97           A11   0.05   0.008   0.003   0.005   0.09   0.015   0.002   0.003   65.28           A12   0.09   0.010   0.004   0.005   0.11   0.014   0.002   0.003   61.81           A13   0.01   0.009   0.003   0.003   0.10   0.016   0.001   0.004   61.63           A14   0.07   0.008   0.003   0.004   0.11   0.010   0.003   0.003   60.30           A15   0.06   0.007   0.003   0.004   0.11   0.010   0.002   0.003   64.06           A16   0.05   0.006   0.004   0.003   0.10   0.019   0.002   0.004   61.72           A17   0.03   0.006   0.003   0.003   0.09   0.013   0.003   0.003   61.41           A18   0.01   0.010   0.004   0.004   0.11   0.015   0.002   0.003   64.50           A19   0.04   0.006   0.003   0.003   0.10   0.015   0.001   0.004   61.76           A20   0.08   0.007   0.003   0.004   0.10   0.015   0.002   0.003   60.21           A21   1.71   0.005   0.003   0.004   0.11   0.014   0.002   0.004   60.26           A22   1.76   0.006   0.004   0.003   0.11   0.010   0.002   0.004   58.76           A23   1.10   0.005   0.004   0.004   0.11   0.015   0.001   0.003   63.49           A24   1.43   0.008   0.004   0.003   0.10   0.015   0.002   0.004   60.24           A25   1.49   0.006   0.003   0.004   0.10   0.015   0.002   0.004   58.79           A26   0.10   0.402   0.004   0.005   0.11   0.014   0.004   0.003   61.38           A27   0.02   0.301   0.003   0.004   0.10   0.017   0.002   0.004   61.33           A28   0.09   0.203   0.004   0.005   0.10   0.015   0.002   0.003   60.00                    
         [0073]    [0073]                                                   TABLE 2-1                       Item   Alloy No.   Cr   Co   Ti   A1   Mo   W   Ta   Nb                   Comparative alloys   B1   14.07   9.31   2.39   2.90   1.50   3.95   4.01   2.47           B2   14.62   8.93   2.44   3.89   2.45   4.46   5.02   3.52           B3   14.45   9.79   3.35   1.91   0.54   4.06   3.97   3.48           B4   14.68   8.60   3.51   3.00   1.50   4.53   5.01   1.48           B5   13.51   8.84   3.54   3.97   2.47   3.57   2.98   2.48           B6   14.22   8.91   4.58   2.07   1.50   3.50   4.95   2.51           B7   13.76   9.65   4.58   2.92   2.54   4.02   2.97   3.51           B8   14.55   9.56   4.64   4.05   0.46   4.50   3.99   1.52           B9   14.26   6.47   2.46   2.10   2.54   4.48   3.98   2.48           B10   13.21   5.27   2.44   3.11   0.46   3.45   4.96   3.47           B11   14.60   6.34   3.37   2.01   1.45   4.57   2.99   3.47           B12   14.11   5.82   3.43   4.00   0.50   3.95   4.96   2.51           B13   14.57   5.67   4.60   2.10   2.45   4.05   4.95   1.49           B14   13.28   6.51   4.41   2.89   0.49   4.51   3.04   2.49           B15   14.40   6.23   4.35   4.04   1.49   3.48   3.99   3.49           B16   14.41   8.90   2.39   2.00   0.52   3.46   2.95   1.49           B17   13.37   6.61   2.50   4.05   1.51   3.97   2.99   1.52           B18   13.91   5.37   3.59   3.06   2.51   3.47   2.95   1.49           B19   14.75   3.74   3.00   2.94   0.98   3.75   1.99   0.50           B20   13.27   7.82   3.53   2.92   0.98   3.93   2.48   1.47           B21   13.48   6.74   3.89   4.09   1.02   4.26   2.52   0.49           B22   14.22   5.93   2.90   3.47   0.99   3.72   3.03   1.47           B23   13.80   3.96   3.44   3.90   1.01   3.94   3.03   0.99           B24   13.42   6.69   3.89   3.07   1.98   3.95   3.00   0.49           B25   13.83   8.36   3.89   3.42   0.95   4.34   1.99   0.99       Conv. Alloys   C1   14.07   9.20   5.03   3.03   3.96   3.92   0.00   0.00           C2   14.18   10.11   4.76   2.95   1.50   3.84   2.79   0.00           C3   13.24   10.10   2.67   4.02   1.52   4.33   4.74   0.00                    
         [0074]    [0074]                                                       TABLE 2-2                           Alloy                                           Item   No.   Hf   Re   P   S   C   B   O   N   Ni                   Comparative alloys   B1   0.01   0.008   0.004   0.004   0.10   0.019   0.002   0.002   59.25           B2   0.05   0.006   0.003   0.005   0.13   0.013   0.001   0.003   54.46           B3   0.03   0.010   0.003   0.003   0.14   0.013   0.002   0.003   58.25           B4   0.05   0.005   0.003   0.005   0.08   0.017   0.001   0.002   57.52           B5   0.03   0.005   0.004   0.005   0.09   0.019   0.002   0.004   58.48           B6   0.02   0.008   0.003   0.003   0.13   0.011   0.003   0.002   57.58           B7   0.04   0.008   0.004   0.005   0.05   0.016   0.002   0.002   55.92           B8   0.07   0.008   0.003   0.003   0.11   0.018   0.003   0.002   56.51           B9   0.07   0.006   0.004   0.004   0.08   0.014   0.002   0.004   61.05           B10   0.05   0.009   0.003   0.004   0.10   0.012   0.002   0.002   63.45           B11   0.04   0.009   0.004   0.004   0.09   0.015   0.001   0.004   61.03           B12   0.01   0.009   0.004   0.003   0.06   0.014   0.002   0.002   60.62           B13   0.08   0.007   0.004   0.005   0.10   0.017   0.002   0.004   59.90           B14   0.06   0.005   0.003   0.003   0.13   0.010   0.003   0.002   62.16           B15   0.08   0.007   0.004   0.003   0.06   0.016   0.002   0.003   58.36           B16   0.07   0.008   0.003   0.003   0.08   0.013   0.002   0.004   63.70           B17   0.00   0.006   0.004   0.005   0.12   0.011   0.003   0.004   63.39           B18   0.05   0.010   0.004   0.004   0.14   0.014   0.002   0.003   62.42           B19   0.01   0.005   0.004   0.005   0.15   0.013   0.001   0.004   68.16           B20   0.09   0.006   0.003   0.004   0.12   0.010   0.002   0.004   63.36           B21   0.00   0.005   0.003   0.005   0.13   0.019   0.002   0.002   63.34           B22   0.05   0.006   0.003   0.005   0.11   0.011   0.002   0.004   64.08           B23   0.08   0.008   0.003   0.003   0.09   0.016   0.001   0.004   65.73           B24   0.05   0.009   0.003   0.004   0.09   0.012   0.002   0.003   63.24           B25   0.06   0.008   0.003   0.004   0.10   0.013   0.002   0.004   62.04       Conv. Alloys   C1   0.00   0.006   0.004   0.005   0.12   0.015   0.001   0.003   60.64           C2   0.09   0.008   0.004   0.004   0.08   0.010   0.001   0.002   59.67           C3   0.01   0.007   0.004   0.005   0.10   0.015   0.002   0.003   59.23                    
         [0075]    [0075]                                                               TABLE 3                                       Solid           solution           Heat                Treatment   Aging condition            Kinds of alloy   No.   Condition   First aging   Second aging   Third aging               Invention   A1˜A28   1480 K/2 h,AC   1366 K/4 h, AC   1325 K/4 h,AC   1116 K/16 h,AC       alloys       Compara.   B1˜B25   1480 K/2 h, AC   1366 K/4 H, AC   1325 K/4 h,AC   1116 K/16 h, AC       alloys       Convent.   C1   1480 K/2 h, AC   1366 K/4 h, AC   1325 K/4 h, AC   1116 K/16 h, AC       alloys   C2   1395 K/2 h   1116 K/24 h, AC   —   —           C3   1433 K/2 h   1116 K/24 h, AC   —   —                    
         [0076]    [0076]                           TABLE 4                                   Evaluation tests   Contents of tests                           Creep rupture test   Test temperature and stress               (1) 1123 K-314 MPa               (2) 1255 K-138 MPa           Oxidation test   Repeating Oxidations in               atmosphere               (1) 1373 K-24 h (20 h × 12 times)           Corrosion resistance test   Corrosion test in high               temperature gas               (1) 1173 K-70 h (7 h × 10 times)               Fuel: Light Oil,               NaCl amount; 80 ppm                        
         [0077]    [0077]                                                                 TABLE 5-1                                           Stability                       Alloy   of           Item   No.   structure   TiEq   MoEq                                        Invention   A1    ◯   4.24   5.98           alloys   A2    ◯   4.64   7.29               A3    ◯   4.64   5.83               A4    ◯   4.81   6.75               A5    ◯   4.78   6.09               A6    ◯   5.00   7.30               A7    ◯   4.18   6.79               A8    ◯   4.15   5.87               A9    ◯   4.84   7.26               A10   ◯   4.48   5.99               A11   ◯   4.87   6.82               A12   ◯   4.44   6.63               A13   ◯   4.31   7.00               A14   ◯   4.92   7.09               A15   ◯   4.72   5.99               A16   ◯   4.86   6.83               A17   ◯   4.54   7.10               A18   ◯   4.47   6.00               A19   ◯   4.70   6.66               A20   ◯   4.67   6.88               A21   ◯   4.62   6.64               A22   ◯   4.89   7.09               A23   ◯   4.46   6.91               A24   ◯   4.70   6.68               A25   ◯   4.62   7.09               A26   ◯   4.60   6.68               A27   ◯   4.40   6.85               A28   ◯   4.67   6.86                        
         [0078]    [0078]                                                                 TABLE 5-2                                           Stability                       Alloy   of           Item   No.   structure   TiEq   MoEq                                        Compara.   B1       4.72   8.24           Alloys   B2       5.58   11.07               B3       6.19   8.36               B4       5.60   8.06               B5       5.61   8.47               B6       7.18   8.54               B7       7.17   9.84               B8       6.84   6.49               B9       4.79   9.55               B10      5.54   8.47               B11      5.95   9.00               B12      5.04   7.76               B13      6.68   8.73               B14      6.50   7.03               B15      7.20   9.03               B16   ◯   3.94   5.43               B17   ◯   4.07   6.74               B18   ◯   5.40   7.95               B19   ◯   3.78   4.51               B20   ◯   4.94   5.86               B21   ◯   4.81   5.08               B22   ◯   4.46   6.08               B23   ◯   4.75   5.69               B24   ◯   5.04   6.14               B25   ◯   4.93   5.28           Conven.   C1    ◯   5.03   6.01           alloys   C2    ◯   5.50   4.98               C3    ◯   3.92   6.29                        
         [0079]    [0079]                                                                                       TABLE 6-1                                               Oxidation   Corrosion           Alloy   Creep rupture time (h)   amount   Amount            Item   No.   1123 K-314 MPa   1255 K-138 MPa   (mg/cm 2 )   (mg/cm 2 )                    Invention alloys   A1   386.0   220.7   −11.26   −0.17           A2   362.5   212.9   −10.46   −0.63           A3   322.7   165.6   −11.79   −0.33           A4   358.1   179.4   −7.24   −0.33           A5   395.7   163.2   −11.54   −0.12           A6   375.6   170.6   −10.78   −0.26           A7   348.8   181.8   −10.82   −0.83           A8   358.5   146.0   −7.17   0.03           A9   333.5   161.8   −10.43   −0.09           A10   371.6   165.8   −8.48   0.03           A11   457.1   203.7   −8.68   −0.04           A12   430.2   192.7   −7.24   −1.93           A13   377.3   169.9   −2.55   −1.43           A14   389.8   214.9   −4.76   −1.64           A15   364.2   181.4   −8.78   −1.68           A16   328.2   170.2   −4.28   −0.83           A17   327.5   198.5   −4.17   −1.05           A18   376.4   187.1   −11.79   −1.62           A19   425.3   247.4   −6.88   −0.22           A20   537.5   225.0   −4.40   −0.43           A21   440.2   240.3   −7.22   −0.33           A22   420.3   220.1   −6.84   −0.74           A23   410.3   198.1   −8.10   −0.62           A24   397.5   200.4   −6.55   −1.20           A25   413.3   188.4   −5.44   −0.31           A26   486.7   213.6   −8.11   −0.56           A27   510.4   240.3   −7.84   −0.89           A28   470.1   220.1   −7.12   −0.11                    
         [0080]    [0080]                                                                                       TABLE 6-2                                               Oxidation   Corrosion           Alloy   Creep Rupture Time (h)   amount   amount            Item   No.   1123 K-314 MPa   1255 K-138 MPa   (mg/cm 2 )   (mg/cm 2 )                    Comparative alloys   B1   432.7   85.7   −11.98   −9.91           B2   0.0   0.0   −2.02   −19.38           B3   17.2   7.4   −42.35   −0.79           B4   375.3   71.5   −13.18   −2.66           B5   67.4   47.0   −6.36   −3.77           B6   22.2   19.5   −66.07   −0.58           B7   0.0   0.0   −35.40   −0.18           B8   42.8   15.2   −6.83   −0.18           B9   11.7   5.3   −58.18   −0.31           B10   109.3   35.8   −13.12   −9.17           B11   12.8   67.1   −64.17   −1.52           B12   130.4   57.7   −4.84   −2.15           B13   18.2   22.8   −55.06   −0.62           B14   74.4   51.8   −24.63   −0.38           B15   0.0   0.0   −1.26   −0.24           B16   35.8   8.0   −49.22   −0.79           B17   281.0   224.6   −8.38   −4.46           B18   334.6   100.7   −15.87   −0.39           B19   22.4   2.3   −14.10   −0.72           B20   92.4   36.4   −27.34   −1.04           B21   281.8   201.5   −5.74   −0.41           B22   242.1   95.9   −12.73   −0.45           B23   177.9   150.2   −6.53   −0.13           B24   270.4   131.6   −24.51   −0.25           B25   294.2   165.2   −13.42   −0.17       Conv. Alloys   C1   387.6   188.3   −130.94   −7.90           C2   159.4   136.3   −29.49   −0.57           C3   530.4   280.3   −3.20   −16.80                    
         [0081]    As is apparent from Table 6, though alloys A1 to A28 of the present invention exhibit almost the same rupture time and rupture strength as those of a conventional alloy (corresponding to U.S. Pat. No. 3,615,376), creep rupture time, oxidation loss and corrosion loss of the alloy of the present invention are greatly reduced and oxidation resistance is greatly improved. When compared with another conventional alloy (corresponding to U.S. Pat. No. 6,416,596B1), creep rupture time is almost two times that of the conventional alloy, whilst oxidation loss and corrosion loss are almost the same as those of conventional alloy. When compared with another conventional alloy (corresponding to U.S. Pat. No. 5,431,750), though the alloy of the present invention is a little bit worse in creep rupture time than the conventional one, oxidation resistance time is almost the same as that of the conventional one, and corrosion loss is greatly reduced and corrosion resistance is greatly improved.  
         [0082]    According to the present invention, there are provided superior alloys that, without sacrificing high temperature creep rupture time of the alloy, have greatly improved oxidation resistance and oxidation resistance properties at high temperatures and have well balanced creep rupture strength, oxidation resistance properties and corrosion resistance.  
         [0083]    The comparative alloys that do not satisfy the alloy compositions of the present invention are inferior in one or more of creep rupture strength, oxidation resistance properties, or oxidation resistance.  
         [0084]    In the above examples, although the description was made with respect to conventional casting alloys, the alloy compositions can be applied to unidirectional casings. The alloys of the present invention containing C and B that are effective for reinforcing grain boundaries and Hf that is is effective for suppressing cracks of grain boundaries at the time of casting, and hence the alloys are suitable for unidirectional castings.  
         [0085]    As having been described, the present invention provides nickel based superalloys that have high temperature creep strength, corrosion resistance and oxidation resistance and are capable of normal casting. Therefore, the alloys are suitable for land-based gas turbines.