Abstract:
A nickel-based DS alloy for directional solidification, includes Cobalt (Co), Chromium (Cr), Molybdenum (Mo), Tungsten (W), Tantalum (Ta), Titanium (Ti), Aluminum (Al), Rhenium (Re), Hafnium (Hf), Boron (B), Carbon (C), and Zirconium (Zr). Further, a component, for example a turbine blade or vane, with such an alloy is provided.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European Patent Office Application No. 11190432.2 EP filed Nov. 24, 2011. All of the applications are incorporated by reference herein in their entirety. 
       FIELD OF INVENTION 
       [0002]    An improved nickel-based superalloy for producing components having columnar grains is provided. 
       BACKGROUND OF INVENTION 
       [0003]    To increase the performance and to achieve a higher efficiency for gas turbines, the thermo-mechanical and oxidative loads to which the turbine blades or vanes are subject are becoming increasingly higher during operation. This requires firstly a higher complexity of the components for better cooling, above all in the cooling-gas passage, and secondly cast alloys with ever greater strength. This is accompanied by the demands for good producibility, above all in the case of casting, and reasonable alloy costs. 
         [0004]    At present, single-crystal materials (SX), such as e.g. the material PWA 1483, CMSX-4, or directionally solidified materials (DS), such as e.g. the material Alloy 247 LC DS, are used in industrial gas turbines for the rotor blades of the first two stages. 
         [0005]    In addition, further 2nd generation DS alloys (Re (rhenium)—content approximately 3%) are commercially available for IGT components, for example PWA 1426, Rene 142 and CM 186 LC. These materials are expensive because of the high Re-content, and their high content of reactive elements, such as e.g. hafnium (approximately 1.2-1.5%). The long solidification time in the case of large rotor blades lead to undesirable and usually unacceptable metal/ceramic reactions with the casting mold both on the inner surface in the cooling-gas passage and on the component surface. Therefore, these materials can only be used with high reworking costs and/or rates of scrap for large DS components. 
         [0006]    In addition, these alloys are additionally susceptible to the formation of brittle phases on account of their high Re (rhenium)—contents at relatively high operating temperatures and long periods under creep loading. In addition, these 2nd generation DS materials have a higher density than those from the 1st generation (without Re) and usually have an inadequate heat treatment window (solution annealing window), which increases the risk of re-crystallization and melting during the required solution annealing and therefore leads to a greater rate of scrap. 
       SUMMARY OF INVENTION 
       [0007]    It is an object to provide a DS alloy for directional solidification and a component including such a DS alloy. The object is achieved by an alloy and a component as claimed in the independent claims. The dependent claims list further advantageous measures which may be combined with one another, as desired, in order to achieve further advantages. 
         [0008]    By a targeted combination of alloying elements, a high-strength DS alloy having thermo-mechanical properties similar to those of the 2nd generation DS alloys has been developed. An improved producibility (castability), higher phase stability, lower density, an adequate heat treatment window for avoiding re-crystallization and melting (solution annealing) and a lower price (Re-content less than 3%) have also been achieved. Compared with the strength potential of known DS alloys, the claimed alloy has a higher strength than Alloy 247 LC DS, and the strength potential of CM 186 LC is achieved, but the negative properties thereof are avoided. The strength potential even gets close to that of the SX alloy PWA 1484 (2nd generation SX comprising 3% Re). 
         [0009]    In addition, the elements which influence the grain boundary strength in the case of DS alloys are also set optimally. 
         [0010]    Advantages:
       a high micro-structural strength/grain boundary strength up to high temperatures, thus reduced cooling and therefore a lower consumption of cooling air in the case of a turbine blade or vane,   good castability resulting in a high throughput and a lower price,   a high phase stability during GT operation resulting in a long service life and thus reduced life cycle costs,   a relatively low density compared to 2nd generation DS alloys resulting in lower specific creep loading and higher thermal loading and/or a longer service life,   good oxidation resistance at elevated temperatures resulting in good emergency operating properties in the case of coating loss and no internal aluminizing is required resulting in lower first time and life cycle costs,   a lower material price in the range of 2nd generation DS alloys which leads to a more favorable component price and an increase in competitiveness, and   an adequate heat treatment window which reduces the risk of re-crystallization and melting (scrap) combined with optimum setting of the microstructure during the solution annealing.       
 
         [0018]    The DS alloy with optimum settings of the Re, Ta, Ti, Hf, C, B and Zr contents achieves the advantages mentioned above. 
         [0019]    In an embodiment, a nickel-based alloy, comprises (in % by weight): 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Cobalt (Co) 
                 9%-11%, in particular 10%, 
               
               
                   
                 Chromium (Cr) 
                 4%-6%, in particular 5%, 
               
               
                   
                 Molybdenum (Mo) 
                 1.7%-2.1%, in particular 1.9%, 
               
               
                   
                 Tungsten (W) 
                 5.5%-6.3%, in particular 5.9%, 
               
               
                   
                 Tantalum (Ta) 
                 6.8%-7.6%, in particular 7.2%, 
               
               
                   
                 Titanium (Ti) 
                 0.8%-1.2%, in particular 1.0%, 
               
               
                   
                 Aluminum (Al) 
                 5.4%-5.9%, in particular 5.6%, 
               
               
                   
                 Rhenium (Re) 
                 1.8%-2.2%, in particular 2.0%, 
               
               
                   
                 Hafnium (Hf) 
                 0.008%-0.12%, in particular 0.10%, 
               
               
                   
                 Boron (B) 
                 0.006%-0.01%, in particular 0.008%, 
               
               
                   
                 Carbon (C) 
                 0.13%-0.15%, in particular 0.12%, and 
               
               
                   
                 Zirconium (Zr) 
                 0.004%-0.006%, in particular 0.005%. 
               
               
                   
                   
               
             
          
         
       
     
         [0020]    It is possible to dispense with Niobium (Nb) and/or Silicon (Si) and/or Gallium (Ga) and/or Germanium (Ge). 
         [0021]    The tantalum (Ta) content is kept as low as possible, since it increases the γ′ solution annealing temperature and lowers the melting temperature and also stabilizes harmful TCP phases. The molybdenum (Mo) content is likewise kept as low as possible, since molybdenum increases the γ′ solution annealing temperature and also stabilizes the harmful TCP phases. 
         [0022]    Titanium (Ti) is used as a substitute for tantalum (Ta). It contributes to the γ′ formation and reduces the γ′ solution annealing temperature, which increases the solution annealing window. 
         [0023]    Boron, carbon, hafnium and zirconium are added as grain boundary stabilizers in a balanced ratio as micro-alloying elements. The grain boundaries are therefore more stable to hot cracking at high temperatures, without the harmful effect of these elements at relatively high contents being brought into action. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  shows a turbine blade or vane. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0025]      FIG. 1  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbomachine, which extends along a longitudinal axis  121 . The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor. 
         [0026]    The blade or vane  120 ,  130  has, in succession along the longitudinal axis  121 , a securing region  400 , an adjoining blade or vane platform  403  and a main blade or vane part  406  and a blade or vane tip  415 . 
         [0027]    As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
         [0028]    A blade or vane root  183 , which is used to secure the rotor blades  120 ,  130  to a shaft or a disk (not shown), is formed in the securing region  400 . 
         [0029]    The blade or vane root  183  is designed, for example, in hammerhead form. Other configurations, such as a fir-tree or dovetail root, are possible. 
         [0030]    The blade or vane  120 ,  130  has a leading edge  409  and a trailing edge  412  for a medium which flows past the main blade or vane part  406 . 
         [0031]    In the case of the blades or vanes  120 ,  130 , superalloys are used in all regions  400 ,  403 ,  406  of the blade or vane  120 ,  130 . The blade or vane  120 ,  130  may be produced by a casting process, by directional solidification, by a forging process, by a milling process or combinations thereof. 
         [0032]    Work-pieces with a single-crystal structure or structures are used as components for machines which, in operation, are exposed to high mechanical, thermal and/or chemical stresses. 
         [0033]    Single-crystal work-pieces of this type are produced, for example, by directional solidification from the melt. This involves casting processes in which the liquid metallic alloy solidifies to form the single-crystal structure, i.e. the single-crystal workpiece, or solidifies directionally. 
         [0034]    In this case, dendritic crystals are oriented along the direction of heat flow and form either a columnar crystalline grain structure (i.e. grains which run over the entire length of the workpiece and are referred to here, in accordance with the language customarily used, as directionally solidified) or a single-crystal structure, i.e. the entire workpiece consists of one single crystal. In these processes, a transition to globular (polycrystalline) solidification needs to be avoided, since non-directional growth inevitably forms transverse and longitudinal grain boundaries, which negate the favorable properties of the directionally solidified or single-crystal component. 
         [0035]    Where the text refers in general terms to directionally solidified microstructures, this is to be understood as meaning both single crystals, which do not have any grain boundaries or at most have small-angle grain boundaries, and columnar crystal structures, which do have grain boundaries running in the longitudinal direction but do not have any transverse grain boundaries. This second form of crystalline structures is also described as directionally solidified microstructures (directionally solidified structures). 
         [0036]    Processes of this type are known for example from U.S. Pat. No. 6,024,792 A1 and EP 0 892 090 A1. 
         [0037]    The blades or vanes  120 ,  130  may likewise have coatings protecting against corrosion or oxidation e.g. (MCrAlX; M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon and/or at least one rare earth element, or hafnium (Hf)). Alloys of this type are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1. 
         [0038]    The density is preferably 95% of the theoretical density. 
         [0039]    A protective aluminum oxide layer (TGO=thermally grown oxide layer) is formed on the MCrAlX layer as an intermediate layer or as the outermost layer. 
         [0040]    The layer preferably has a composition Co-30Ni-28Cr-8Al-0.6Y-0.7Si or Co-28Ni-24Cr-10Al-0.6Y. In addition to these cobalt-based protective coatings, it is also possible to use nickel-based protective layers, such as Ni-10Cr-12Al-0.6Y-3Re or Ni-12Co-21Cr-11Al-0.4Y-2Re or Ni-25Co-17Cr-10Al-0.4Y-1.5Re. 
         [0041]    It is also possible for a thermal barrier coating, which is preferably the outermost layer, to be present on the MCrAlX, consisting for example of ZrO 2 , Y 2 O 3 -ZrO 2 , i.e. un-stabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. 
         [0042]    The thermal barrier coating covers the entire MCrAlX layer. Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
         [0043]    Other coating processes are possible, e.g. atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may include grains that are porous or have micro-cracks or macro-cracks, in order to improve the resistance to thermal shocks. The thermal barrier coating is therefore preferably more porous than the MCrAlX layer. 
         [0044]    Refurbishment means that after they have been used, protective layers may have to be removed from components  120 ,  130  (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the component  120 ,  130  are also repaired. This is followed by recoating of the component  120 ,  130 , after which the component  120 ,  130  can be reused. 
         [0045]    The blade or vane  120 ,  130  may be hollow or solid in form. If the blade or vane  120 ,  130  is to be cooled, it is hollow and may also have film-cooling holes  418  (indicated by dashed lines). 
         [0046]    While specific embodiments have been described in detail, those with ordinary skill in the art will appreciate that various modifications and alternative to those details could be developed in light of the overall teachings of the disclosure. For example, elements described in association with different embodiments may be combined. Accordingly, the particular arrangements disclosed are meant to be illustrative only and should not be construed as limiting the scope of the claims or disclosure, which are to be given the full breadth of the appended claims, and any and all equivalents thereof. It should be noted that the term “comprising” does not exclude other elements or steps and the use of articles “a” or “an” does not exclude a plurality.