Abstract:
Many known solder alloys according to prior art utilize silicon or boron as melting point reducers, which, however, form brittle phases that have an undesirable effect on the thermo-mechanical properties. The invention relates to a solder ally that comprises gallium and/or germanium, preferably forms the Y′ phase and has improved mechanical properties.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2008/052324, filed Feb. 26, 2008 and claims the benefit thereof. The International Application claims the benefits of European application No. 07005303.8 EP filed Mar. 14, 2007, European application No. 07007925.6 EP filed Apr. 18, 2007, and European application No. 07011676.9 EP filed Jun. 14, 2007, all of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to solder alloys and to a process for repairing a component. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is sometimes necessary to repair components after they have been produced, for example after casting or after they have been used and cracks have formed. 
         [0004]    There are various repair processes for this purpose, for example the welding process; in this process, however, it is additionally necessary to melt a substrate material of the component, and this may result in damage particularly to cast and directionally solidified components and in the evaporation of constituents of the substrate material. 
         [0005]    A soldering process is carried out at temperatures which are lower than the temperature for the welding process and therefore lower than the melting temperature of the substrate material. 
         [0006]    Nevertheless, the solder should have high strength in order that the crack filled with solder or the depression does not weaken the entire component at the high operating temperatures. 
       SUMMARY OF THE INVENTION 
       [0007]    Therefore, it is an object of the invention to provide a solder alloy and a process for repairing a component which overcome the problem mentioned above. 
         [0008]    The object is achieved by a solder consisting of a solder alloy and by a process as claimed in the independent claims. 
         [0009]    The solder alloy comprises: 
         [0000]    gallium (Ga) and/or germanium (Ge), and
 
optionally
 
chromium (Cr),
 
cobalt (Co),
 
aluminum (Al),
 
tungsten (W),
 
nickel (Ni),
 
and has one of the following compositions,
 
where G=Ga and/or Ge:
 
       Ni—Cr-G, Ni—Co-G, 
     Ni—W-G, Ni—Al-G, 
     Ni—Cr—Co-G, 
     Ni—Cr—W-G, 
     Ni—Cr—Al-G, 
     Ni—Co—W-G, 
     Ni—Co—Al-G, 
     Ni—W—Al-G, 
     Ni—Cr—Co—W-G, 
     Ni—Cr—Co—Al-G, 
     Ni—Cr—W—Al-G, 
     Ni—Co—W—Al-G, 
     Ni—Cr—Co—W—Al-G. 
       [0010]    The dependent claims list further advantageous measures which can advantageously be combined with one another in any desired way, theses measures are the following:
       the solder alloy does not contain any silicon,   the solder alloy does not contain any carbon,   the solder alloy does not contain any iron,   the solder alloy does not contain any manganese,   the solder alloy comprises gallium (Ga) and no germanium (Ge),   the solder alloy comprises germanium (Ge) and no gallium (Ga),   the solder alloy comprises gallium (Ga) and germanium (Ge),   in the solder alloy, the gallium (Ga) or germanium (Ge) content is ≧3% by weight, in particular the gallium or germanium content is 3% by weight,   in the solder alloy, the gallium (Ga) or germanium (Ge) content is ≧6% by weight, in particular the gallium or germanium content is 6% by weight,   in the solder alloy, the gallium (Ga) or germanium (Ge) content is ≦28% by weight, in particular the gallium or germanium content is ≦18% by weight,   in the solder alloy, the gallium (Ga) or germanium (Ge) content is ≦13% by weight, in particular the gallium or germanium content is 13% by weight,   in the solder alloy, the gallium (Ga) or germanium (Ge) content is ≦8% by weight, in particular the gallium or germanium content is 8% by weight,   the solder alloy comprises 18% by weight to 28% by weight, in particular 20% by weight, germanium (Ge), especially 26% by weight Ge,   the solder alloy comprises 21% by weight to 25% by weight germanium (Ge), in particular 23% by weight germanium (Ge),   the solder alloy comprises 28% by weight to 35% by weight gallium (Ga),   the solder alloy contains chromium (Cr), in particular at least 0.1% by weight,   the solder alloy contains cobalt (Co), in particular at least 0.1% by weight,   the solder alloy contains aluminum (Al), in particular at least 0.1% by weight,   the solder alloy contains tungsten (W), in particular at least 0.1% by weight,   the solder alloy contains titanium (Ti), in particular at least 0.1% by weight,   the solder alloy contains molybdenum (Mo), in particular at least 0.1% by weight,   the solder alloy contains tantalum (Ta), in particular at least 0.1% by weight,   the solder alloy consists of nickel (Ni) and gallium (Ga) or germanium (Ge) and a single further alloying element selected exclusively from the group consisting of chromium (Cr), cobalt (Co), tungsten (W) and aluminum (Al),   the solder alloy consists of nickel (Ni) and gallium (Ga) or germanium (Ge) and two further alloying elements selected exclusively from the group consisting of chromium (Cr), cobalt (Co), tungsten (W) and aluminum (Al),   the solder alloy consists of nickel (Ni) and gallium (Ga) or germanium (Ge) and three further alloying additions selected exclusively from the group consisting of chromium (Cr), cobalt (Co), tungsten (W) and aluminum (Al),   the solder alloy consists of nickel (Ni), chromium (Cr), cobalt (Co), tungsten (W), aluminum (Al) and gallium (Ga) or germanium (Ge),   the solder alloy does not contain any boron (B) and/or any zirconium (Zr),   the solder alloy comprises nickel (Ni) as the highest proportion by weight,   the solder alloy comprises nickel (Ni) as the highest proportion by volume,   in the solder alloy, the chromium content is 2% by weight-10% by weight, in particular 3% by weight-9% by weight, especially 8% by weight,   in the solder alloy, the aluminum content is 1% by weight-5% by weight, in particular 2% by weight-4% by weight, especially 3% by weight,   in the solder alloy, the tungsten content is 2% by weight-6% by weight, in particular 3% by weight-5% by weight, especially 4% by weight,   in the solder alloy, the cobalt content is 2% by weight-10% by weight, in particular 3% by weight-9% by weight, especially 8% by weight,   the solder alloy does not contain any silicon,   the solder alloy does not contain any carbon,   the solder alloy does not contain any iron,   the solder alloy does not contain any manganese,   the solder alloy does not contain any tungsten,   in the solder alloy, the aluminum content is between 1.0% by weight and 2.0% by weight, in particular 1.5% by weight,   in the solder alloy, the tungsten content is between 1% by weight and 3% by weight, in particular 2% by weight,   in the solder alloy, the cobalt content is between 3% by weight and 5% by weight, in particular 4% by weight,   in the solder alloy, the chromium content is between 3% by weight and 5% by weight, in particular 4% by weight,   in the solder alloy, the solder alloy does not comprise any chromium (Cr),   in the solder alloy, the solder alloy does not comprise any cobalt (Co),   in the solder alloy, the solder alloy does not comprise any aluminum (Al),   in the solder alloy, the solder alloy consists of nickel, germanium, tungsten and aluminum,   in the solder alloy, the solder alloy consists of nickel, germanium, cobalt and tungsten,   in the solder alloy, the solder alloy consists of nickel, germanium, chromium and tungsten,   in the solder alloy, the solder alloy consists of nickel, germanium, chromium and cobalt,   in the solder alloy, the solder alloy consists of nickel, germanium, cobalt and aluminum,   in the solder alloy, the solder alloy consists of nickel, germanium, chromium and aluminum.       
 
         [0062]    The process comprises repairing a component using a solder at a temperature of at least 1140° C., especially at least 1160° C. 
         [0063]    The dependent claims list further advantageous measures for the process which can advantageously be combined with one another in any desired way, these measures are the following:
       the soldering process is carried out under isothermal conditions,   the soldering process is carried out by means of a temperature gradient,   the solder is directionally solidified, in particular in single crystal form,   the solder is used for the alloys PWA 1483, PWA 1484 or Rene N5,   a substrate of the component is directionally solidified, in particular in single crystal form,   the temperature is 1160° C.,   the temperature is 1180° C.,   the temperature is 1200° C.,   the temperature is 1230° C.,   the temperature is 1260° C.,   the temperature is 1280° C.,   the temperature is at most 1280° C., in particular at most 1160° C.,   an overall pressure of less than 10 mbar (1000 Pa), in particular of about 1 mbar (100 Pa), is set in a process chamber,   the overall pressure in the process chamber is set at more than 0.1 mbar (10 Pa), in particular at more than 1 mbar (100 Pa),   the process chamber is purged with an inert gas, in particular for at least 10 hours, in particular preferably for 48 hours, before the component is heated in the process chamber with the solder,   the throughput of the purging operation is between 0.2 l/min and 1 l/min,   the throughput of the purging operation is 1 l/min,   an inert gas, in particular with a degree of purity of 6.0, is filtered through a gas cleaning cartridge before entering into the process chamber,   the duration of the soldering is at least 10 hours, in particular at least 48 hours,   the solder ( 10 ) is solidified in polycrystalline form (CC), in particular in CC components.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0084]    The invention is explained with reference to the following drawings. In the drawings: 
           [0085]      FIG. 1  shows two cross-sectional views of a component during and after treatment with the solder according to the invention, 
           [0086]      FIG. 2  shows a perspective view of a turbine blade or vane, 
           [0087]      FIG. 3  shows a perspective view of a combustion chamber, 
           [0088]      FIG. 4  shows a gas turbine, and 
           [0089]      FIG. 5  shows a list of superalloys. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0090]      FIG. 1  shows a component  1  which is treated with a solder  10  consisting of a solder alloy according to the invention. 
         [0091]    The component  1  comprises a substrate  4  which, particularly in the case of components for high temperature applications, in particular in the case of turbine blades or vanes  120 ,  130  ( FIG. 2 ) or combustion chamber elements  155  ( FIG. 3 ) for steam or gas turbines  100  ( FIG. 4 ), consists of a nickel-base or cobalt-base superalloy ( FIG. 5 ). 
         [0092]    The solder  10  can preferably be used for all the alloys according to  FIG. 5 . 
         [0093]    These may preferably be the known materials PWA 1483, PWA 1484 or Rene N5. 
         [0094]    The solder  10  is also used in blades or vanes for aircraft. 
         [0095]    A crack  7  or depression  7  which is to be filled by soldering is present in the substrate  4 . The cracks  7  or depressions  7  preferably have a width of about 200 μm and may have a depth of up to 5 mm. 
         [0096]    In this case, the solder  10  consisting of a solder alloy is applied into or close to the depression  7 , and the solder  10  is melted by heat treatment (+T) below a melting temperature of the substrate  4  and completely fills the depression  7 . 
         [0097]    The solder alloy  10  is based on nickel and therefore the largest proportion of said alloy is preferably taken up by nickel (Ni). 
         [0098]    A binary system of Ni—Ge or Ni—Ga is used with preference. 
         [0099]    The gallium (Ga) content is preferably at least 0.1% by weight. The germanium content is likewise preferably at least 0.1% by weight. Even these small proportions influence the soldering behavior of nickel or a nickel alloy. 
         [0100]    In addition to a remainder of nickel and gallium and/or germanium, the further constituents of chromium (Cr), cobalt (Co), aluminum (Al) and titanium (Ti), tungsten (W), molybdenum (Mo) or tantalum (Ta) may preferably be present which, if used, are each preferably used in a proportion of at least 0.1% by weight. 
         [0101]    The chromium content is preferably in a range from 2% by weight-10% by weight, in particular in a range from 3% by weight-9% by weight, with particularly preferred exemplary embodiments having a chromium content of 4% by weight or 8% by weight, and therefore preferred chromium values are also in a range from 3% by weight to 5% by weight or 7% by weight to 9% by weight, preferably 8% by weight. 
         [0102]    The chromium content is also preferably 4% by weight. 
         [0103]    The aluminum content is preferably in a range from 1% by weight-5% by weight, particularly preferably in a range from 2% by weight-4% by weight. A particularly good exemplary embodiment has a solder alloy with an aluminum content of 3% by weight. 
         [0104]    The tungsten content is preferably in a range from 2% by weight-6% by weight, particularly preferably in a range from 3% by weight-5% by weight. Particularly good results have been achieved with a tungsten content of 4% by weight. 
         [0105]    The cobalt content is in a range from 2% by weight-10% by weight, particularly preferably in a range from 3% by weight-9% by weight. Particularly preferred exemplary embodiments have a cobalt content of 4% by weight or 8% by weight, and therefore particularly preferred cobalt contents are 3% by weight to 5% by weight or 7% by weight to 9% by weight, particularly 8% by weight. The cobalt content is also preferably 4% by weight. 
         [0106]    The gallium or germanium content is preferably at least 3% by weight, particularly preferably at least 6% by weight. The germanium or gallium content may preferably be limited to a maximum value of 18% by weight. The maximum gallium or germanium content is likewise preferably 13% by weight and very particularly preferably 8% by weight. 
         [0107]    The gallium (Ga) content in a nickel-base superalloy as the solder alloy is preferably between 28% by weight and 35% by weight. 
         [0108]    The germanium content is preferably between 18% by weight and 28% by weight, in particular 20% by weight, 23% by weight, 26% by weight or 27% by weight, particularly in the case of binary systems, i.e. NiGe20, NiGe23 or NiGe26, in particular for solidification in single crystal form. 
         [0109]    The above list of the solder constituents of nickel, chromium, cobalt, tungsten, aluminum, gallium or germanium is preferably conclusive. 
         [0110]    Preference is given to using either only gallium or germanium. 
         [0111]    The text which follows lists the conclusive composition of advantageously used alloys, where the alloy comprises either only germanium or only gallium or else germanium and gallium (G=gallium and/or germanium, i.e. only Ga or only Ge or Ga and Ge): 
       Ni—Cr-G 
     Ni—Co-G 
     Ni—W-G 
     Ni—Al-G 
     Ni—Cr—Co-G 
     Ni—Cr—W-G 
     Ni—Cr—Al-G 
     Ni—Co—W-G 
     Ni—Co—Al-G 
     Ni—W—Al-G 
     Ni—Cr—Co—W-G 
     Ni—Cr—Co—Al-G 
     Ni—Cr—W—Al-G 
     Ni—Co—W—Al-G 
     Ni—Cr—Co—W—Al-G. 
       [0112]    The solder  10  preferably does not contain any boron. Likewise, the solder  10  preferably does not contain any zirconium. The addition of rhenium may likewise preferably be omitted. Likewise, preference is given to not using any hafnium. 
         [0113]    The addition or the presence of silicon and/or carbon is preferably avoided since they form brittle phases in the solder. 
         [0114]    The addition or the presence of iron and/or manganese is likewise preferably avoided since these elements form low-melting phases or non-oxidizing phases. 
         [0115]    The solder  10  may be joined to the substrate  4  of the component  1 ,  120 ,  130 ,  155  in an isothermal process or a temperature gradient process. A gradient process is preferably suitable when the substrate  4  has a directional structure, for example an SX or DS structure, such that the solder  10  then also has a directional structure. However, a directionally solidified structure in the solder may also be provided in an isothermal process. 
         [0116]    Equally, the component  1  does not need to have a directionally solidified structure (but rather a CC structure). 
         [0117]    The solders in CC substrates of components may likewise be soldered and solidified in a CC structure, the solders then being solidified in polycrystalline form (CC). 
         [0118]    The following solders are of particular interest especially for the polycrystalline solidification of the solders: 
       NiGe 
     NiGeW4Al3 
     NiGeCo8W4 
     NiGeCr8W4 
     NiGeCr8Co8W4Al3 
     NiGeCr8Co8 
     NiGeCo8Al3 
     NiGeCr8Al3 
     NiGeCr4Co4W2Al1.5. 
       [0119]    In this case, the germanium content is from 20% by weight-30% by weight, in particular 26% by weight or 27% by weight. 
         [0120]    During the melting (isothermal process or gradient process), use is preferably made of an inert gas, in particular argon, which reduces the vaporization of chromium from the substrate  4  at the high temperatures, or a reducing gas (argon/hydrogen) is used. 
         [0121]    The solder  10  may also be applied to a large area of a surface of a component  1 ,  120 ,  130 ,  155  in order to thicken the substrate  4 , in particular in the case of hollow components. The solder  10  is preferably used to fill cracks  7  or depressions  7 . 
         [0122]    When a solder  10  is soldered in vacuo, which is often carried out when the solder  10  or the component  1 ,  120 ,  130 ,  155  oxidizes, the use of inert gases (Ar, He, Ar/He, H 2  . . . ) and/or the use of a vacuum preferably results in the problem of constituents of the component  1 ,  120 ,  130 ,  155  or of the solder  10  vaporizing at an excessively low process pressure. 
         [0123]    At an excessively high oxygen partial pressure p 02 , the solder  10  or the component  1 ,  120 ,  130 ,  155  oxidizes. 
         [0124]    The process according to the invention therefore also preferably proposes carrying out the soldering process in vacuo in a process chamber, preferably in a furnace at an oxygen partial pressure p 02  of at most 10 −6  mbar (10 −4  Pa). 
         [0125]    The oxygen partial pressure p 02  is preferably at least 10 −7  mbar (10 −5  Pa). The overall process pressure is preferably at most 100 mbar (10 000 Pa), in particular at most 10 mbar (1000 Pa). The overall process pressure is preferably at least 0.1 mbar (10 Pa). Particularly good soldered joins have been achieved at a pressure of 1 mbar (100 Pa). 
         [0126]    These pressure values are achieved, in particular, due to the fact that the process chamber has a vacuum in its interior, is preferably permanently pumped out and, before the soldering, is preferably purged with a pure inert gas, preferably argon (Ar) (Ar 5.0, preferably Ar 6.0). This is preferably carried out for at least 10 hours, in particular for 48 hours with a throughflow rate of preferably between 0.2 l/min and 1 l/min. 
         [0127]    In this case, preference is given to using argon 6.0 (this means an oxygen content of 5×10 −7  in the process gas) which, however, is preferably filtered through a gas cleaning cartridge, such that the oxygen and water content is reduced by a factor of 100 so as to achieve an oxygen content of 5×10 −9  in the process gas introduced into the process chamber. 
         [0128]    Argon is likewise preferably present within the pressure values described above during the soldering operation. 
         [0129]    The temperature during the soldering process is at least 1140° C., in particular at least 1160° C. Further advantageous soldering temperatures are 1160° C., 1180° C., 1200° C., 1230° C. and 1260° C. The maximum temperature is preferably 1280° C., in particular at most 1260° C. 
         [0130]    The duration of the soldering treatment is preferably at least 10 hours, in particular 48 hours. 
         [0131]      FIG. 2  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbomachine, which extends along a longitudinal axis  121 . 
         [0132]    The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor. 
         [0133]    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 . 
         [0134]    As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
         [0135]    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 . 
         [0136]    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. 
         [0137]    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 . 
         [0138]    In the case of conventional blades or vanes  120 ,  130 , by way of example solid metallic materials, in particular superalloys, in particular the superalloys according to  FIG. 5 , are used in all regions  400 ,  403 ,  406  of the blade or vane  120 ,  130 . 
         [0139]    Superalloys of this type are known, for example, from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949; these documents form part of the disclosure with regard to the chemical composition of the alloy. 
         [0140]    The blade or vane  120 ,  130  may in this case be produced by a casting process, by means of directional solidification, by a forging process, by a milling process or combinations thereof. 
         [0141]    Workpieces 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. 
         [0142]    Single-crystal workpieces 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. 
         [0143]    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. 
         [0144]    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). 
         [0145]    Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1; these documents form part of the disclosure with regard to the solidification process. 
         [0146]    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 EP0 486 489 B1, EP0 786 017 B1, EP0 412 397 B1or EP 1 306 454 A1, which are intended to form part of the present disclosure with regard to the chemical composition of the alloy. 
         [0147]    The density is preferably 95% of the theoretical density. 
         [0148]    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). 
         [0149]    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-base protective coatings, it is also preferable to use nickel-base protective layers, such as Ni-10Cr-12Al-0.6Y-3Re or Ni-12Co-21Cr-11Al-0.4Y-2Re or Ni-25Co-17Cr-10A1-0.4Y-1.5Re. 
         [0150]    It is also possible for a thermal barrier coating, which is preferably the outermost layer and consists for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide, to be present on the MCrAlX. 
         [0151]    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). 
         [0152]    Other coating processes are possible, for example 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. 
         [0153]    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. 
         [0154]    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). 
         [0155]      FIG. 3  shows a combustion chamber  110  of a gas turbine. The combustion chamber  110  is configured, for example, as what is known as an annular combustion chamber, in which a multiplicity of burners  107 , which generate flames  156 , arranged circumferentially around the axis of rotation  102  open out into a common combustion chamber space  154 . For this purpose, the combustion chamber  110  overall is of annular configuration positioned around the axis of rotation  102 . 
         [0156]    To achieve a relatively high efficiency, the combustion chamber  110  is designed for a relatively high temperature of the working medium M of approximately 1000° C. to 1600° C. To allow a relatively long service life even with these operating parameters, which are unfavorable for the materials, the combustion chamber wall  153  is provided, on its side which faces the working medium M, with an inner lining formed from heat shield elements  155 . 
         [0157]    On the working medium side, each heat shield element  155  made from an alloy is equipped with a particularly heat-resistant protective layer (MCrAlX layer and/or ceramic coating) or is made from material that is able to withstand high temperatures (solid ceramic bricks). 
         [0158]    These protective layers may be similar to the turbine blades or vanes, i.e. for example 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, which are intended to form part of the present disclosure with regard to the chemical composition of the alloy. 
         [0159]    It is also possible for a, for example, ceramic thermal barrier coating to be present on the MCrAlX, consisting for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. 
         [0160]    Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
         [0161]    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. 
         [0162]    Refurbishment means that after they have been used, protective layers may have to be removed from heat shield elements  155  (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the heat shield element  155  are also repaired. This is followed by recoating of the heat shield elements  155 , after which the heat shield elements  155  can be reused. 
         [0163]    Moreover, a cooling system may be provided for the heat shield elements  155  and/or their holding elements, on account of the high temperatures in the interior of the combustion chamber  110 . The heat shield elements  155  are then, for example, hollow and may also have cooling holes (not shown) opening out into the combustion chamber space  154 . 
         [0164]      FIG. 4  shows, by way of example, a partial longitudinal section through a gas turbine  100 . 
         [0165]    In the interior, the gas turbine  100  has a rotor  103  with a shaft  101  which is mounted such that it can rotate about an axis of rotation  102  and is also referred to as the turbine rotor. 
         [0166]    An intake housing  104 , a compressor  105 , a, for example, toroidal combustion chamber  110 , in particular an annular combustion chamber, with a plurality of coaxially arranged burners  107 , a turbine  108  and the exhaust-gas housing  109  follow one another along the rotor  103 . 
         [0167]    The annular combustion chamber  110  is in communication with a, for example, annular hot-gas passage  111 , where, by way of example, four successive turbine stages  112  form the turbine  108 . 
         [0168]    Each turbine stage  112  is formed, for example, from two blade or vane rings. As seen in the direction of flow of a working medium  113 , in the hot-gas passage  111  a row of guide vanes  115  is followed by a row  125  formed from rotor blades  120 . 
         [0169]    The guide vanes  130  are secured to an inner housing  138  of a stator  143 , whereas the rotor blades  120  of a row  125  are fitted to the rotor  103  for example by means of a turbine disk  133 . 
         [0170]    A generator (not shown) is coupled to the rotor  103 . 
         [0171]    While the gas turbine  100  is operating, the compressor  105  sucks in air  135  through the intake housing  104  and compresses it. The compressed air provided at the turbine-side end of the compressor  105  is passed to the burners  107 , where it is mixed with a fuel. The mix is then burnt in the combustion chamber  110 , forming the working medium  113 . From there, the working medium  113  flows along the hot-gas passage  111  past the guide vanes  130  and the rotor blades  120 . The working medium  113  is expanded at the rotor blades  120 , transferring its momentum, so that the rotor blades  120  drive the rotor  103  and the latter in turn drives the generator coupled to it. 
         [0172]    While the gas turbine  100  is operating, the components which are exposed to the hot working medium  113  are subject to thermal stresses. The guide vanes  130  and rotor blades  120  of the first turbine stage  112 , as seen in the direction of flow of the working medium  113 , together with the heat shield elements which line the annular combustion chamber  110 , are subject to the highest thermal stresses. 
         [0173]    To be able to withstand the temperatures which prevail there, they may be cooled by means of a coolant. 
         [0174]    Substrates of the components may likewise have a directional structure, i.e. they are in single-crystal form (SX structure) or have only longitudinally oriented grains (DS structure). 
         [0175]    By way of example, iron-base, nickel-base or cobalt-base superalloys are used as material for the components, in particular for the turbine blade or vane  120 ,  130  and components of the combustion chamber  110 . 
         [0176]    Superalloys of this type are known, for example, from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949; these documents form part of the disclosure with regard to the chemical composition of the alloys. 
         [0177]    The blades or vanes  120 ,  130  may also have coatings which protect against corrosion (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, scandium (Sc) and/or at least one rare earth element or hafnium). Alloys of this type are known from EP0 486 489 B1, EP0 786 017 B1, EP0 412 397 B1 or EP 1 306 454 A1, which are intended to form part of the present disclosure with regard to the chemical composition. 
         [0178]    A thermal barrier coating, consisting for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide, may also be present on the MCrAlX. 
         [0179]    Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
         [0180]    The guide vane  130  has a guide vane root (not shown here), which faces the inner housing  138  of the turbine  108 , and a guide vane head which is at the opposite end from the guide vane root. The guide vane head faces the rotor  103  and is fixed to a securing ring  140  of the stator  143 .