CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP02/14499, filed Dec. 18, 2002 and claims the benefit thereof. The International Application claims the benefits of European Patent application No. 02001348.8 EP filed Jan. 18, 2002, both of the applications are incorporated by reference herein in their entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a turbine with at least four stages in accordance with claim  1  and to the use of a turbine blade with a reduced density in accordance with claim  9 . 
     The use of ceramic guide vanes in gas turbines is known from U.S. Pat. No. 3,992,127. Ceramic guide vanes are used because the ceramic has good high-temperature properties. Particularly high temperatures, which only ceramics are able to withstand, occur in particular in the first row downstream of the combustion chamber (first turbine stage), with the turbine blades and vanes in the first row being the smallest. 
     U.S. Pat. No. 5,743,713 has disclosed a ceramic blade which is inserted into a metallic rotor disk of a turbine. 
     U.S. Pat. No. 4,563,128 has disclosed a ceramic blade which has a metallic core surrounded on the outside by ceramic and extending as far as a radial end of the blade. The metallic core forms a very high proportion of the volume. 
     Hitherto, ceramic rotor blades have only been used, by virtue of their high thermal stability, in the temperature-critical stage or stages of a turbine, whereas in the subsequent stages it has been customary to use metallic rotor blades (in particular made from Ni-based alloys or from TiAl alloys). 
     A significant improvement to the efficiency of gas turbines can be achieved if, at least from the fourth stage onward, the turbine rotor blades are increased in size by, for example, approximately 20% compared to conventional dimensions. This increase in size from the fourth stage onward, however, leads to a considerable increase in the centrifugal forces at the blades if the rotational speed remains unchanged, and these forces represent unacceptable loads on these blades and on the disks to which the blades are secured. 
     SUMMARY OF THE INVENTION 
     Therefore, it an object of the invention to provide a turbine with an increased efficiency compared to a turbine with conventional blading. 
     The object is achieved by virtue of the fact that the turbine, in the fourth stage, in each case has rotor blades with a length of at least 50 cm which contain a high proportion of a material with a density of at most 4 g/cm 3 , and are, for example, made from ceramic, with the result that the mass is significantly reduced compared to standard metallic blading of conventional dimensions. This allows the blade length, or at least the length of the main blade section, to be lengthened considerably compared to metallic blades. 
     It is even possible to use solid-ceramic or hollow-ceramic blades which are secured to metallic disks of the turbine rotor, as is known from U.S. Pat. No. 5,743,713. 
     It is also advantageous to use ceramic rotor blades which have a metallic core which is surrounded by ceramic. In this case, the proportion by volume of the ceramic is very high, so that the mass is greatly reduced compared to a purely metallic blade with an optional thin ceramic protective layer. 
     A further advantage of a more lightweight blade is that the mechanical loading on the disk to which the blade is secured is lower during rotation on account of the lower mass attached thereto. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures diagrammatically depict the invention, which is explained in more detail below with further details and advantageous refinements. 
       In the drawing: 
         FIG. 1  shows a gas turbine, 
         FIG. 2  shows a partial region of a gas turbine with a fourth rotor blade stage, 
         FIG. 3  shows a rotor blade and a rotor disk, 
         FIG. 4  shows a section on line IV-IV in  FIG. 3 , 
         FIG. 5   a, b  show further exemplary embodiments of a rotor blade. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  diagrammatically depicts a longitudinal section through a turbine, for example a gas turbine  41 . However, the invention is not restricted to a gas turbine. 
     A compressor  47 , a combustion chamber  50  and a turbine part  53  are arranged in succession along a turbine shaft which includes a tie rod  4 . The turbine part  53  has a hot-gas duct  56 . Gas turbine blades and vanes  13 ,  16  are arranged in the hot-gas duct  56 . Rings of guide vanes and rings of rotor blades are provided alternately. The gas turbine blades and vanes  13 ,  16  are cooled, for example, by combined air and/or steam cooling. For this purpose, by way of example, compressor air is removed from the compressor  47  and fed to the gas turbine blades and vanes  13 ,  16  via an air passage  63 . Steam is also fed to the gas turbine blades and vane  13 ,  16  via a steam feed  66 , for example. This steam preferably originates from a steam turbine of a combined-cycle gas and steam process. 
       FIG. 2  shows an excerpt from a gas turbine  41 . The gas turbine  41  has a turbine shaft with a tie rod  4  which rotates about an axis  7 . A plurality of guide vanes  13  and a plurality of rotor blades  16 , which are arranged, for example, in the hot-gas duct  56 , extend in the radial direction  19 , which runs perpendicular to the axis  7 . There are at least four rows of rotor blades and, for example, four rows of guide vanes, i.e. there is a total of four stages. The first row of guide vanes may, for example, may be replaced by a special burner arrangement. Just one of the blades  16  in the fourth stage is illustrated here, by way of example. 
     The rotor blades  16  are, for example, secured to metal disks ( 25 ,  FIG. 3 ) on the turbine shaft, held together by the tie rod  4 , and rotate with the tie rod  4  about the axis  7 . 
     The guide vanes  13  are secured in a rotationally fixed position to a casing  10  of the gas turbine  41 . 
     A hot gas  22  flows in the direction of the axis  7 , from the left to the right in the drawing, as is diagrammatically indicated by an arrow. 
     The fourth row of rotor blades, as seen in the direction of flow  22 , is denoted by V 4 . The rotor blades in the fourth stage are in each case rotor blades  16  which have a high proportion by volume of their material made up of a material with a density of at most 4 g/cm 3  and are made, for example, from ceramic and have a length of at least 50 cm, in particular of at least 65 cm. 
     Since the density of ceramic materials is in the range from 1.5 to 3.5 g/cm 3 , and is therefore well below the densities of nickel-base alloys, at 8 g/cm 3 , and of TiAl alloys, at approximately 4.5 g/cm 3 , a ceramic rotor blade of this type has a considerable reduction in mass compared to a corresponding metallic rotor blade, so that, when these rotor blades are rotating, lower centrifugal forces occur, in particular at the outer radial end  37  of the rotor blade  16 , thereby inducing loading in particular on the root of the rotor blade  16  and its anchoring in the turbine shaft. 
     By lengthening the turbine rotor blades in the fourth row by, for example, approximately 20% , it is possible to considerably increase the efficiency of gas turbines. Ceramic rotor blades are, for example, made completely from ceramic, in which case the ceramic may advantageously comprise various layers of ceramics. For example, it is possible to use fiber-reinforced CMC oxide ceramics or fiber-reinforced CMC nonoxide ceramics, nonoxidic ceramics, such as for example carbon fibers or SiC fibers in a corresponding carbon or silicon carbide matrix. It is also possible to use oxide systems, e.g. mullite fibers or aluminum oxide fibers in a mullite matrix. 
     The ceramics may in turn be coated with a protective layer  36  ( FIG. 4   a ) to prevent corrosion and oxidation, such as those which are known from metallic turbine blades: yttrium-stabilized zirconia, boron nitride, spinels. 
       FIG. 3  shows a rotor blade  16  with a length L between platform  17  and radial end of the rotor blade  16  which is formed, for example, entirely from ceramic and is inserted into a metallic rotor disk  25  in a manner fixed in terms of rotation. The metallic disk  25  is connected to the tie rod  4  and rotates therewith. 
     The diameter of the disk  25  is no greater than usual and is also not exposed to the highest temperatures within the hot-gas duct  56 , and consequently metal can continue to be used as material for the disk  25 , in the same way as in a conventional turbine. 
     It is also possible to use what are known as hybrid turbine blades, which still have a metallic core but this core is surrounded by a ceramic, as is known, for example, from U.S. Pat. No. 4,563,128. The content of disclosure of this document relating to the structure of the ceramic turbine blade is expressly incorporated in the content of disclosure of the present application. Further types of hybrid blades are conceivable. 
       FIG. 4  shows an example of a hybrid blade  16 . A main blade section  28  at its outer surface consists of ceramic  39 . In the interior, there is a metallic core  31 , for example formed from a nickel and/or cobalt superalloy. The metallic core  31 , by way of example, also forms a root part  34  of the blade  16 . 
     In the radial direction  19 , the metallic core  31  does not extend all the way to the radial end  37  of the blade  16 , but rather, for example, only extends over for example 70% of the length of the main blade section  28  in the radial direction  19 , since otherwise the loads caused by the centrifugal forces at the intended rotational speed of the turbine would exceed the mechanical strength of the metallic core or of the blade root or of the anchoring in the turbine shaft. 
     The metallic core  31  may at least in part be formed from metallic foam, in order to save further weight. 
     The proportion by volume of the material formed by the ceramic is amounts to least 40% or, for example, even exceeds that of the metallic core  31 , so that the blade  16  has a high proportion by volume of its material formed by ceramic. 
     The proportion of ceramic  39  may also be located predominantly at the end  37  of the blade  16 , since that is where the centrifugal forces are highest ( FIG. 5   a ). 
     A remaining part  38  of the blade  16  consists of metal, for example of a nickel and/or cobalt superalloy. The hybrid blade  16  may also be of internally hollow design, in order to further reduce its weight. 
     It is also possible, as illustrated in  FIG. 5   b , to provide a skeleton  40  made from metal, for example from a nickel and/or cobalt superalloy, into which ceramic parts are introduced. 
     The skeleton  40  comprises, for example, a leading edge  70 , which the medium strikes first in the direction of flow, a trailing edge  73 , the root part  34  and the tip  76 , as well as the radial end  37 . 
     The rotor blade  16  may also be internally hollow and cooled by air and/or steam cooling with or without film-cooling bores. 
     It has not hitherto been known that ceramic rotor blades with a length that is considerably increased compared to conventional dimensioning, on account of their lower density and the associated reduction in the centrifugal forces, can advantageously be used to increase the turbine efficiency.
The invention relates to a turbine comprising at least four stages and to the use of a rotor blade with a reduced mass. In prior art, rotor blades in the fourth stage of a gas turbine, which exceed 50 cm in length, cause problems relating to mechanical strength, as centrifugal forces of too great a magnitude occur during the rotation of the rotor blades. An inventive rotor blade in the fourth row of a gas turbine has a reduced density as a result of a high proportion of a ceramic, thus reducing the centrifugal force.