Abstract:
An improved nickel-based superalloy having high corrosion and oxidation resistance and good compatibility with a thermal barrier coating. The enhanced oxidation resistance and compatibility with the thermal barrier coating results from the inclusion of two or more rare earth elements. The superalloy is useful for the fabrication of components for a gas turbine.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
       [0001]     The U.S. Government has a paid-up license in the invention and the right in limited circumstances to require that patent owner to license others on reasonable terms as provided for by the terms of DE-FC26-05NT42644 awarded by the Department of Energy. 
     
    
     CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0002]     This application is a continuation-in-part of international patent application PCT/EP2005/057043 filed on Dec. 21, 2005, and claiming priority of Sweden application 0403162-1 filed on Dec. 23, 2004, which international application was published in English as WO 2006/067189 on Jun. 29, 2006.  
       FIELD OF THE INVENTION  
       [0003]     The invention relates to a nickel-based superalloy with very high corrosion resistance and enhanced oxidation resistance and more particularly, to a nickel-based superalloy for directionally solidified and conventionally cast components suited for use in gas turbine engines.  
       BACKGROUND OF THE INVENTION  
       [0004]     Nickel-base superalloys have a very good material strength at high temperatures. These properties permit their use in components for gas turbine engines where the retention of excellent mechanical properties at high temperatures is required. The use of these alloys at increasingly higher temperatures requires that a coating be applied to the superalloy component for thermal protection. The coating typically consists of applying a bondcoat to the superalloy and then a thermal barrier coating (TBC) to the bondcoat. Typical bond coats are alloys of the type MCrAlX where M is Ni, Co, or Fe and X is commonly Y, Zr, or Hf. The bondcoat tends to degrade during prolonged high temperature exposure. The degraded bondcoat does not adequately adhere the thermal barrier coating to the superalloy component. As a result, spallation of the TBC occurs with complete loss of thermal protection to the component. The rate at which the bondcoat degrades depends upon the composition of the superalloy to which it is applied. Generally alumina forming superalloys exhibit longer bondcoat lifetimes than chromia forming superalloys. However, it is often preferable to use high chromium containing superalloys for very high corrosion resistance. A need exists for a superalloy with a lower propensity to promote bondcoat degradation and significantly enhance the resistance of the TBC to spallation.  
       SUMMARY OF THE INVENTION  
       [0005]     This application is directed to a nickel-based superalloy that has high corrosion and oxidation resistance and good compatibility with a thermal barrier coating deposited thereon. The nickel-based superalloy may be formed from materials in the following weight percentages: 21.0 to 24.0 Cr; 18.0 to 20.0 Co; 0 to 0.5 Mo; 1.5 to 2.5 W; 1.0 to 2.0 Ta; 1.5 to 2.3 Al; 3.4 to 4.0 Ti; 0.7 to 1.2 Nb; 0.05 to 0.3 Hf; 0.05 to 0.3 Si; 0.002 to 0.008 B; 0.010 to 0.040 Zr; 0.10 to 0.20 C; 0.001 to 0.1 of a mixture of two or more rare earth elements selected from the group of La, Ce, Nb, Dy, Pr, Sm, and Gd; and the balance formed from Ni. Preferably, the superalloy may include the following materials in the following weight percentages: 22.0 to 22.8 Cr; 18.5 to 19.5 Co; 0 to 0.2 Mo; 1.8 to 2.2 W; 1.3 to 1.5 Ta; 1.8 to 2.0 Al; 3.6 to 3.8 Ti; 0.9 to 1.1 Nb; 0.1 to 0.25 Hf; 0.15 to 0.25 Si; 0.004 to 0.006 B; 0.020 to 0.030 Zr; 0.13 to 0.17 C; 0.01 to 0.05 of a mixture of two or more rare earth elements selected from the group of La, Ce, Nb, Dy, Pr, Sm, and Gd; and the balance formed from Ni. The superalloy may also be formed from the following materials in weight percentages including: 22.4 Cr; 19.0 Co; 2.0 W; 1.4 Ta; 1.9 Al; 3.7 Ti; 1.0 Nb; 0.2 Hf; 0.2 Si; 0.005 B; 0.025 Zr; 0.15 C; 0.02 of a mixture of two or more rare earth elements selected from the group of La, Ce, Nb, Dy, Pr, Sm, and Gd; and the balance formed from Ni. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0006]     This invention is directed to a high chromium superalloy that promotes superior corrosion and oxidation resistance and an improved compatibility with a TBC applied to the superalloy via a bondcoat. In one embodiment, the superalloy may be formed from materials in the following weight percentages: 21.0 to 24.0 Cr; 18.0 to 20.0 Co; 0 to 0.5 Mo; 1.5 to 2.5 W; 1.0 to 2.0 Ta; 1.5 to 2.3 Al; 3.4 to 4.0 Ti; 0.7 to 1.2 Nb; 0.05 to 0.3 Hf; 0.05 to 0.3 Si; 0.002 to 0.008 B; 0.010 to 0.040 Zr; 0.10 to 0.20 C; 0.001 to 0.1 of a mixture of two or more rare earth elements selected from the group of La, Ce, Nb, Dy, Pr, Sm, and Gd; and the balance formed from Ni. The inclusion of rare earth elements selected from the group of La, Ce, Nb, and Dy provides enhanced coating performance. Furthermore, the desired coating performance is a result of the use of two or more rare earth elements rather than a single element.  
         [0007]     A chromium content of at least 21 weight percent results in excellent levels of high temperature corrosion resistance. Although the aluminum content is relatively low compared to levels that are generally present in superalloys with high oxidation resistance, oxidation resistance is enhanced by the presence of the rare earth elements, the silicon, and the hafnium present in the superalloy.  
         [0008]     The silicon in the alloy permits the formation of SiO 2  at the surface oxide layer to provide oxidation resistance. However, the level of silicon must be kept at levels below 0.2 weight percent, a level where the silicon content is detrimental to the performance of the alloy. The addition of the hafnium at levels similar to that of the silicon compensates for the limitation in silicon level without the detrimental performance resulting from excessive silicon levels.  
         [0009]     Particularly the addition of the rare earth elements dramatically improves the oxidation resistance. The presence of rare earth elements is believed to promote the diffusion of aluminum to the surface increasing the proportion of alumina in the scale relative to alloys where no rare earth elements are present.  
         [0010]     The presence of the rare earth elements enhances the coating life. This enhancement is attributed to the ability of the rare earth elements to form sulfides and oxysulfides fixing sulfur impurities which prevents their diffusion to the surface permitting the degradation of the alumina scale on the superalloy adjacent to the bondcoat.  
         [0011]     A preferred superalloy for high corrosion resistance and an improved oxidation resistance may be formed from materials in the following weight percentages: 22.0 to 22.8 Cr; 18.5 to 19.5 Co; 0 to 0.2 Mo; 1.8 to 2.2 W; 1.3 to 1.5 Ta; 1.8 to 2.0 Al; 3.6 to 3.8 Ti; 0.9 to 1.1 Nb; 0.1 to 0.25 Hf; 0.15 to 0.25 Si; 0.004 to 0.006 B; 0.020 to 0.030 Zr; 0.13 to 0.17 C; 0.01 to 0.05 of a mixture of two or more rare earth elements selected from the group of La, Ce, Nb, Dy, Pr, Sm, and Gd; and the balance formed from Ni. A most preferred superalloy composition may be formed from materials in the following weight percentages: 22.4 Cr; 19.0 Co; 2.0 W; 1.4 Ta; 1.9 Al; 3.7 Ti; 1.0 Nb; 0.2 Hf; 0.2 Si; 0.005 B; 0.025 Zr; 0.15 C; 0.02 of a mixture of two or more rare earth elements selected from the group of La, Ce, Nb, Dy, Pr, Sm, and Gd; and the balance formed from Ni.  
         [0012]     Alternatives for the alloy composition and other variations within the range provided will be apparent to those skilled in the art. Variations and modifications can be made without departing from the scope and spirit of the invention as defined by the following claims.