Abstract:
A nickel-based superalloy that forms a chromia scale in an oxidizing environment is disclosed. The alloy provides good oxidation resistance at temperatures below 900° C. in a dry or moist atmosphere. The superalloy is suited for components of gas or steam turbine engines including blades and vanes.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a nickel-base superalloy that forms a protective chromia layer in an oxidizing atmosphere. 
       BACKGROUND OF THE INVENTION 
       [0002]    Nickel-base superalloys have a very good material strength at elevated temperatures. These properties permit their use in components for gas turbine engines where the retention of excellent mechanical properties at high temperatures is required for typical lifetimes in excess of 100,000 hours. Components within an industrial gas turbine are exposed to a range of temperatures depending upon where the component is located in the turbine. For example, last stage turbine blades see intermediate temperatures and do not require, and are not covered by a thermal barrier coating, and are protected by the surface oxide layer that forms upon exposure to the oxidizing environment. 
         [0003]    The metallurgy of superalloys is a sophisticated and well developed field. Optimization of the composition of superalloys consists of defining the amounts of elements which are desirably present, and the amounts of elements which are desirably absent. These impurities can in some cases be completely eliminated from the composition through the judicious selection of melt stock material; however some elements cannot be readily eliminated. One impurity which has long been recognized as being detrimental is sulfur. Sulfur was initially identified as being detrimental to mechanical properties, and its presence in alloy compositions was limited for that reason. However, the sulfur levels which do not present significant loss of mechanical properties, a bulk property, can in some cases still be highly detrimental to oxidation resistance, a surface property. 
         [0004]    Oxidation resistance of superalloys is primarily due to the presence of an adherent surface oxide scale. The composition and nature of oxide scales depends on the composition of the alloy and the environment to which the superalloy component operates. Typically, the oxide is either alumina, or chromia. These oxides are formed by a selective oxidation of the component in the alloy. A continuous oxide scale of predominately or exclusively one metal occurs where a metal at or above a critical concentration in the alloy oxidizes preferentially over more noble metals, such that the sufficiently high concentration less noble element diffuses to and into the scale, forming more of the selective oxide while the more noble metal diffuses from the scale into the alloy. The composition and thickness of the protective oxide scale formed depends on a number of factors including the relative concentration of the metals in the alloy, the relative nobilities of the metals in the substrate, and oxygen solubility and diffusivity in the alloy. As the temperature changes, the diffusion rates and solubilities of the metals and oxygen will change. Typically the formation of an alumina scale is favored by higher temperatures. Some alloys form chromia scales at lower temperatures and alumina scales at higher temperatures where aluminum no longer precipitates internally. Generally maximum protection via alumina occurs by formation at a temperature range, in excess of 1,000° C. The selective oxidation can also be enhanced by the incorporation of reactive species such as rare earth elements. 
         [0005]    Moisture affects the degradation and debonding of the protective oxide. This is particularly critical for steam turbines, where superalloys are replacing steel as the temperature at which the turbine operates increases. The temperatures at which a steam turbine operates are typically lower than the temperatures where gas turbine operates. The presence of water vapor generally lowers the oxidation stability of the superalloy, as the oxide scale is either poorly formed, or is not stable to the water vapor containing oxidizing environment. Selective oxidation to alumina is not favored in a water containing atmosphere, particularly at lower temperatures. At higher temperatures, in excess of 900° C., water vapor can promote the formation of volatile species that remove chromia scale and can ultimately result in the loss of oxidation protection by a chromia scale. 
         [0006]    Hence the identification of superalloys that provides good oxidation resistance to components, particularly for components that must operate in the presence of water vapor. 
       SUMMARY OF THE INVENTION 
       [0007]    This invention is directed to superalloys that form highly adherent chromia surface layers when exposed to an oxidizing environment at elevated temperatures. The superalloy may be usable in high temperature environments, such as in use as a turbine vane or turbine blade of a turbine engine. In one embodiment, the superalloy may be configured for oxidation resistance components used in gas or steam turbine engines. The superalloy may be formed from materials in the following weight percentages: 10 to 25 Cr; 5 to 25 Co; 2.0 to 6.0 Mo; 0.5 to 4.0 Al; 1.0 to 4.0 Ti; 0 to 3 Hf; 0 to 10 Fe; 0 to 1 Si; 0 to 0.10 B; 0 to 0.20 Zr; 0 to 0.20 C; 0 to 0.5 total of at least one rare earth elements selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; with less than 100 ppm S; and the balance formed from Ni excluding incidental impurities. A preferred composition may be formed from materials in the following weight percentages: 18 to 21 Cr; 12.0 to 15.0 Co; 3.5 to 5.0 Mo; 1.2 to 1.6 Al; 2.75 to 3.25 Ti; 0 to 0.5 Hf; 0 to 2 Fe; 0 to 0.5 Si; 0.003 to 0.010 B; 0.02 to 0.08 Zr; 0.03 to 0.10 C; 0 to 0.2 of one or more rare earth elements selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; with less than 10 ppm S; and the balance formed from Ni. A most preferred superalloy composition may be formed from materials in the following weight percentages: 18.5 Cr; 13.0 to 14.0 Co; 4.0 to 4.4 Mo; 1.30 to 1.50 Al; 2.80 to 3.20 Ti; 0.05 to 0.15 Hf; 0 to 0.2 Fe; 0.05 to 0.15 Si; 0.008 B; 0.06 Zr; 0.07 C; 0.02 of a mixture of one or more rare earth elements selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; with less than 4 ppm S; and the balance formed from Ni. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0008]    This invention is directed to superalloys that form highly adherent chromia surface layers when exposed to an oxidizing environment at elevated temperatures. The superalloy may be usable in high temperature environments, such as in use as a turbine vane or turbine blade of a turbine engine. In one embodiment, the superalloy may be configured for oxidation resistance components used in gas or steam turbine engines. 
         [0009]    In one embodiment, the superalloy may form and maintain a well adhered protective chromia scale for use at intermediate temperatures, which provides oxidation resistance when exposed to a dry or moist gas and is suitable for components used in a gas or steam turbine engine. The superalloy may be formed from materials in the following weight percentages: 10 to 25 Cr; 5 to 25 Co; 2.0 to 6.0 Mo; 0.5 to 4.0 Al; 1.0 to 4.0 Ti; 0 to 3 Hf; 0 to 10 Fe; 0 to 1 Si; 0 to 0.10 B; 0 to 0.20 Zr; 0 to 0.20 C; 0 to 0.5 total of at least one rare earth elements selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; with less than 100 ppm S; and the balance formed from Ni. 
         [0010]    A preferred superalloy for high corrosion resistance and an improved oxidation resistance may be formed from materials in the following weight percentages: 18 to 21 Cr; 12.0 to 15.0 Co; 3.5 to 5.0 Mo; 1.2 to 1.6 Al; 2.75 to 3.25 Ti; 0 to 0.5 Hf; 0 to 2 Fe; 0 to 0.5 Si; 0.003 to 0.010 B; 0.02 to 0.082 Zr; 0.03 to 0.10 C; 0 to 0.2 of one or more rare earth elements selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; with less than 10 ppm S; and the balance formed from Ni. A most preferred superalloy composition may be formed from materials in the following weight percentages: 0.8.5 Cr; 13.0 to 14.0 Co; 4.0 to 4.4 Mo; 1.30 to 1.50 Al; 2.80 to 3.20 Ti; 0.05 to 0.15 Hf; 0 to 0.2 Fe; 0.05 to 0.15 Si; 0.05 to 0.15 B; 0.06 Zr; 0.07 C; 0.02 of a mixture of one or more rare earth elements selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; with less than 4 ppm S; and the balance formed from Ni. 
         [0011]    The superalloy of the present invention is intended to be used for components where a chromia scale provides oxidation resistance. It is therefore intended that components produced from this superalloy be used at intermediate temperatures generally in the range of 450 to 750° C. and are not intended for service at temperatures of above 900° C. The turbine components prepared from the inventive superalloy can be used in the presence of a dry gas or one that includes water vapor. Therefore, the components from the inventive superalloy can be used in either a gas or steam turbine engine. 
         [0012]    The inventive superalloy has chromium (Cr) content in excess of 10 weight percent. Below this level the solution concentration in the alloy is generally insufficient to support the formation of a chromia scale with little or no other metal oxides included in the scale. To assure an excellent well adhered chromia scale the scale should be almost exclusively chromia with little content of other metals. The preferred level of 18 to 21 weight percent Cr assures that a well adhered chromia scale forms. 
         [0013]    Aluminum (Al) is included in the superalloy at levels of 1 to 4 weight percent. The level of Al is insufficient to form an alumina scale rather than remain primarily as an alloy element in the gamma prime phase at the intermediate temperatures where components of the inventive superalloy are used. 
         [0014]    Titanium (Ti) is included at 1.0 to 4.0 weight percent in the inventive alloy and in general will reside in the gamma prime phase of the superalloy where it acts as a solid-solute strengthener. In most cases titania will not be present in the chromia scale. However, some titania can be included in the chromia scale when the scale is formed near the upper temperature limits for use of the inventive superalloy. The titania can reside at the gas/chromia interface and act as a physical barrier to the loss of volatile chromium oxide species. 
         [0015]    Sulfur (S) is preferably absent from the superalloy, but is generally present as an impurity in the superalloy. It is necessary to keep the S at very low levels, below 100 ppm and is preferably below 10 ppm as the presence of S promotes spalling of oxide scales. 
         [0016]    One or more rare earth elements selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd can be included in the inventive superalloy. The inclusion of the rare earths aids in the formation, adherence, and maintenance of the chromia scale. The rare earth elements also selectively combine with sulfur to form refractory sulfides in the superalloy, preventing sulfur migration to the scale where it is detrimental to chromia adhesion to the superalloy. 
         [0017]    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.