Abstract:
A superalloy with enhanced coating compatibility. The alloy is a nickel-based alloy and may be used in various applications, such as the production of gas turbine components, due to its high strength. The alloys are highly oxidation resistant and have enhanced coating compatibility. The alloys may be used as an underlying substrate with a variety of different coatings, including thermal and environmental barrier coatings. The alloys may be produced using known casting and or alloy mastering techniques.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention is directed generally to alloys, and more particularly to alloys with enhanced coating compatibility.  
       BACKGROUND  
       [0002]     Nickel-base superalloys are alloys having more nickel than any other element, and contain a group of elements that provide solid solution or gamma-prime strengthening. The gamma prime phase forms during cooling from the solution treatment temperature. Other secondary phases may form during heat treatment or during service. Nickel-base superalloys are the currently preferred alloy choice for making the components of aircraft and industrial-gas turbine engines that are exposed to the highest temperatures. Examples include turbine blades, turbine vanes, turbine blade rings, combustion system components, discs, some shafts, some rotors, and interstage seals.  
         [0003]     A nickel-base alloy and an article of manufacture containing a substrate formed of such a nickel-base alloy is may be seen in the book “Superalloys II”, edited by C. T. Sims, N. S. Stoloff and W. C. Hagel (editors), John Wiley &amp; Sons, New York 1987. Of particular relevance in this context are chapter 4 “Nickel-base alloys”, pages 97-134, chapter 7 “Directionally Solidified Superalloys”, pages 189-214, and chapter 20 “Future of Superalloys”, pages 549-562. The book discloses particular embodiments of such nickel-base alloys, termed as “superalloys”. These superalloys are characterized by their superior mechanical properties and their ability to retain these properties to temperatures amounting up to 90% of the respective melting temperatures.  
         [0004]     A nickel-base superalloy may be characterized in general terms as set out above. In several instances, a nickel-base superalloy contains a continuous matrix composed of a solid solution of chromium in nickel and a precipitate granularly dispersed in and coherent with the matrix and composed of an intermetallic nickel compound. To specify the precipitate as coherent with the matrix means that crystalline structures of the matrix are continued into the precipitate. Thus, there are, in general, no physical boundaries between the matrix and the grains of the precipitate. Instead, an interface between the matrix and the precipitate will be characterized by a local change in chemical composition through a continuous, however strained, crystal lattice.  
         [0005]     It is beneficial for nickel-base superalloys to exhibit acceptable mechanical properties at both low and high temperatures, such as good strength, good fatigue resistance, low creep rates, sufficient ductility, and acceptable density. It is also beneficial for the alloys to have good corrosion and oxidation resistance in a harsh combustion-gas environment. Further, it is beneficial for the superalloys to have good stability in both extended exposure at elevated temperature and during cyclic heating and cooling patterns. These properties may be achieved through the careful selection of the alloying elements and the processing of the material. A number of superalloy compositions have been developed to supply the appropriate combinations of these properties for various applications in the gas turbine environment.  
         [0006]     Nickel-based superalloys are widely used for hot section components in both aero and industrial gas turbines as they retain their excellent mechanical properties to high temperatures. However, to operate at increasingly higher temperatures it is sometimes necessary to apply a coating to the superalloy for thermal protection. The thermal protection system typically includes a bondcoat and a thermal barrier coating (TBC). The bondcoat provides an interfacial layer between the superalloy and the TBC. During prolonged high temperature exposure the bondcoat degrades and this degradation eventually leads to the spallation of the TBC and loss of thermal protection of the coating. The rate at which the bondcoat degrades depends greatly on the composition of the superalloy substrate. The alumina forming superalloys generally exhibit better bondcoat compatibility and consequently longer coating lives than the chromia forming superalloys.  
         [0007]     Accordingly, what is needed is an alloy having better coating compatibility. Also what is needed is an alloy having enhanced coating compatibility that may be formed using conventional alloy forming methods. Also what is needed is an alloy that may be used in a variety of different applications, such as those in gas turbine engine components.  
       SUMMARY OF THE INVENTION  
       [0008]     This present invention provides an alloy having enhanced coating compatibility. The alloys of the present invention are nickel-based alloys and may be used in various applications, such as the production of gas turbine components, due to the high strength and/or oxidation resistance of the alloys. The alloys of the present invention include large quantities of aluminum which promotes the formation of a stable alumina scale when exposed to high temperatures in an oxidizing environment. The alloys of the present invention also include one or more rare earth elements selected from lanthanum, yttrium, cerium, or a combination thereof. The rare earth element improves oxidation resistance and/or enhances the compatibility of the alloy with various coatings. The alloys of the present invention may be used as an underlying substrate with a variety of different coatings, including thermal and environmental barrier coatings. The alloys may be produced using known vacuum or inert environment casting and or alloy mastering techniques. The alloys may be used in investment cast components produced by conventional casting, directional solidification, or single crystal casting techniques.  
         [0009]     These and other embodiments are described in more detail below. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]     The present invention is more particularly described in the following description and examples that are intended to be illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art. As used in the specification and in the claims, the singular form “a,” “an,” and “the” may include plural referents unless the context clearly dictates otherwise. Also, as used in the specification and in the claims, the term “comprising” may include the embodiments “consisting of” and “consisting essentially of.” 
         [0011]     The present invention provides an alloy. The alloy is designed to have enhanced coating compatibility, especially for thermal barrier coatings (TBC). In addition, the alloys of the present invention may be designed to have excellent oxidation resistance. The base superalloy, the TBC, and any bondcoat may be considered as a system wherein each part of the system may be matched for optimum performance and/or life of the system. Traditionally, coatings have been developed to match existing superalloy compositions and little work has been undertaken to optimize the superalloy composition for coating compatibility and/or optimum system performance.  
         [0012]     The alloys of the present invention contain relatively large amounts of aluminum that promotes the formation of a stable alumina scale when exposed to a high temperature an oxidizing environment. The alloys of the present invention also contain small amounts of one or more rare earth elements in combined amounts of up to about 0.12% by weight of the alloy.  
         [0013]     The addition of small amounts of rare earth elements lanthanum (La), yttrium (Y), and/or cerium (Ce) in small amounts has been found to dramatically improve the oxidation resistance and/or enhance the compatibility of the alloy with various coatings. The increase in coating life, though the addition of one or more rare earth elements, is attributed to the ability of these elements to form sulfides and oxi-sulfides that reduce the residual sulfur content and that help to prevent the diffusion of sulfur atoms to the alumina scale. Sulfur has been cited as being one of the elements most detrimental to the protective alumina scale. One reason for this is that sulfur reduces the adherence of the alumina scale by weakening the Van der Waal&#39;s bond between the scale the superalloy base material.  
         [0014]     Accordingly, in one embodiment of the present invention, the alloys include small amounts lanthanum, yttrium, cerium, or a combination thereof. The alloys may include one or more rare earth elements. Accordingly, based upon the alloy, the amounts of the other components, and/or the selected characteristics of the final alloy, the alloy may include only one of lanthanum, yttrium or cerium. In alternative embodiments, the alloy may include only two of lanthanum, yttrium or cerium. In other alternative embodiments, the alloy may include all three of lanthanum, yttrium and cerium.  
         [0015]     The amounts of the at least one rare earth elements included in the alloy may vary depending on one or more factors including, but not limited to, the alloy, the amounts of the other components, and/or the selected characteristics of the final alloy. In one embodiment, the total amount of lanthanum, yttrium, cerium, or a combination thereof in the alloy is from about 0.001 to about 0.12 percent, by weight, of the alloy. In another embodiment, the total amount of lanthanum, yttrium, cerium, or a combination thereof in the alloy is from about 0.005 to about 0.05 percent, by weight, of the alloy. In yet another embodiment, the total amount of lanthanum, yttrium, cerium, or a combination thereof in the alloy is from about 0.015 to about 0.025 percent, by weight, of the alloy.  
         [0016]     In addition to the at least one rare earth element, the alloys of the present invention include various other elements. The amounts of each element are selected to achieve an alloy having enhanced compatibility to coatings. The alloys of the present invention, in various embodiments, include varying amounts of chromium (Cr), cobalt (Co), molybdenum (Mo), tungsten (W), tantalum (Ta), aluminum (Al), titanium (Ti), boron (B), zirconium (Zr), carbon (C), hafnium (Hf), and nickel (Ni).  
         [0017]     In one aspect, the alloys of the present invention include chromium. In one embodiment, the amount of chromium in the alloy is from about 7.2 to about 9.3 percent, by weight, of the alloy. In another embodiment, the amount of chromium in the alloy is from about 7.6 to about 8.5 percent, by weight, of the alloy. In yet another embodiment, the amount of chromium in the alloy is from about 7.8 to about 8.3 percent, by weight, of the alloy.  
         [0018]     In another aspect, the alloys of the present invention include cobalt. In one embodiment, the amount of cobalt in the alloy is from about 8.5 to about 10.0 percent, by weight, of the alloy. In another embodiment, the amount of cobalt in the alloy is from about 8.8 to about 9.7 percent, by weight, of the alloy. In yet another embodiment, the amount of cobalt in the alloy is from about 9.1 to about 9.5 percent, by weight, of the alloy.  
         [0019]     In still another aspect, the alloys of the present invention include molybdenum. In one embodiment, the amount of molybdenum in the alloy is from about 0.2 to about 0.8 percent, by weight, of the alloy. In another embodiment, the amount of molybdenum in the alloy is from about 0.3 to about 0.7 percent, by weight, of the alloy. In yet another embodiment, the amount of molybdenum in the alloy is from about 0.4 to about 0.6 percent, by weight, of the alloy.  
         [0020]     In yet another aspect, the alloys of the present invention include tungsten. In one embodiment, the amount of tungsten in the alloy is from about 8.8 to about 10.2 percent, by weight, of the alloy. In another embodiment, the amount of tungsten in the alloy is from about 9.2 to about 9.8 percent, by weight, of the alloy. In yet another embodiment, the amount of tungsten in the alloy is from about 9.4 to about 9.6 percent, by weight, of the alloy.  
         [0021]     In still another aspect, the alloys of the present invention include tantalum. In one embodiment, the amount of tantalum in the alloy is from about 2.8 to about 3.7 percent, by weight, of the alloy. In another embodiment, the amount of tantalum in the alloy is from about 3.0 to about 3.5 percent, by weight, of the alloy. In yet another embodiment, the amount of tantalum in the alloy is from about 3.1 to about 3.3 percent, by weight, of the alloy.  
         [0022]     In yet another aspect, the alloys of the present invention include aluminum. In one embodiment, the amount of aluminum in the alloy is from about 5.0 to about 6.0 percent, by weight, of the alloy. In another embodiment, the amount of aluminum in the alloy is from about 5.2 to about 5.8 percent, by weight, of the alloy. In yet another embodiment, the amount of aluminum in the alloy is from about 5.5 to about 5.6 percent, by weight, of the alloy.  
         [0023]     In still another aspect, the alloys of the present invention include titanium. In one embodiment, the amount of titanium in the alloy is from about 0.4 to about 1.2 percent, by weight, of the alloy. In another embodiment, the amount of titanium in the alloy is from about 0.6 to about 0.9 percent, by weight, of the alloy. In yet another embodiment, the amount of titanium in the alloy is from about 0.7 to about 0.8 percent, by weight, of the alloy.  
         [0024]     In yet another aspect, the alloys of the present invention include boron. In one embodiment, the amount of boron in the alloy is from about 0.005 to about 0.03 percent, by weight, of the alloy. In another embodiment, the amount of boron in the alloy is from about 0.008 to about 0.025 percent, by weight, of the alloy. In yet another embodiment, the amount of boron in the alloy is from about 0.01 to about 0.02 percent, by weight, of the alloy.  
         [0025]     In still another aspect, the alloys of the present invention include zirconium. In one embodiment, the amount of zirconium in the alloy is from about 0.003 to about 0.03 percent, by weight, of the alloy. In another embodiment, the amount of zirconium in the alloy is from about 0.006 to about 0.025 percent, by weight, of the alloy. In yet another embodiment, the amount of zirconium in the alloy is from about 0.01 to about 0.02 percent, by weight, of the alloy.  
         [0026]     In yet another aspect, the alloys of the present invention include carbon. In one embodiment, the amount of carbon in the alloy is from about 0.03 to about 0.13 percent, by weight, of the alloy. In another embodiment, the amount of carbon in the alloy is from about 0.05 to about 0.11 percent, by weight, of the alloy. In yet another embodiment, the amount of carbon in the alloy is from about 0.07 to about 0.09 percent, by weight, of the alloy.  
         [0027]     In still another aspect, the alloys of the present invention include hafnium. In one embodiment, the amount of hafnium in the alloy is from about 1.0 to about 2.0 percent, by weight, of the alloy. In another embodiment, the amount of hafnium in the alloy is from about 1.2 to about 1.8 percent, by weight, of the alloy. In yet another embodiment, the amount of hafnium in the alloy is from about 1.4 to about 1.6 percent, by weight, of the alloy.  
         [0028]     The balance of the alloy, i.e. from about 55 to about 70 percent, by weight, is nickel.  
         [0029]     The alloys of the present invention may be produced by known vacuum or inert environment casting and or alloy mastering techniques. The alloys may be used in investment cast components produced by conventional casting, directional solidification or single crystal casting techniques.  
         [0030]     One example of a conventional casting technique is equiaxed investment casting in which there is a thin-shell ceramic mold comprised of a pour cup, sprue, runners, gates and component patterns. The alloy is pre-melted in a crucible and subsequently poured into the ceramic mold where is solidifies with an equiaxed grain structure. Upon completion of alloy solidification, the ceramic shell is broken away from the cast components. The cast components are cut away from the gating system and finished into a final casting.  
         [0031]     One example of a directional solidification casting method is directionally solidified investment casting in which there is a thin-shell ceramic mold comprised of a pour cup, sprue, runners, gates, starter blocks and component patterns arranged such that the bottom of the mold is open at the starter blocks. The alloy is pre-melted in a crucible and subsequently poured into the ceramic mold which sits on a ‘chill plate’. Solidification of the molten alloy initiates at the starter blocks and is subsequently controlled by the removal of heat from the bottom of the mold to the top, resulting in a ‘directionally solidified’ grain structure. Upon completion of alloy solidification, the ceramic shell is broken away from the cast components. The cast components are cut away from the gating system and starter blocks and finished into a final casting.  
         [0032]     The superalloys of the present invention may be used in the formation of a wide variety of different articles. Examples of articles that may be made by the present invention include, but are not limited to, gas turbine components such as turbine blades, turbine vanes, turbine blade rings, combustion system components, heat shield elements, or a combination thereof.  
         [0033]     The alloys of the present invention have enhanced coating compatibilities. As such, coatings may be applied either directly to the alloy, or onto a bondcoat that has been applied to the alloy. The coating that may be applied may be any coating capable of being applied to an alloy. Examples of coatings that may be applied include, but are not limited to, bondcoats, overlay coatings, environmental barrier coatings, thermal barrier coatings, or a combination thereof.  
         [0034]     The foregoing is provided for purposes of illustrating, explaining, and describing embodiments of this invention. Modifications and adaptations to these embodiments will be apparent to those skilled in the art and may be made without departing from the scope or spirit of this invention.