Patent Application
20050031482
The present invention relates to materials designed to withstand high temperatures. More particularly, this invention relates to heat-resistant alloys for high-temperature applications, such as, for instance, gas turbine engine components of aircraft engines and power generation equipment. The present invention further relates to methods for repairing articles for high temperature applications.
There is a continuing demand in many industries, notably in the aircraft engine and power generation industries where efficiency directly relates to operating temperature, for alloys that exhibit sufficient levels of strength and oxidation resistance at increasingly higher temperatures. Gas turbine airfoils on such components as vanes and blades are usually made of materials known in the art as “superalloys.” The term “superalloy” is usually intended to embrace iron-, cobalt-, or nickel-based alloys, which include one or more additional elements to enhance high temperature performance, including such non-limiting examples as aluminum, tungsten, molybdenum, titanium, and iron. The term “based” as used in, for example, “nickel-based superalloy” is widely accepted in the art to mean that the element upon which the alloy is “based” is the single largest elemental component by atom fraction in the alloy composition. Generally recognized to have service capabilities limited to a temperature of about 1200° C., conventional superalloys used in gas turbine airfoils often operate at the upper limits of their practical service temperature range. In typical jet engines, for example, bulk average airfoil temperatures range from about 900° C. to about 1000° C., while airfoil leading and trailing edge and tip temperatures can reach about 1150° C. or more. At such elevated temperatures, the oxidation process consumes conventional superalloy parts, forming a weak, brittle metal oxide that is prone to chip or spall away from the part.
Erosion and oxidation of material at the edges of airfoils lead to degradation of turbine efficiency. As airfoils are worn away, gaps between components become excessively wide, allowing gas to leak through the turbine stages without the flow of the gas being converted into mechanical energy. When efficiency drops below specified levels, the turbine must be removed from service for overhaul and refurbishment. A significant portion of this refurbishment process is directed at the repair of the airfoil leading and trailing edges and tips. For example, damaged material is removed and then new material built onto the blade by any of several methods, such as, for example, welding with filler material, welding or brazing new sections onto the existing blade, or by plasma spraying or laser deposition of metal powders onto the blade. The performance of alloys commonly used for repair is comparable or inferior to that of the material of the original component, depending upon the microstructure of the repaired material, its defect density due to processing, and its chemistry. Furthermore, in current practice, the original edge material is made of the same material as the rest of the original blade, often a superalloy based on nickel or cobalt. Because this material was selected to balance the design requirements of the entire blade, it is generally not optimized to meet the special local requirements demanded by conditions at the airfoil leading or trailing edges. However, maximum temperatures, such as those present at airfoil tips and edges, are expected in future applications to be over about 1300° C., at which point many conventional superalloys begin to melt. Clearly, new materials for repair and manufacture must be developed to improve the performance of repaired components and to exploit efficiency enhancements available to new components designed to operate at higher turbine operating temperatures.
These and other needs are addressed by embodiments of the present invention. One embodiment is an alloy comprising, in atom percent, at least about 50% rhodium (Rh); at least about 5% of a metal selected from the group consisting of platinum (Pt), palladium (Pd), and combinations thereof; from about 5% to about 24% ruthenium (Ru); and from about 1% to about 40% chromium (Cr); wherein the alloy comprises less than about 50% by volume of an A3-structured phase, and wherein the quantity defined by the expression ([Cr]+2[Ru]) is in the range from about 25% to about 50%, where [Ru] and [Cr] are the atom percentages of ruthenium and chromium in the alloy, respectively.
Another embodiment is an article for use in a high temperature, oxidative environment, comprising the alloy of the present invention.
A third embodiment is a method for repairing an article. The method comprises providing an article, providing a repair material comprising the alloy of the present invention, and joining the repair material to the article.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
The description herein employs examples taken from the gas turbine industry, particularly the portions of the gas turbine industry concerned with the design, manufacture, operation, and repair of aircraft engines and power generation turbines. However, the scope of the invention is not limited to only these specific industries, as the embodiments of the present invention are applicable to many and various applications that require materials resistant to high temperature and aggressive environments. Unless otherwise noted, the temperature range of interest where statements and comparisons are made concerning material properties is from about 1000° C. to about 1300° C. The term “high temperature” as used herein refers to temperatures above about 1000° C.
The alloy of the present invention balances several competing material requirements, including, for example, cost, strength, ductility, and oxidation resistance. In accordance with one embodiment of the present invention, the alloy comprises, in atom percent, at least about 50% rhodium (Rh), at least about 5% of a metal selected from the group consisting of platinum (Pt), palladium (Pd), and combinations thereof, from about 5% to about 24% ruthenium (Ru), and from about 1% to about 40% chromium (Cr). The alloy comprises less than about 50% by volume of an A3-structured phase, which is a solid solution containing, among other elements, Ru and Cr, and is commonly referred to in the art as “epsilon phase,” or ε. The presence of this phase strengthens the alloy at the cost of some ductility. The remainder of the alloy comprises a comparatively ductile Al-structured, or face-centered cubic (FCC) phase. In order to achieve a desirable balance of properties, the composition of the alloy is maintained such that a quantity defined by the expression ([Cr]+2[Ru]) is in the range from about 25% to about 50%, where [Ru] and [Cr] are the atom percentages of ruthenium and chromium in the alloy, respectively. By adding Pd, Cr, and Ru in the above proportions to Rh, the present inventors have discovered a material having suitable high temperature strength (due to solution strengthening obtained from the alloying elements, and in some cases further strengthening due to the presence of the A3-structured phase) with sufficient ductility (due to the substantial proportion of the A1-structured phase) to be formed into useful shapes. Moreover, the cost of the alloy is reduced by adding these alloying elements to the Rh without reducing the oxidation resistance of the material below required levels. Those skilled in the art will appreciate that Pd is less expensive and less dense than Pt, and so in many applications where weight and cost are important considerations, optimal compositions often minimize the use of Pt in favor of Pd. However, Pt may be used in place of some or all of the Pd addition where very high environmental resistance is desired above all other characteristics.
The mix of the above properties and others such as modulus of elasticity can be controlled by varying the relative proportions of constituent elements. For example, Cr additions tend to lower the alloy density while increasing the thermal expansion coefficient, and Ru additions tend to increase strength and modulus. Moreover, maintaining the Cr and Ru in accordance with the expression described above controls the amount of A3-structured phase in the alloy, allowing further control over the strength and ductility of the material. In certain embodiments, the alloy comprises, in atom percent, from about 7% to about 20% ruthenium, and from about 1% to about 25% chromium. In particular embodiments, the alloy comprises, in atom percent, from about 8% to about 20% ruthenium, and from about 1% to about 10% chromium. Maintaining alloy composition within these ranges tends to minimize the amount of A3-structured phase, thereby maximizing the ductility, and therefore the formability, of the alloy.
Alloys set forth herein as embodiments of the present invention are suitable for production using any of the various known methods of metal production and forming. Conventional casting, powder metallurgical processing, directional solidification, and single-crystal solidification are non-limiting examples of methods suitable for forming ingots of these alloys. Thermal and thermo-mechanical processing techniques common in the art for the formation of other alloys, including, for instance, forging and heat treating, are suitable for use in manufacturing and strengthening the alloys of the present invention.
Another embodiment is an article for use in a high temperature, oxidative environment. The article comprises the alloy described above. The article may be one that has been repaired, or it may be a newly manufactured article. In some embodiments, the article comprises a component of a gas turbine engine, such as, for example, a turbine blade, vane, or a combustor component. Referring to
A further embodiment of the present invention is a method for repairing an article. In this method, an article is provided. The article, in certain embodiments, comprises a component of a gas turbine engine, including, for example, a blade, a vane, or a combustion component. A repair material is provided, and this repair material comprises the alloy described above for previous embodiments of the present invention. This repair material is joined to the article. In some embodiments, joining is accomplished, at least in part, by disposing a coating comprising the repair material onto the article being repaired. Suitable methods for disposing the coating include, for example, thermal spraying, plasma spraying, HVOF spraying, and laser deposition. In other embodiments, the repair material is joined to the substrate by one or more conventional joining processes, including, for example, welding, brazing, or diffusion bonding. Regardless of whether the repair material is in the form of a coating or a solid section, it may be disposed at any section of the article deemed to require the performance characteristics of the repair material. These sections include, for example, the leading and trailing edges of airfoils, and blade tips.
While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations, equivalents, or improvements therein may be made by those skilled in the art, and are still within the scope of the invention as defined in the appended claims.
