Patent Application
20060147335
This invention relates generally to metals and metal alloys used in high temperature applications. More specifically, the invention relates to niobium-silicide compositions which are useful for a variety of turbine engine components.
Various types of metals and metal alloys are used for high temperature equipment, e.g., turbine engines and other machinery. Choice of a particular metal depends in large part on the projected temperature-exposure of the component, along with other specified requirements for the component—strength, creep resistance, oxidation resistance, environmental resistance, weight requirements, and the like.
Gas turbine engines provide a good example of how component requirements can vary throughout a single (albeit complex) piece of equipment. In a typical gas turbine engine, air is compressed in a compressor and mixed with fuel and ignited in a combustor for generating hot combustion gases. The gases flow downstream through a high pressure turbine (HPT) having one or more stages, including a turbine nozzle and rotor blades. The gases then flow to a low pressure turbine (LPT) which typically includes multi-stages with respective turbine nozzles and rotor blades. Nickel-based superalloys are often the materials of choice for the “hot” sections of the turbine, where metal temperatures as high as about 1150° C. are typical. Titanium alloys, which are lighter than the nickel alloys, are often used in the compressor sections of the turbine engines, where temperatures are lower, e.g., less than about 600° C.
As turbine engine operating temperatures increase to satisfy greater demands in efficiency, new materials for the higher-temperature environment have been developed. Examples are the refractory metal intermetallic composite (RMIC) materials. Many of these are based on niobium (Nb) and silicon (Si), and are described, for example, in U.S. Pat. No. 5,932,033 (Jackson and Bewlay); U.S. Pat. No. 5,942,055 (Jackson and Bewlay); and U.S. Pat. No. 6,419,765 (Jackson, Bewlay, and Zhao).
The RMIC composites usually have a multi-phase microstructure. For example, the microstructure may comprise a metallic Nb-base phase and one or more intermetallic metal silicide phases. As described in U.S. Pat. No. 5,833,773 (Bewlay and Jackson), the metal silicide phase sometimes includes an M3Si silicide and an M5Si3 silicide, where M is Nb, Ti or Hf. The materials are considered to be composites that combine high-strength, low-toughness silicides with a lower-strength, higher-toughness Nb-based metallic phase. They often have melting temperatures of up to about 1700° C., and possess a relatively low density as compared to many nickel alloys. These characteristics make such materials very promising for potential use in applications in which the temperatures exceed the current service limit of the nickel-based superalloys.
As mentioned above, some sections of a turbine engine do not require the very high-temperature capabilities possessed by many of the niobium-silicide alloys. For example, the low-pressure turbine sections often are exposed to temperatures in the range of about 600° C.-1000° C. While these temperature requirements are not as demanding as in the case of the hot sections of the turbine, other characteristics of the components may assume greater importance. For example, the low pressure turbine components may have higher damage tolerance requirements than those which can be satisfied by using the typical niobium-silicide alloys intended for hot-section applications. Moreover, the lower-temperature components may still require a relatively high level of strength, along with other attributes, like oxidation resistance and creep resistance.
It should thus be apparent that new niobium-silicide alloys which exhibit a more appropriate balance of properties for selected, temperature-based applications would be welcome in the art. The compositions should provide good performance at intermediate operating temperatures, e.g., about 600° C.-1000° C., in terms of one or more properties like strength, ductility, and creep resistance. Moreover, the compositions should be lighter than many of the nickel-based superalloys employed at the LPT and HPT operating temperatures.
One embodiment of this invention is directed to a refractory composition comprising niobium and silicon. The amount of silicon present is less than about 9 atom %, based on total atomic percent for the composition. In some embodiments, the composition is characterized by a microstructure comprising a metallic Nb-base phase and at least one metal silicide phase of the formula M3Si or M5Si3, wherein M is at least one element selected from the group consisting of Nb, Hf, Ti, Mo, Ta, W, a platinum group metal, and combinations thereof.
Another embodiment relates to a turbine engine component (e.g., a gas turbine), as described herein. The component comprises an alloy of niobium and silicon, wherein the amount of silicon present is less than about 9 atom %, based on total atomic percent. (As used herein, “alloy” is meant to describe a solid or liquid mixture of two or more metals, or one or more metals with one or more non-metallic elements).
Further details regarding the various features of this invention are found in the remainder of the specification.
The refractory composition of this invention comprises niobium and silicon. Silicon is present at less than about 9 atom %, based on total atomic percent in the composition. In some embodiments, the amount of silicon present is at least about 0.5 atom %. Moreover, in other preferred embodiments, silicon is present in the range of about I atom % to about 8.5 atom %, and most often, about 5 atom % to about 8.5 atom %. However, in other preferred embodiments, the silicon is present at a level of about 1 atom % to about 5 atom %. In general, for selected turbine component applications, the presence of these relatively small amounts of silicon appears to lead to considerable improvements in the ductility and “damage tolerance” of the components, when they are employed at intermediate temperature ranges, e.g., about 600° C. to about 1000° C.
In preferred embodiments, the refractory composition further comprises at least one element selected from the group consisting of titanium, hafnium, chromium, and aluminum. The choice and selected amount for each of these elements depends on a variety of factors. However, performance requirements for a particular end use are usually most important.
Titanium is usually employed to improve high-temperature oxidation resistance. The presence of Ti can also improve the intrinsic ductility of the metallic phase. When present, the level of Ti is usually in the range of about 5 atom % to about 45 atom %, based on total atomic percent for the composition. In some preferred embodiments, Ti is present at a level in the range of about 10 atom % to about 30 atom %. Moreover, in some especially preferred embodiments, Ti is present at a level in the range of about 15 atom % to about 25 atom %.
Hafnium can serve as a solid solution-strengthener of the Nb-based metallic phase. Hf can also reduce the internal oxidation of the metal phase, as well as improving creep performance. When present, the level of Hf is usually in the range of about 1 atom % to about 20 atom %, based on total atomic percent for the composition. In some preferred embodiments, Hf is present at a level in the range of about 2 atom % to about 15 atom %. In some especially preferred embodiments, the level of Hf is about 2 atom % to about 10 atom %.
Chromium is usually present to improve oxidation resistance. For these niobium-silicide compositions, the presence of Cr can promote the formation of a silicon-modified chromium-based Laves-type phase, as described in U.S. Pat. No. 5,942,055 (Jackson et al), which is incorporated herein by reference. The presence of the Laves phase can be a desirable characteristic, in regard to oxidation resistance.
When present, the level of Cr is usually in the range of about 1 atom % to about 25 atom %, based on total atomic percent for the composition. In some preferred embodiments, Cr is present at a level in the range of about 5 atom % to about 15 atom %. Moreover, in some especially preferred embodiments, Cr is present at a level in the range of about 5 atom % to about 10 atom %.
Aluminum may also be present in the niobium-silicide compositions, as mentioned above. Al can also improve oxidation resistance. When present, the level of Al is usually in the range of about 1 atom % to about 20 atom %, based on total atomic percent for the composition. In some preferred embodiments, Al is present at a level in the range of about 5 atom % to about 15 atom %. In some of the especially preferred embodiments, Al is present at a level in the range of about 5 atom % to about 10 atom %.
In some instances, the refractory composition described herein further comprises at least one platinum group metal. As used herein, the term “platinum group metal” is meant to describe the following: rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), ruthenium (Ru), rhodium (Rh), and palladium (Pd). The platinum group metals can be used to enhance various properties, such as strength (e.g., tensile strength), oxidation resistance, formability, ductility, toughness, fatigue resistance, and creep resistance.
The level of the platinum group metal can vary significantly, depending on end use requirements, e.g., as they relate to the properties mentioned above. The platinum group metal is usually present in the range of about 1 atom % to about 30 atom %. In some preferred embodiments, the platinum group metal is present at a level of about 1 atom % to about 25 atom %. In some especially preferred embodiments, the range is from about 1 atom % to about 15 atom %. A preferred group of the platinum group metals comprises Pt, Re, and Ru. Re and Ru are sometimes of special interest, because of their ability to significantly strengthen these low-silicon compositions, while providing acceptable ductility.
Rhenium is an especially preferred platinum group metal for some embodiments of the present invention. An appropriate level of Re will depend on the factors set forth above. When used, Re is usually present at a level of up to about 20 atom %, based on total atomic percent for the refractory composition. A preferred range of Re for many embodiments is about 1 atom % to about 15 atom %. An especially preferred range is about 1 atom % to about 12 atom %.
In some embodiments, the refractory composition further comprises at least one element selected from the group consisting of tungsten (W), tantalum (Ta), and molybdenum (Mo). These elements are often helpful in increasing the tensile strength of the metallic phase, and the creep strength of both the metallic phase and the intermetallic phase. However, their presence may also result in a heavier alloy product—especially in the case of tantalum and tungsten. Moreover, at certain levels, these alloys could adversely affect oxidation resistance. Thus, the appropriate amount of each of these elements will depend on a variety of end use requirements.
Usually, W, Ta, and Mo are individually present at a level of less than about 30 atom %, based on total atomic percent for the composition. In preferred embodiments, they are present at a level in the range of about 1 atom % to about 25 atom %. In some especially preferred embodiments, they are individually present at a level in the range of about 1 atom % to about 20 atom %. As a group, their total level is usually less than about 40 atom %, and more often, less than about 30 atom %.
In some embodiments, the refractory composition further comprises at least one rare earth element, i.e., lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. (For the purpose of this disclosure, yttrium is also considered to be a part of the rare earth group). The rare earth elements can further improve oxidation resistance (e.g., internal oxidation resistance), as well as adherence of the oxide scale to the parent component. They can also improve ductility.
The rare earth elements are usually employed at relatively small levels, e.g., less than about 10 atom %, based on total atomic percent for the composition. In preferred embodiments, each rare earth element, when included, is present at a level in the range of about 0.1 atom % to about 5 atom %. Preferred rare earth elements for some embodiments are yttrium, terbium, dysprosium, and erbium.
Other elements may also be included in the refractory composition. Examples include at least one of boron (B), carbon (C), germanium (Ge), zirconium (Zr), vanadium (V), tin (Sn), nitrogen (N), iron (Fe) or indium (In). These elements are usually employed (individually) at levels in the range of about 0.1 atom % to about 15 atom %, based on total atomic percent for the composition, although the level of Zr can be as high as about 20 atom %. The presence of these elements enhances one or more characteristics. For example, an interstitial element like B can improve oxidation resistance, as can V. Moreover, C can improve creep resistance, as well as tensile strength. The addition of N can help stabilize an Nb5Si3 phase in the alloy, as described in pending application Ser. No. 10/932,128 (RD-27,311-1). (That patent application was filed on Sep. 1, 2004 for Bernard Bewlay et al, and is incorporated herein by reference). Some of these elements can also raise the temperature range-of-stability for one or more phases in the refractory product.
Non-limiting, exemplary ranges can be provided for these elements, based on total atomic % in the composition:
Table 1 lists some of the more specific compositions which fall within the scope of this invention, and which are preferred in some embodiments. All quantities are in atom percent, and based on 100 atom % for the entire composition. (Moreover the compositions may comprise other elements as well).
*PGM = platinum group metal
As mentioned above, the refractory compositions of this invention (i.e., in alloy form) are sometimes characterized by a multi-phase microstructure. In general, the microstructure comprises a metallic Nb-base phase and at least one metal silicide phase of the formula M3Si or M5Si3, wherein M is at least one element selected from the group consisting of Nb, Hf, Ti, Mo, Ta, W, a platinum group metal, and combinations thereof. Very often, the metal silicide phase comprises an Nb3Si phase or an Nb5Si3 phase, or a combination of the two phases. Some of the alloys described herein include other phases as well. For example, they may further include a chromium-based Laves-type phase, modified with silicon. Such a phase promotes oxidation resistance, as described in U.S. Pat. No. 5,932,033 (Jackson and Bewlay), which is incorporated herein by reference.
The selection of phases and element constituents in alloys made from the refractory compositions is aimed at achieving a balance of properties which are important for a particular end use application. The primary properties were mentioned above, e.g., strength (fracture strength and rupture strength), toughness, density, oxidation resistance, and creep resistance. As described in U.S. Pat. No. 5,833,773, all of the elements mentioned above partition to varying degrees between different phases of the alloy.
Methods for preparing the refractory compositions and alloys of this invention are generally known in the art. Non-limiting illustrations of preparation techniques are provided in examples in the following patents: U.S. Pat. No. 6,419,765 (Jackson et al); U.S. Pat. No. 5,833,773 (Bewlay et al); and U.S. Pat. No. 5,741,376 (Subramanian et al), all of which are incorporated herein by reference. Frequently, the alloy constituents, in elemental form, are combined by melting in a crucible by an appropriate technique, such as arc melting, electron beam melting, plasma melting, and induction skull melting. However, other techniques (or combinations of techniques) can be used in preparing the alloy compositions. For example, powder metallurgical techniques such as grinding or atomization (e.g., gas atomization) could be employed, as well as vapor deposition.
The alloy product can be processed and formed into a desired article by a variety of techniques. For example, a molten alloy product can be cast in a suitable apparatus. Mold assemblies for casting are well known in the art. One example is provided in U.S. Pat. No. 6,676,381 (Subramanian et al), which is incorporated herein by reference. However, many casting techniques could be employed. Moreover, those skilled in the art are familiar with various operational details regarding any particular casting technique. In some preferred embodiments, the molten metal is solidified by a directional solidification (DS) technique. DS techniques are well-known in the art (e.g., the Bridgman technique), and described, for example, in U.S. Pat. Nos. 6,059,015 and 4,213,497 (Sawyer), which are incorporated herein by reference.
A variety of other techniques (alone or in combination) can also be used to process the alloy products. Non-limiting examples include extrusion (e.g., hot-extrusion), forging, hot isostatic pressing, and rolling. Those skilled in the art are familiar with details regarding appropriate thermomechanical treatments of the alloys.
The low-silicon refractory compositions can be formed into a variety of components. Many of them could be used in turbines, e.g., land-based turbines, marine turbines, and aeronautical turbines, although non-turbine applications are also possible. These components can greatly benefit from the enhancements in strength, ductility, and creep resistance at selected operational temperatures. Thus, another embodiment of this invention is directed to such components. Specific, non-limiting examples of the turbine components are buckets, nozzles, blades, rotors, vanes, stators, shrouds, and combustors.
While preferred embodiments have been set forth for the purpose of illustration, the foregoing description should not be deemed to be a limitation on the scope of the invention. Accordingly, various modifications, adaptations, and alternatives may occur to one skilled in the art without departing from the spirit and scope of the claimed inventive concept. All of the patents, patent applications (including provisional applications), articles, and texts which are mentioned above are incorporated herein by reference.
